A typical shop diagnostic fee is over $70. If you found this useful, give back to those that take the time to answer your questions. As little as $3 would be appreciated.
Dec 302016

2006 GMC SierraHello, I have a 2006 GMC Sierra 1500 5.3L Ext cab, the AC intermittently working. It will always work when I first start the truck and then blow warm air. It will blow cold shorter times during hotter days. The compressor is not running when it blows warm. I have checked the charge on the system and it is good. I have replaced the low pressure switch and checked the continuity is good. Not sure what the next step would be, the relay module or the High pressure switch? or icing up the line and needs purged? or the wire connection at the low pressure switch?


AC Intermittently Working

My initial thoughts would be to check the high speed fan and or the high speed fan relay. The system is charged and the compressor is cycling as it should. So lets look to make sure the fan is cooling the Freon in the condenser.

It may be possible that the Electronic Blend Door Actuator is not functioning properly which would only allow warm air to pass through vents. The Air Conditioning lines going into the firewall would be cold if this is the case. Most common to replace actuator.

Dec 232016

2005 GMC YukonI have a 2005 GMC Yukon XL that I have just replaced the fuel pump in. After getting the old pump out and installing the new one, reconnecting all the hoses and lines and getting the tank back up in place, I turned the key to the on position about 4-5 times to re pressurize the system. After that the truck started right up. Ran perfect. The wife used to it to run a few errands, and later ran to the store, still no problems.

Next day, I went to run to the gas station and it didn’t start right up. I just turned the key to the on position a couple times and it started. As I proceeded to drive down the road, I stepped on the gas pretty good and it like fell flat. It wouldn’t rev up. I left it run as I ran into the store and on my way back it seemed to slowly rev up more and more.

Fuel Problem

Later that afternoon we went to leave and took like 15-20 minutes to get started. And every time I would put it in gear it would die. So I left it till the next day and now I can’t get it to start, sounds like it wants to but wont. I can still here the fuel pump turn on when I turn the key to the on position but, it sounds like its got air bubbles in it?? Like its pushing air. I ended up replacing the pressure line because I couldn’t get the old fuel line out of the the plastic connector. It took some time but I did end up finally getting the old return line out and was able to use the original plastic connector.

I just wondering if that original return connector isn’t holding pressure? Or if I have some other line that isn’t seated properly?? Or, do I have a faulty fuel pump or a clogged fuel filter?? Any help would be greatly appreciated! Thank You!!


Fuel Problem Test

Anytime you have a “crank no start” problem you first want to determine what the engine is not getting. Once you know for sure its not getting spark or fuel you can concentrate on that system. Since you are pretty certain you have a fuel problem. A quick test can be done. Spray starting fluid in the air filter while someone cranks the engine over. Hold the gas pedal at least half way while doing so. If the engine fires right up, you have proven there is a fuel related problem. About the only way to get air into the system would be if the fuel tank was empty or the fuel pump was faulty.

Fuel Filter

The fuel filter should be replaced anytime the fuel pump is replaced. It is  a normal maintenance part and will only help the situation. Of course the fuel system needs to re pressurized after replacing the fuel filter similar to what you did when you replace the fuel pump earlier. The in-line fuel filter is located on the rail behind the composition sensor for the 5.3L engine. The 4.8L,6.0L and 8.1L engines do not have an external fuel filter, just the one in the tank connected tot he fuel pump.

Fuel Pressure Testing

Fuel Problem Pressure Gauge

When dealing with the fuel system it is important to know the fuel pressure of the system. Without the correct fuel pressure the fuel injectors will not fire. The fuel system pressure should be between 55-62 psi for gasoline engines and is slightly lower for ethanol engines at 48-54 psi. Connect the fuel pressure gauge tot he fuel rail test port located under the engine sight shield. In conclusion when the fuel pressure is below the specified reading some restriction may be involved. Consequently replace the fuel filter and see if the reading increases. If not the fuel pump may need to be replaced as a result.

Dec 162016

GMC SafariMy 2000 GMC Safari van clunks and jerks when I stop or start. And does the same when I turn the wheel all the way either direction.


