Mar 222020

GMC Yukon

I have replaced 2 rear brake lines from ABS Module to rear brake. Plus replaced Master cylinder with active control. Bled the farther rear line, then the next rear left. Then Bled the right front brake and last front left. But I’m not getting pressure to the brake petal, it goes to the floor. What can be the problem. I don’t have a scan tool for ABS Module. Can it be a fuse? or is it the module? or do I continue to bleed the lines?
Thank you for your help it is very much appreciated!

Did you bench bleed the master cylinder before installation?

Lets will assume you did. The brakes on these are pickier than any other brakes I have worked with in 30 years. I have had a time trying to get a brake pedal on the 2006 Chevy trucks. I replaced the brake brake lines with stainless steel and couldn’t get a pedal no matter how many times I tried to bleed them. But fortunately I am not one to give up easy.

I even dove into the thinking it was the ABS Module bleed issue. No it was not. A work around is just getting it to activate and it is considered bled. This is of course if you do not have a scan tool.

How to get the brakes to bleed

It turns out that the only way I could get the rear wheel cylinders to bleed all the air out was to over adjust the rear brake shoes (This was also done after replacing the rear brake shoes). I had to tighten the shoes all the way and then bleed the brakes. Then test and adjust the rear shoes. I know it sounds odd, but you will see these shoes like to be tight.

After doing this the brakes worked just fine. Before this I was having to pump the brake pedal to get it to stop at all. Yes, I too replaced the master cylinder.

2006 GMC Envoy Stabilitrac warning message

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Dec 042019

2006 GMC Envoy

Have warning light Readout says service stabilatrac ?

First of all,  you will find this issue has been addressed in the below articles. Secondly, the most common failure part that would cause this issue is the Steering Position Sensor. The video below shows how to replace it. Lastly, I have found additional information in the form of a TSB and added it below.



How to fix the StabiliTrak Service Light on by replacing the Steering Position Senor

Will not shift out of second gear

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Oct 182019

1995 GMC Yukon

My 1998 Yukon 4×4 slt 5.7l will not shift out of second gear. If I drive it hard n hot rod it then it seems to shift Tru the gears like it’s supposed to. The fluid looks dark as well I’m not shur what the problem may be. Cud u please help

First thing to do would be to check to see if there are any codes stored in the computer that might aim us in the correct direction. Post any codes found and we can dig further into that.

Stuck in 2nd gear

Some have suggested that a faulty VSS – Vehicle Speed Sensor may cause this issue. Others have run into valve body issues. Still yet there have been claims of faulty shift solenoids causing the same issue. And another has claimed faulty ignition switch caused the vehicle to not shift out of 2nd gear. Others claim a faulty fuse issue. (see video).

The transmission on this vehicle will intermittently default
to second gear. I’ve had other GM trucks do this with ign.
switch problems i.e. low voltage to term. 20 in fuse panel.
However, when this problem occurs the voltage is normal. It
also doesn’t exhibit the speedometer/tachometer problems
normally associated with ign.switch problems on these
vehicles. I did note that when the problem occurs the security
light comes on with no starting problems and I lose scan tool
data stream. Any help would be greatly appreciated!

____________________________ FIX ___________________________

The problem turned out to be the ignition switch, even though
the symptoms were a little different than the ones I’d had in
the past. Voltage drop to 10v on fuse #20 when the fault was
present. Thanks for the help!

Source: Chevy Mechanic: Mike

2012 GMC Acadia Radio Removal

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Oct 072019

2012 GMC Acadia

This is specifically a question about the interior of the vehicle.
My wife’s 2012 GMC Acadia front-wheel drive with a little over 71k miles. The top of the center console where it connects to the dashboard came loose and I pressed it back into place. Except for the right side – it is being stubborn.
1. Is this something my wife should be concerned about?
2. Is it something that I can fix myself?

The instrument panel center accessory trim plate just snaps into place. To remove it calls for use of a suitable flat bladed trim tool. If you or someone has removed it to do a repair or work on a radio or something they may not have installed the internal components correctly causing it to be a bit stubborn.

