DTC Advanced Diagnostics: P0125

DTC P0125:Engine Coolant Temperature (ECT) Sensor 1 Malfunction/Slow Response

General Description

The powertrain control module (PCM) supplies voltage to the engine coolant temperature signal circuit (about 5 V) through a pull-up resistor. As the engine coolant cools, the engine coolant temperature (ECT) sensor 1 resistance increases, and the PCM detects a high signal voltage. As the engine coolant warms, the ECT sensor 1 resistance decreases, and the PCM detects a low signal voltage. If the ECT sensor 1 output voltage does not reach a specified temperature at which closed loop control for stoichiometric air/fuel ratio starts within a set time, depending on the initial engine coolant temperature after starting the engine, the PCM detects a malfunction and stores a DTC.

Monitor Execution, Sequence, Duration, DTC Type

Execution	Once per driving cycle
Sequence	None
Duration	20 minutes or less
DTC Type	Two drive cycles, MIL on

Enable Conditions

Condition	Minimum	Maximum
Initial engine coolant temperature [ECT SENSOR 1]	–	0 °F (-18 °C)
Fuel feedback	Other than during fuel cut-off operation

[ ]: HDS Parameter

Malfunction Threshold

The engine running time before the engine coolant temperature [ECT SENSOR 1] reaches 0 °F (-18 °C), based on the initial engine coolant temperatures [ECT SENSOR 1], is as follows:

Initial engine coolant temperature [ECT SENSOR 1]	Engine running time
-54 °F (-48 °C)	5 minutes or more
-35 °F (-37 °C)	5 minutes or more
0 °F (-18 °C)	1 minute or more

Possible Cause

NOTE: The causes shown may not be a complete list of all potential problems, and it is possible that there may be other causes.

	ECT sensor 1 failure
	Thermostat stuck open

Confirmation Procedure

Operating Condition

1.	Start the engine under Enable Conditions (see "Initial engine coolant temperature [ECT SENSOR 1]").
2.	Let the engine idle for at least 20 minutes.

With the HDS

None.

Diagnosis Details

Conditions for setting the DTC

When a malfunction is detected during the first drive cycle with the engine coolant temperature and intake air temperature at engine start-up within the specified temperature range, a Pending DTC is stored in the PCM memory. If the malfunction returns in the next (second) drive cycle with the engine coolant temperature and intake air temperature at engine start-up within the specified temperature range, the MIL comes on and a Confirmed DTC and the freeze data are stored.

Conditions for clearing the DTC

The MIL is cleared if the malfunction does not return in three consecutive trips in which the diagnostic runs. The MIL, the Pending DTC, the Confirmed DTC, and the freeze data can be cleared with the scan tool Clear command or by disconnecting the 12 volt battery.