Safety Warning
DIY auto repair can cause serious injury, fire, or vehicle damage. These guides are for informational purposes only. Always follow OEM torque specs, wear PPE, and consult a certified mechanic if you are unsure. You are solely responsible for your safety.
SYS.OK|MANUAL.DB● LIVE
HOMESERVICE MANUALSGMC1999FORWARD CONTROL 5.7 R, AUTOMATICREPAIR AND DIAGNOSISEXTERNAL PAGESDIFFERENT CARSECTION 68 (ENGINE CONTROL SYSTEM SELF-DIAGNOSTICS - 3.1L ENGINE)DIAGNOSTIC TESTSDTC P0125: ECT EXCESSIVE TIME TO ENTER CLOSED LOOPDIAGNOSTIC PROCEDURES
1999 GMC Forward Control 5.7 R, Automatic
Diagnostic Procedures
1999 GMC Forward Control 5.7 R, AutomaticSECTION Diagnostic Procedures
WARNING: This page is about a different car, the 2002 Pontiac Grand Prix, 2002 Chevrolet Malibu, and 2002 Buick Century. However, it is still accessible from the selected car via links, so may be relevant.
- Perform diagnostic system check - engine controls. See DIAGNOSTIC SYSTEM CHECK - ENGINE CONTROLS under SELF-DIAGNOSTIC SYSTEM. After performing diagnostic system check - engine controls, go to next step.
- Is the cooling system low on coolant? If yes, check and repair cause of coolant loss. If no, go to next step.
- Turn ignition off. Disconnect ECT sensor. Connect a 3-amp fused jumper wire between the signal circuit and the low reference circuit of ECT sensor harness connector. Turn ignition on, with engine off. With a scan tool, observe the ECT sensor temperature. Does the scan tool indicate that the ECT sensor temperature is more than 280°F (138°C)? If yes, go to step 7. If no, go to next step.
- Turn ignition off. Disconnect ECT sensor. Connect a 3-amp fused jumper wire between the signal circuit of the ECT sensor harness connector and a known-good ground. Turn ignition on, with engine off. Observe the ECT sensor temperature with a scan tool. Does the scan tool indicate that the ECT sensor temperature is more than 280°F (138°C)? If yes, go to next step. If no, go to step 6.
- Test ECT sensor low reference circuit for a high resistance or an open. Repair the circuit as necessary. After repairs, go to step 10.
- Test ECT sensor signal circuit for a high resistance or an open. Repair the circuit as necessary. After repairs, go to step 10.
- Remove ECT sensor. Suspend the ECT sensor and a thermometer in a pan of water. Place the pan on a burner or hot plate. Test the resistance of the ECT sensor through the specified range of 50-194°F (10-90°C). Does the resistance and temperature closely match? Refer to appropriate SENSOR RESISTANCE table in appropriate SENSOR OPERATING RANGE CHARTS article. If yes, go to next step. If no, go to step 9.
- Reinstall ECT sensor. When complete, diagnose thermostat operation.
- Replace ECT sensor. After replacing sensor, go to next step.
- Clear DTCs with scan tool. Turn ignition off for 30 seconds. Start engine. Operate the vehicle within the Conditions for Running the DTC. See CONDITIONS FOR RUNNING DTC . Does the DTC run and pass? If yes, go to next step. If no, see DIAGNOSTIC AIDS .
- Using scan tool, observe the stored information and Capture Info. If scan tool displays any undiagnosed DTCs, diagnose affected DTCs. See DIAGNOSTIC TROUBLE CODE DEFINITIONS . If no DTCs are displayed, system is okay.
RENDER: 1.0x
NO RELATED
Recommended Tools & Savings
Use the Manual With the Right Hardware
Pair factory procedures with proven DIY tools so the instructions are easier to execute.
Affiliate disclosure: We may earn a commission at no extra cost to you.
When to See a Mechanic
Stop DIY work and contact a certified mechanic immediately if any of the following apply:
- • You smell fuel, burning insulation, or see smoke.
- • Brakes feel soft, pull hard to one side, or make grinding noises.
- • The engine overheats, stalls repeatedly, or misfires under load.
- • You are missing required tools, torque specs, or safe lifting equipment.
- • You are not confident in the next step or safety outcome.