2012 GMC Arcadia Radio Navigation Removal

Fouling spark plugs and burning oil 2006 GMC Sierra 2500HD 8.1L

 Auto Repair Questions, Chevy, GMC  Comments Off on Fouling spark plugs and burning oil 2006 GMC Sierra 2500HD 8.1L
Oct 052019

2007 GMC Sierra

Engine burning oil

This question is in regard to a 2006 GMC Sierra 2500HD (8.1L gas engine) with 197,000 miles on it. It had around 60k or 70k miles when I bought it in 2012. It has had a rough life since then – lots of hauling and towing. It has had a long-running problem of burning oil (I would have to add some oil every month), but the oil-burning has suddenly gotten much worse, and I am adding oil every few days.

Every mechanic I’ve ever taken the truck to always says “that’s just what old trucks do, they burn oil”. Another problem (which I assume is related) is that the spark plugs get ruined frequently and have to be changed. I’ve had to change them at 124k miles, 130k (wires only), 170k, 183k, and 197k. At 197k, I told the mechanic I was tired of spending so much on spark plugs, and we decided to try a cheaper plug and see what happens. This, in addition to a full tune-up/fluid exchanges, fixed the symptoms I was seeing (intermittent shudder when idling, ‘check engine’ light coming on when going uphill over 45mph, engine misfire code P0300).

But then I only made it 1200 miles before those same symptoms returned. Same P0300 code, one ruined spark plug, cylinder 2. A mechanic replaced that one plug, and I made it another 80 miles before things went south again yesterday. This time it’s 2 plugs, cylinder 2 and another cylinder (he didn’t tell me which one). So we’re replacing all plugs with the better double platinum ones.

Any ideas what is going on? Am I just doomed to replace the plugs every 15k miles from now on? I love this truck and don’t want to get rid of it. For anyone interested, there are more details below, in case they provide any additional clues. Thanks in advance!

Full Maintenance History

  • oil changes are done every 5k miles
  • oil cooler lines replaced in 2013
  • major tune-up at 124k miles (transmission flush, coolant flush, 4×4 service (front/rear differentials, transfer case), spark plugs, fuel induction system service, air filter replaced)
  • 130k, replaced spark plug wires, cleaned mass air flow sensor
  • 132k, replaced fuel pump assembly
  • 169k, code P0171, cleaned mass air flow sensor
  • 170k, replaced spark plugs, plug wires (mechanic said plugs were really bad and covered in oil)
  • 174k, replaced air filter
  • 183k, replaced spark plugs and air filter
  • 1955xx miles, replaced serpentine belt, belt tensioner, and fan clutch; mechanic said spark plugs are misfiring/worn out (but I couldn’t afford to replace them yet); mechanic said “we took off the valve covers and looked inside of the cylinder heads, in case the valves are gummed up. But we didn’t really find anything except the piston rings are a bit burned out.” He recommended trying Lucas oil stabilizer and Lucas fuel cleaner (which I did) and using premium gas and higher quality synthetic oil from now on (which I did). He said the engine is burning lean, so better fuel will help that. Told me to get a tune-up as soon as I can.
  • at 197k, ‘check engine’ light came on every time going uphill over 45mph; code was P0300 (engine misfire; no other codes); idle is very rough; replaced spark plugs/wires with cheaper copper plugs as an experiment, did fuel exchanges (differentials, transfer case, transmission (& filter), coolant, power steering fluid); cleaned the air filter
  • 1200 miles later, replaced 1 spark plug; added “Engine Restorer 8-Cylinder” to the oil, as recommended by the mechanic
  • 80 miles later (today), replacing all spark plugs with the right kind. Mechanic says it will take a while of driving at high speeds for the “Engine Restorer” to work?

I travel a lot, so I rarely get to use the same mechanic. Whenever I go to a new mechanic, they usually tell me that my last mechanic was BS-ing me and/or didn’t know what he was talking about. I’m a car dummy, so I have to just take the word of these mechanics, and I never really know when I’m getting the run-around.