Difficult to guess without some testing. I would recommend placing the front end securely on jack stands. Turn the wheels from side to side to try and reproduce the issue. Inspect all moving components. Look for loose ball joints and wheel bearings while still listening for clunks. Wiggle the tire from top to bottom and side to side while someone looks for anything loose.


With the vehicle on the ground, in park and the engine running turn the wheel all the way in either direction. Doing this should help recreate the issue and allow someone on the outside to listen for the problem area. Once the area is located a more in depth inspection will be necessary.

Dec 082016

2007 GMC SierraKeep getting a ‘stabiliTrak service and service traction’ error message. The ‘brake pedal sensor switch’ has been replaced and now the message appears only when the engine is idling for more than a few minutes (disappears once I start driving). What could cause the message to keep appearing?


My first guess would be a faulty sensor or sensor connection. However some hands on testing would be needed to locate the issue.

This vehicle is equipped with a BOSCH ABS/EBD/TCS/VSES brake system. The electronic brake control module (EBCM) and the brake pressure modulator valve (BPMV) is serviced separately. The BPMV uses a 4 circuit configuration to control hydraulic pressure to each wheel independently.

The following vehicle performance enhancement systems are provided.

    • Electronic Brake Distribution (EBD)
    • Power Brake Booster Solenoid Vacuum Supply
    • Traction Control System (TCS)
    • Vehicle Stability Enhancement System (VSES)

StabiliTrak® Indicator Light

Stabilitrak LightIf equipped, this warning light should come on briefly when the engine is started.

If the warning light does not come on then, have it fixed so it will be ready to warn you if there is a problem. If it stays on, or comes on when you are driving, there may be a problem with your StabiliTrak® system and your vehicle may need service. When this warning light is on, the system is off and will not limit wheel spin. Adjust your driving accordingly.

This light will also flash when the StabiliTrak® system is active.

If the StabiliTrak® system warning light comes on and stays on for an extended period of time when the system is turned on, your vehicle needs service.

Service StabiliTrak Message

 The message center displays the service stability system message when any one of many VSES-disabling DTCs is set. The EBCM sends a serial data message to the IPC requesting this display.

Service Traction Control Message

 The message center displays the service traction control system message when any one of many traction control – disabling DTCs is set. The EBCM sends a serial data message to the IPC requesting this display.

StabiliTrak Off Message

 The message center displays the stabiliTrak off message when one or more of the following conditions exists.
    • The transfer case is shifted into 4 LO. The EBCM sends a serial data message to the IPC requesting illumination.
    • The driver manually disables the VSES and engine torque reduction by pressing the traction control switch. The EBCM sends a serial data message to the IPC requesting illumination.
    • The estimated temperature of any solenoid coil exceeds an acceptable limit. The EBCM sends a serial data message to the IPC requesting this display.
    • The EBCM detects a failed brake switch. The EBCM sends a serial data message to the IPC requesting this display. A DTC sets when this condition exists.
    • VSES sensor initialization time is excessive. The EBCM sends a serial data message to the IPC requesting this display.
    • Serial data communication between the EBCM and any of several other control modules is interrupted. The EBCM sends a serial data message to the IPC requesting this display or the IPC displays the message when communication with the EBCM is interrupted.
    • The PCM is not able to perform engine torque reduction. The EBCM sends a GMLAN message to the IPC requesting this display. DTCs set when this condition exists.
    • The EBCM detects an excessively low or excessively high ignition voltage. The EBCM sends a GMLAN message to the IPC requesting this display.

Vehicle Stability Enhancement System (VSES)

 Vehicle stability enhancement system (VSES) provides added stability during aggressive maneuvers. Yaw rate is the rate of rotation about the vehicle’s vertical axis. The VSES is activated when the electronic brake control module (EBCM) determines that the desired yaw rate does not match the actual yaw rate as measured by the yaw rate sensor.

The desired yaw rate is calculated by the EBCM using, primarily, the following inputs.

