DTC Descriptors
DTC P0125: Engine Coolant Temperature (ECT) Insufficient for Closed Loop Fuel ControlDTC P0128: Engine Coolant Temperature (ECT) Below Thermostat Regulating Temperature
Diagnostic Fault Information
Important: Always perform the Diagnostic System Check - Vehicle prior to using this diagnostic procedure.
Circuit | Short to Ground | High Resistance | Open | Short to Voltage | Signal Performance |
|---|---|---|---|---|---|
ECT Sensor Signal | P0117 | P0118 | P0118 | P0118 | P0116, P0125, P0128 |
Low Reference | -- | P0118 | P0118 | P0118 | P0125, P0128 |
Typical Scan Tool Data
Circuit | Normal Range | Short to Ground | Open | Short to Voltage |
|---|---|---|---|---|
ECT Sensor Signal | -39 to +120°C (-38 to +248°F) | 150°C (302°F) | -40°C (-40°F) | -40°C (-40°F) |
Low Reference | -- | -- | -40°C (-40°F) | -40°C (-40°F) |
Circuit/System Description
The engine coolant temperature (ECT) sensor is a variable resistor that measures the temperature of the engine coolant. The engine control module (ECM) supplies 5 volts to the ECT signal circuit and supplies a ground to the low reference circuit.
The purpose of both of these rationality diagnostics is to use the ECT sensor to determine if the engine coolant will heat up at the correct rate, and also meet the calibrated target temperatures under various operating conditions.
The ECM uses the start-up ECT, and the start-up intake air temperature (IAT) to begin the diagnostic calculation. The air flow into the engine is accumulated , and vehicle speed, distance and engine run time are also factored in, to determine if the ECT does increase normally, and reach the calibrated target temperatures.
Conditions for Running the DTC
| • | DTCs P0101, P0102, P0103, P0112, P0113, P0116, P0117, P0118, P0502, P0503 are not set. |
| • | The start-up IAT is between -7 to +55°C (+19 to +131°F). |
| • | The start-up ECT is less than 65°C (149°F) when the start-up IAT is less than 10°C (50°F). |
| OR |
| • | The start-up ECT is less than 75°C (167°F) when the start-up IAT is greater than 10°C (50°F). |
| • | The engine run time is between 30 seconds and 30 minutes. |
| • | The vehicle has traveled more than 0.8 kilometers (0.5 miles) at more than 8 km/h (5 mph). |
| • | The mass air flow (MAF) is between 2-30 g/s. |
| • | This DTC runs once per ignition cycle within the enabling conditions. |
Conditions for Setting the DTC
P0125
The ECM detects that the minimum ECT of 8°C (46°F) has not been met.
P0128
The ECM detects that the minimum ECT of 80°C (176°F) has not been met with a start-up IAT of greater than 10°C (50°F).
OR
The ECM detects that the minimum ECT of 70°C (158°F) has not been met with a start-up IAT of less than 10°C (50°F).
Action Taken When the DTC Sets
| • | The control module illuminates the malfunction indicator lamp (MIL) on the second consecutive ignition cycle that the diagnostic runs and fails. |
| • | The control module records the operating conditions at the time the diagnostic fails. The first time the diagnostic fails, the control module stores this information in the Failure Records. If the diagnostic reports a failure on the second consecutive ignition cycle, the control module records the operating conditions at the time of the failure. The control module writes the operating conditions to the Freeze Frame and updates the Failure Records. |
| • | The ECM will command the cooling fans ON. |
| • | A message center or an indicator may not function. |
Conditions for Clearing the MIL/DTC
| • | The control module turns OFF the malfunction indicator lamp (MIL) after 3 consecutive ignition cycles that the diagnostic runs and does not fail. |
| • | A current DTC, Last Test Failed, clears when the diagnostic runs and passes. |
| • | A history DTC clears after 40 consecutive warm-up cycles, if no failures are reported by this or any other emission related diagnostic. |
| • | Clear the MIL and the DTC with a scan tool. |
Reference Information
Schematic Reference
Connector End View Reference
Electrical Information Reference
Scan Tool Reference
Circuit/System Verification
- If the ignition has been OFF for 8 hours or more, the ECT and the IAT, should be within 11°C (20°F) of each other and also the ambient temperature. Turn ON the ignition, with the engine OFF, and use a scan tool to observe the IAT and the ECT sensor parameters.
- Inspect the ECT sensor terminals for corrosion, and for engine coolant leaking through the sensor.
- Use the scan tool to verify the proper operation of the engine cooling system fans.
- Turn OFF the ignition.
- Inspect the cooling system surge tank for the proper engine coolant level. Refer to Cooling System Leak Testing and Cooling System Draining and Filling .
- Verify the proper heat range, and the operation of the thermostat. Refer to Thermostat Diagnosis .
Caution: Under pressure, the temperature of the solution in the radiator can be considerably higher, without boiling. Removing the radiator cap while the engine is hot (pressure is high), will cause the solution to boil instantaneously, with explosive force. The solution will spew out over the engine, fenders, and the person removing the cap. Serious bodily injury may result. Flammable antifreeze, such as alcohol, is not recommended for use at any time. Flammable antifreeze could cause a serious fire.
Important: A critical analysis of the operation of the thermostat is necessary, to properly diagnose these DTCs.
Circuit/System Testing
- Turn OFF the ignition, and all electrical accessories. Allow sufficient time for all of the control modules to power down before taking a resistance measurement.
- Disconnect the ECT sensor.
- Measure for a proper value of less than 5 ohms of resistance between the ECT low reference circuit and a good ground.
- Disconnect the ECM.
- Measure for a proper value of less than 5 ohms of resistance on the ECT sensor signal circuit.
- Remove and test the ECT sensor. Observe the component testing procedure below.
| ⇒ | If the resistance is more than 5 ohms, test the low reference circuit for a high resistance. |
| ⇒ | If the ECT low reference circuit tests normal, replace the ECM. |
| ⇒ | If the resistance is more than 5 ohms, repair the high resistance in the ECT sensor signal circuit. |
| ⇒ | If the resistance measurements of the ECT sensor are out of range, replace the sensor. |
| ⇒ | If the ECT sensor tests normal and the DTC continues to set, replace the ECM. |
Component Testing
Measure and record the resistance of the ECT sensor at various ambient temperatures, and then compare those measurements to the Temperature vs Resistance Table. Refer to Temperature Versus Resistance .
Repair Instructions
Important: Always perform the Diagnostic Repair Verification after completing the diagnostic procedure.
| • | Control Module References for ECM replacement, setup, and programming |