Thanks to anyone who made it this far in my post!
(attached photo is the cheapo copper spark plug after 1200 miles)

Oil Fouled Spark Plug

Print a copy of your history and present it to your next mechanic, they will appreciate knowing what you have already gone through.

Chevy engines burn oil. This is true but they shouldn’t foul the plug in 1200 miles. You have something else going on.

Definitely fouling the spark plug. So the engine is going to store the P0300 – Engine Misfire code when this happens. That is normal during this situation. There are several things that can cause this code and situation.  The good news would be that you have tried a few things that tell us which cylinders are the problem cylinder.

Compression Test

The next thing to do would be to have a compression test performed. This information can sometimes explain a lot of whats going on. And this will confirm the problem cylinder/s.

Head Gasket

If two cylinders next to each other are significantly lower than the rest there is a possibility of a blown head gasket. However a blown head gasket would also be followed by white exhaust smoke and coolant loss. Since you didn’t mention any of those symptoms we will move on.

Fuel Injector

If there is specifically one cylinder that keeps fouling out the spark plug there is a chance of the fuel injector being stuck OPEN(ON). When this occurs the cylinder rings will wash out(unseat) and foul out that spark plug. After time the the excessive fuel being dumped into the cylinder with washed out rings will allow the fuel to seep into the crankcase and contaminate the engines oil. When this happens you will notice excessive exhaust smoke and start fouling out additional spark plugs.

When looking at the oil on the dipstick it may also have a fuel smell. A quick test can be to remove the spark plugs from the effected cylinders and cycle the fuel pump on and off a few times. While keeping at a distance to prevent fuel from shooting out at you, look to see if the cylinder fills with fuel. If you find the fuel injector is stuck open you will need to replace it and and then perform an oil change to remove any contaminates oil.

Hole or Crack in Piston

You didn’t mention it but i want to cover engine ping. If you experience the engine pinging when climbing a hill or under heavy acceleration you may have caused internal damage. Continually driving the vehicle under these conditions will burn a hole in the piston. If this is the case you will have 0 compression and oil foul the spark plug quickly.

Worn Valve Guides and seals

When the valve seals or guides are worn you will see a puff of smoke at start up. Sometimes lasting until the engine reaches operating temperature. This usually occurs after the vehicle has had time to set and the oil drains down past the guide seals and into the cylinders.

Engine Misfire Causes

  • Faulty spark plug (s)
  • Faulty ignition coil (s) – Engine Coil
  • Ignition coils harness is open or shorted
  • Ignition coils poor electrical connection
  • Clogged or faulty fuel injectors
  • Injectors harness is open or shorted
  • Injectors poor electrical connection
  • Improper spark plugs
  • Insufficient cylinders compression
  • Incorrect fuel pressure
  • Intake air leak


P023F Fuel Pump Secondary Circuit 2016 GMC Sierra SLE z71

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Jun 252019

pulled the codes po23f fuel relay
cant find the relay

P023F – Fuel Pump Secondary Circuit Open

Code P023F possible causes:
  • Bad fuel pump relay
  • Short to voltage on FP voltage supply circuit or feedback circuit
  • Short to ground on PCM
  • FP relay control circuit PCM
  • FP relay control driver shorted to ground internally

There are several fuel pump fuses and relays located in the underhood fuse block.


  • 18 – Fuel Pump Relay (20A) – Fuse(2 pin micro fuse)
  • 41 – Fuel Pump Relay – Secondary (20A) – Fuse(2 pin micro fuse)
  • 59 – Fuel Pump Relay – Primary(Micro Relay)
  • 66 – Fuel Pump Relay – Secondary(Micro Relay)
Dec 092018

2006 GMC Envoy

Did the 2006 GMC Envoy XL-SLT ever come with Tow Hooks either front or rear? I cant get a positive answer from the local GMC Dealer. My almost former Chevy Trailblazer LT model had them. The Envoy has the Towing pkg but no towing hooks of which I’m surprised>

Envoy did not come with factory tow hooks….