    • The position of the steering wheel
    • The speed of the vehicle
    • The lateral, or sideways acceleration of the vehicle

The difference between the desired yaw rate and the actual yaw rate is the yaw rate error, which is a measurement of oversteer or understeer. When a yaw rate error is detected, the EBCM attempts to correct the vehicle’s yaw motion by applying brake pressure to one or more of the wheels. The amount of brake pressure which is applied varies, depending on the correction required. The engine torque may be reduced also, if it is necessary to slow the vehicle while maintaining stability.

VSES activations generally occur in turns during aggressive driving. When braking during VSES activation, the pedal may pulsate. The brake pedal pulsates at a higher frequency during VSES activation than during ABS activation.

VSES Sensors Initialization

 The vehicle stability enhancement system (VSES) sensors values may vary slightly due to differences in temperature, sensor mounting, connector resistances, manufacturing, etc. Since VSES is a very sensitive and precise control system, it is imperative that the electronic brake control module (EBCM) be able to accurately equate a given sensor voltage with an actual unit of measurement. For example, the yaw rate signal of one vehicle may be 2.64 volts at +18.0 deg/sec yaw rate while the yaw rate signal of another vehicle may be 2.64 volts at +17.5 deg/sec yaw rate. Therefore, at the beginning of each ignition cycle, the EBCM must perform an initialization procedure to observe how the VSES sensors are correlated with each other and also to determine what each sensor value is when the applicable unit of measurement equals 0. This voltage is referred to as the sensor bias voltage. Although some activation of the VSES system may occur if required to prior to full initialization, the system does not give optimum performance until the sensors are fully initialized.

The following VSES sensors require initialization:

    • The yaw rate sensor
    • The lateral accelerometer
    • The master cylinder pressure sensor
    • The steering wheel position sensor

When the vehicle speed is greater than 25 km/h (15 mph), full sensor initialization must occur during 3 km (1.8 mi) of driving or 1 km (0.6 mi) of straight and stable driving, whichever occurs first. Although an attempt at initialization may fail due to driving conditions, such as driving on a very winding road, failed initialization is usually caused by a sensor bias voltage which is not within an acceptable range. Often, a DTC sets soon after a failed initialization attempt. The message center displays the stability system disabled message when sensor initialization fails.

Dec 082016

2007 GMC SierraI keep getting an engine light for my gas cap. It is not a GM part, which costs $35. Is there a sensor for the gas cap that can be disconnected?


The check engine light comes on when a Diagnostic Trouble Code has been detected. A trouble code for a small evap(evaporative system) leak comes up and the most common reason for this code would be a loose or failed gas cap. If you have just filled the fuel tank I would recommend tightening the cap and driving it a few days to see if it doesn’t cycle back off. The Gas cap can be tested with the proper equipment. Also the evap leak can be located with a smoke machine.

Check Gas Cap Message

 The control module sends a class 2 message to the driver information center (DIC) illuminating the Check Gas Cap message when a malfunction in the evaporative emission (EVAP) system and a large leak test fails.

There is not a sensor that can be disconnected that pertains to the gas cap.

EVAP System Operation

 The evaporative emission (EVAP) control system limits fuel vapors from escaping into the atmosphere. Fuel tank vapors are allowed to move from the fuel tank, due to pressure in the tank, through the vapor pipe, into the EVAP canister. Carbon in the canister absorbs and stores the fuel vapors. Excess pressure is vented through the vent line and EVAP vent solenoid valve to the atmosphere. The EVAP canister stores the fuel vapors until the engine is able to use them. At an appropriate time, the control module will command the EVAP purge solenoid valve ON, allowing engine vacuum to be applied to the EVAP canister. With the EVAP vent solenoid valve OFF, fresh air is drawn through the vent solenoid valve and the vent line to the EVAP canister. Fresh air is drawn through the canister, pulling fuel vapors from the carbon. The air/fuel vapor mixture continues through the EVAP purge pipe and EVAP purge solenoid valve into the intake manifold to be consumed during normal combustion. The control module uses several tests to determine if the EVAP system is leaking.

