p0125. scanner says one thing, repair book says another

P0125 INSUFFICIENT COOLANT TEMPERATURE FOR CLOSED LOOP FUEL CONTROL

This code is a code that has to wait until the engine warms to actually test for it and see it. It’s a closed loop code. When you first start the engine cold it is in what is called open loop. Once the engine reaches operating temperature it goes into closed loop which is when the PCM test the circuit and sets the code. It is most likely the O2 sensor heater that has gone bad but here is the description and diagnostic testing you can run to confirm if you’d like.

SELF-DIAGNOSTICS – V6 -1999 Toyota Tacoma Page 1 of 2

DTC P0125: INSUFFICIENT COOLANT TEMPERATURE FOR CLOSED LOOP FUEL
CONTROL

NOTE: Bank No. 1 refers to bank which includes cylinder No. 1.
Bank No. 2 refers to bank without cylinder No. 1. Heated
oxygen sensor No. 1 refers to sensor closest to engine
block.

Circuit Description

Heated oxygen sensor monitors exhaust gas oxygen content which
delivers an input signal to ECM. ECM uses input signal to determine
fuel injection system operation. Heated oxygen sensors include a
heater. DTC is set when heated oxygen sensor No. 1 does not output a
rich condition once engine is at normal operating temperature,
engine speed is more than 1500 RPM and vehicle speed is 25-62 MPH
for at least 2 minutes. Possible causes are:

lHeated oxygen sensor No. 1 circuit is open or shorted.
lVehicle has run out of fuel.
lFuel system malfunction.
lDefective fuel injector.
lAir induction system malfunction.
lEGR system malfunction (if equipped).
lExhaust system leak.
lDefective heated oxygen sensor(s) No. 1.
lDefective ECM.

Diagnostic Aids

After confirming DTC P0125, use scan tool to access CURRENT DATA to
confirm voltage output of heated oxygen sensor No. 1. If voltage
displayed on scan tool is always less than 0.1 volt, circuit may be
open or shorted. Using scan tool, read freeze frame data. Freeze
frame records engine conditions when malfunction is detected.

Diagnosis & Repair

1.If any other DTCs are displayed, diagnose and repair those DTCs
first and retest. If only DTC P0125 is displayed, go to next
step.
2.Connect scan tool to Data Link Connector (DLC) No. 3. See Fig.
1 . Start and warm engine to normal operating temperature. Using
scan tool, monitor heated oxygen sensor No. 1. Snap accelerate
engine to about 4000 RPM 3 times. Heated oxygen sensor No. 1 should indicate a rich signal (.45 volt or more) at least once.
If a rich signal is indicated for heated oxygen sensors No. 1 at
least once, go to step 10 . If heated oxygen sensor No. 1 does
not display a rich signal at least once, go to next step.
3.Check for open or short in wiring between heated oxygen sensor
No. 1 and ECM. If problem does not exists, go to next step. If
problem exists, repair wiring as necessary. See appropriate
wiring diagram in WIRING DIAGRAMS article.
4.Using scan tool, check if a misfire has occurred by monitoring
DTCs and DATA LIST. If a misfire has occurred, perform DTC
P1300: IGNITOR CIRCUIT . If misfire has not occurred, go to next
step.
5.Ensure oil dipstick, oil filler cap, PCV system and all other
air induction system components are intact and operating
properly. If problem exists, repair as necessary. If problem
does not exist, go to next step.
6.Check EGR system components (if equipped). See EMISSION SYSTEMS
& SUB-SYSTEMS in SYSTEM & COMPONENT TESTING – V6 & V8 article.
If problem exists, repair as necessary. If problem does not
exists or vehicle is not equipped with EGR system, go to next
step.
7.Check fuel pressure. See FUEL SYSTEM in BASIC DIAGNOSTIC
PROCEDURES – V6 & V8 article. If fuel pressure is not within
specification, repair as necessary. If fuel pressure is within
specification, go to next step.
8.Check fuel injectors. See FUEL SYSTEM in SYSTEM & COMPONENT
TESTING – V6 & V8 article. If problem exists, repair as
necessary. If problem does not exist, go to next step.
9.Check exhaust system for leaks. If problem exists, repair as
necessary. If problem does not exist, replace defective heated
oxygen sensor No. 1.
10.Perform test drive confirmation, then go to next step. See TEST
DRIVE CONFIRMATION under DTC P0130: HEATED OXYGEN SENSOR NO. 1
CIRCUIT.
11.Clear and recheck for DTCs. If DTC P0125 is displayed again,
replace ECM. If DTC P0125 is not displayed again, go to next
step.
12.Vehicle either ran out of fuel or problem is intermittent. Check
component and ECM connections.