Envoy is the “Luxury* edition

Trailblazer is the more “sport” edition

That’s the reason behind Envoy’s not getting factory tow hooks, even the Denali did not have them.

I know, as I asked every place I went when shopping around for my Envoy, they never had that option available. I was told that if I wanted tow hooks, I’d have to buy the Trailblazer.

Source: [email protected]

Radio, Door Locks, and Windows quit working 2003 GMC Yukon

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Oct 252018

2005 GMC Yukon

Instrument Cluster, Radio, Door Locks, and Windows quit working on my 2003 GMC Yukon XL SLT 4×4. Believe it to be in the BCM. Need schematics, Connector Charts, and Diagnostic connector Diagram to Help in Diagnosing. Thanks, Patches

Here is the information that I have including wiring diagrams. A blown fuse that is used by all systems described would be the first thing I would look for. And it would be the most common cause if they all went out at the same time. Since the vehicle is more than 10 years old, rusted ground connections may also be a factor.

Body Control System Description and Operation

BCM Functions

The Body Control Module (BCM) performs multiple body control functions. The BCM can control devices directly connected to its outputs based on input information. The BCM evaluates this information and controls certain body control systems by commanding outputs on or off. The BCM control inputs can be:

  • Sensors and switches that are directly connected to the BCM
  • Class 2 serial data received from other control modules connected to the class 2 serial data link.

The BCM is also capable of controlling other vehicle systems that are not directly wired to the BCM. The BCM does this by sending specific messages on the class 2 serial data link. The control module capable of performing the required function will respond to the BCM message.

The BCM controls these functions:

  • Audible warnings
  • Automatic door locks
  • Automatic headlamp control
  • Interior lighting
  • Keyless entry (AUO option)
  • Passlock theft deterrent
  • Retained Accessory Power (RAP)

Serial Data Power Mode

On vehicles that have several control modules connected by serial data circuits, one module is the power mode master (PMM). On this vehicle the PMM is the Body Control Module (BCM). The BCM uses 3 signals from the ignition switch. These are the Ignition 0, Ignition 1, and Accessory.

To determine the correct power mode the BCM uses:

  • The state of these signals
  • Next , the sequence of switch closures received by the BCM
  • Finally, the status of the engine run flag

The chart indicates the power modes detected and transmitted by the BCM:

Correct Ignition Switch Inputs

Ignition Key Position

Power Mode Parameter

Engine Running

Sampled Ignition Signal State


Ignition 1

Ignition 0

OFF Off-Awake or RAP No Inactive Off Off
UNLOCK Unlock or RAP Unlock No Inactive Off On
ACC Accessory No Active Off On
ON Run No Active On On
START Crank No Inactive On On
ON Run Yes Active On On

Fail-safe Operation

Since the operation of the vehicle systems depends on the power mode, there is a fail-safe plan in place should the BCM fail to send a power mode message. The fail-safe plan covers modules with discrete ignition signal inputs as well as those modules using exclusively serial data control of power mode.

Serial Data MessagesThe modules that depend exclusively on serial data messages for power modes stay in the state dictated by the last valid BCM message until they receive the engine status from the PCM. If the BCM fails, the modules monitor the serial data circuit for the engine run flag serial data. If the engine run flag serial data is true, indicating that the engine is running, the modules fail-safe to “Run”. In this state the modules and their subsystems can support all operator requirements. If the engine run flag serial data is false, indicating that the engine is not running, the modules fail-safe to “OFF-AWAKE”. In this state the modules are constantly checking for a change status message on the serial data circuits and can respond to both local inputs and serial data inputs from other modules on the vehicle.