Large Leak Test

 This tests for large leaks and restrictions to the purge path in the evaporative emission (EVAP) system. When the enabling criteria has been met, the control module commands the EVAP vent solenoid valve ON and the EVAP purge solenoid valve ON, allowing vacuum into the EVAP system. The control module monitors the fuel tank pressure (FTP) sensor voltage to verify that the system is able to reach a predetermined level of vacuum within a set amount of time.
Nov 242016

GMC SafariI’m trying to figure I have brake lights and directional light no nightlife no night light I change the receptacle with only two wires in the with only two wires in the rain and what am I doing wrong?


Check the fuses and then the wiring for good ground and current. Then check the switch, wiring harness and connections for no night light.

fuse box diagram 1986 GMC Safari Van - No Night Light

Light operation – No Night Light

The headlight switch is a push button switch to turn the lights on and off. It is is located on the left side of the instrument panel. A rheostat dial, located with the headlight/parking light switch, is used to control the illumination of the instrument panel. It is also used to control the night light.

A dimmer switch (part of the combination switch), to control the Hi and Lo beam operation, is located in the steering column; the lights are changed by pulling the lever toward the driver.

Nov 232016

2012-GMC TerrainThe entire display and radio quit working, all fuses are good,? thanks


The fuses should be Hot at all times. This means you should see battery voltage at each fuse on both sides of the fuse. Double Check these fuses.

Radio Quit – Check These Fuses First

  1. F5DA – 10 amp – located in the instrument panel fuse block(center console)
  2. F31DA – 30 amp – located in the instrument panel fuse block(center console)
  3. F7DA – 5 amp – located in the instrument panel fuse block(center console)
  4. F15DA – 10 amp – located in the instrument panel fuse block(center console)
  5. F19DA – 10 amp – located in the instrument panel fuse block(center console)
  6. F9DA – 20 amp – located in the instrument panel fuse block(center console)
  7. F33DA – 10 amp – located in the instrument panel fuse block(center console)

If all the fuses show good and but you do not see battery voltage at the fuse, trace the wiring going tot he fuse and repair. If you see battery voltage at the fuses on both side, check wiring going to the display and radio. Check the connections. Repair as needed. If wiring and connections check out, it may necessary to replace the radio and or the display.

You may want purchase a full set of wiring diagrams to assist in tracing the circuit.

Nov 202016

1998 GMC SonomaCan the lower intake manifold gaskets be replaced without removing the upper intake from the lower? Or can I pull the entire intake as an entire unit?


Yes. The intake manifold may be removed as an assembly. Do not remove the specific intake manifold components unless component service is required.

intake manifold

Intake Manifold Removal

  • Remove the evaporative emission (EVAP) canister purge solenoid valve harness.
  • Push the quick disconnect clip and hold in place.
  • Pull outward on the harness elbow.
  • Take off the engine coolant temperature (ECT) sensor wire connector (if equipped) from the engine wiring harness bracket.
  • Remove the lower manifold bolts.
  • Remove the manifold assembly.
  • Discard the lower manifold gaskets.

Important The intake manifold may be removed as an assembly. Do not remove the specific  manifold components unless component service is required.
Do not allow dirt or debris to enter the fuel system. Ensure that the ends of the fuel system are properly sealed.
Do not disassemble the Central Sequential Fuel Injection (SFI) unit, unless service is required.

• Inspect the lower intake manifold for the following:

– Cracks or damage

– Damage to the gasket sealing surfaces

– Damage to the threaded bolt holes

– Restricted exhaust gas recirulation (EGR) system passages

– Restricted cooling system passages

Oct 312016

1990 GMC SierraI just had a mechanic put in a used rebuilt set of gears in the rear end of my 1991 GMC 1/2 ton 4×4 truck. The problem we are having is when you drive it, it starts out fine then there is a jolt and it acts like it wants to go in revere.
He brought it back to his shop and picked it up and spun the wheels, they seemed fine and then Wham, loud noise and the wheels stop and wanted to go in revere. He is stumped he has never came across this before what could of happened?


Is the Driveshaft disconnected when spinning the wheels up in the air?

Does the rear end have a Locker Unit? Limited Slip? Or Posi-traction?

Sounds like something has backed off and is periodically catching. Remove Drive Shaft. Drain rear end fluid and remove cover. spin and inspect the work. Make sure the axle clips are not backing out or damaged.

1991 GMC K1500 Rear End Cover

rear end