Discrete Ignition SignalsThose modules that have discrete ignition signal inputs also remain in the state dictated by the last valid BCM message received on the serial data circuits. They then check the state of their discrete ignition input to determine the current valid state. If the discrete ignition input is active, B+, the modules will fail-safe to the “RUN” power mode. If the discrete ignition input is not active, open or 0 voltage, the modules will fail-safe to “OFF-AWAKE”. In this state the modules are constantly checking for a change status message on the serial data circuits and can respond to both local inputs and serial data inputs from other modules on the vehicle.

Body Control Module Programming


The procedures below are designed to set-up the body control module (BCM) correctly during BCM related service. Before you start, read these procedures carefully and completely.


The following procedures must be followed:

  1. BCM Programming
  2. Programming Theft Deterrent System Components, if equipped with Passlock System.

Programming the BCM


After the procedure is completed, the personalization settings of the BCM are set to a default setting. Inform the customer that the personalization must be set again.

Programming Theft Deterrent System Components


This procedure has to be performed only for vehicles equipped with Passlock System.

Perform the Programming Theft Deterrent System Components in Theft Deterrent after successfully finishing the Setup New BCM procedure. If the Programming Theft Deterrent System Components in the Theft Deterrent procedure is not performed after a BCM replacement, one of the following conditions will occur:

  • The vehicle will not be protected against theft by the PASSLOCK system.
  • The engine will not crank nor start.


After programming, perform the following to avoid future misdiagnosis:

  1. Turn the ignition OFF for 10 seconds.
  2. Connect the scan tool to the data link connector.
  3. Turn the ignition ON with the engine OFF.
  4. Use the scan tool in order to retrieve history DTCs from all modules.
  5. Clear all history DTCs.

> > Need more, access a 2003 GMC auto repair manual now!

Passlock Key No Start 2002 GMC Envoy 4.2

 Auto Repair Questions, GMC  Comments Off on Passlock Key No Start 2002 GMC Envoy 4.2
Jul 092018

GMC Envoy

Passlock Key No Start

I recently bought this 2002 GMC Envoy. It was sitting for around 6 months after the previous owner had a problem with it and couldn’t afford to fix it. My Envoy did not come with any sort of chipped key and the key I have will work in the ignition but not to unlock the doors. It cranks but won’t start. And the shifter locks and wont go into any gear. Is it the passkey security issue with it? I’ve check fuel pump and it’s working but no fuel at the fuel rail. So I’m figuring it’s got to be something to do with the security system. Is there a way to bypass that for now so I can at least get it started and check for it to run before spending all the money on a new passkey? Any hints or tips would sure help. Thank you very much.

Vehicle Theft Deterrent (VTD) Description and Operation

Passlock™ System

The Passlock™ System is provided in order to prevent vehicle theft if the ignition lock cylinder is forced to rotate or the ignition switch is operated while separated from the ignition lock cylinder case. If starting is attempted without authorization from the Passlock™ System, the powertrain control module (PCM) will disable engine starting. Start disable may be in the form of fuel disable or starter disable, depending on engine application.

The components of the Passlock™ System are as follows:

  • The ignition lock cylinder and key
  • Ignition lock cylinder case, including the Passlock™ sensor
  • The ignition switch
  • The body control module (BCM)
  • The security indicator on the instrument cluster
  • The powertrain control module (PCM)Ignition Lock Cylinder Case, Including the Passlock™ Sensor

The ignition lock cylinder fits inside the ignition lock cylinder case and operates the ignition switch when turned by a key with the proper mechanical cut. When the ignition key is used to turn the ignition lock cylinder to crank, start, a magnet on the lock cylinder passes close to the Passlock™ sensor within the ignition lock cylinder case. The magnet activates the security hall effect sensor in the Passlock™ sensor which completes a circuit from the security sensor signal circuit through a resistor to the security sensor low reference circuit. The resistance value will vary from vehicle to vehicle.

If a magnet from outside of the ignition lock cylinder case is used to attempt to steal the vehicle, the tamper hall effect sensor will be activated. This completes a circuit from the security sensor signal circuit through a tamper resistor to the security sensor low reference circuit bypassing the security resistor. If the ignition switch is forced to rotate without the correct key, or if the ignition lock cylinder is removed by force, the Passlock™ sensor will be damaged and will not operate.

Ignition Switch

The ignition switch contains the wiring and electrical switching portion of the column mounted ignition assembly. The switch includes wiring pigtails which connect it to the base of column connector, the Passlock™ sensor on the ignition lock cylinder case, and other components. The wiring for the Passlock™ sensor is unaffected by ignition switch position. The electrical switch portion is operated by the key and lock cylinder when they are rotated within the ignition lock cylinder case. The ignition switch operates the crank relay regardless of the status of the Passlock™ System.

Body Control Module (BCM)

The body control module (BCM) contains the logic of the Theft Deterrent System. The BCM provides the battery positive voltage to operate the Passlock™ sensor. The BCM also measures the voltage of the security sensor signal circuit. The voltage measured will indicate whether the Passlock™ sensor has been activated and whether the resistance value from the sensor is a valid value or the tamper value. If voltage measured is in the valid range, the BCM compares this voltage, voltage code, to a previously learned voltage code. If the voltage codes match, the BCM sends a class 2 message containing a password to the powertrain control module (PCM). When the voltage codes do not match, or the voltage is in the tamper range, or there is a circuit fault, the BCM will not send the correct password to the PCM, and the vehicle will not start.

Powertrain Control Module

The powertrain control module (PCM) contains the remainder of the logic of the Theft Deterrent System. If a class 2 message containing a valid password is received from the body control module (BCM), the PCM will continue to allow the fuel injectors to operate. The PCM will allow the fuel injectors to operate until it decides there is no valid password coming from the BCM. If the PCM does not receive a class 2 message, or receives a class 2 message with an incorrect password, the engine will crank and will not run or will start and stall immediately.

Theft System Indicator

The IPC illuminates the theft deterrent indicator as determined by the theft deterrent system. The IPC receives a class 2 message from the BCM requesting illumination.

  • The vehicle theft deterrent (VTD) system requests the IPC to illuminate the indicator only when the ignition switch is ON.
  • The content theft deterrent (CTD) system requests the IPC to illuminate the indicator only when the ignition switch is in the OFF or ACC positions or during RAP.
  • The body control module performs the displays test at the start of each ignition cycle. The indicator illuminates for approximately 3 seconds.


Fuel Lockout Cycle

When it receives a password which is incorrect or a password which indicates tamper and the powertrain control module (PCM) disables the fuel injectors, the fuel injectors remain disabled for 10 minutes even if the ignition switch is turned from the RUN position to the OFF position.

Changing the Passlock™ Components

The following components contain codes or passwords, or must learn codes or passwords for the Passlock™ System to allow the vehicle to start:

  • The ignition lock cylinder case
  • The body control module (BCM)
  • The powertrain control module (PCM)

If any of these parts are replaced, a learn procedure must be performed. Refer to Programming Theft Deterrent System Components . If parts are replaced and a learn procedure is not performed, the engine will crank and will not run or will start and stall immediately.

2004 GMC Sierra blower doesn’t work – Solved

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Jun 012018

GMC Sierra

Heater and Air Conditioning blower doesn’t work.
Causes, and location of blower motor?

Manual Heating and Air Conditioning

If the blower motor only comes on in High speed, replace the blower motor resistor. If the blower motor doesn’t work at all, you will need to replace the blower motor and possibly the resistor also.

Automatic Climate Control

If the blower motor only comes on in High speed, replace the blower motor controller.

Blower motor location

Under the passenger side dash. Access from inside the cab.

Blower motor location diagram

Blower Motor Replacement

  • If equipped, remove the sound insulator panel.
  • Remove the blower motor insulating cover screws.
  • Disconnect the electrical connector from the blower motor.
  • Remove the blower motor insulating cover.

Blower Motor

  • Pull the retaining tab down while turning the blower motor counterclockwise in order to disengage the blower motor from the heater/ventilation module.