Powertrain Controls - ECM/PCM

DTC 15 Engine Coolant Temperature Sensor:

Engine Coolant Temperature Sensor Wiring Diagram:

Circuit Description:


The engine coolant temperature sensor uses a thermistor to control the signal voltage to the Electronic Control Module (ECM). The ECM applies a voltage on CKT 410 to the sensor. When the engine is cold, the sensor (thermistor) resistance is high, therefore the ECM will see high signal voltage. As the engine warms, the sensor resistance becomes less, and the voltage drops. At normal engine operating temperature the voltage will measure about 1.5 to 2.0 volts at the ECM.


Engine coolant temperature is one of the inputs used to control:

^ Fuel delivery.
^ EVAP canister purge.
^ Ignition Control (IC).
^ Exhaust Gas Recirculation (EGR).
^ Idle Air Control (IAC).
^ Cooling fan operation.
^ Torque Converter Clutch (TCC).


Test Description: Numbers below refer to circled numbers on the diagnostic chart.

1. DTC 15 will set if engine coolant temperature voltage indicates a coolant temperature less than -38.5~C (-37.3~F).

2. This test simulates a DTC 14. If the ECM recognizes the low signal voltage, (high temperature) and the "Scan" reads 130~C (266~F), the ECM and wiring are OK.

3. This test will determine if CKT 410 is open. There should be 5 volts present at sensor connector if measured with a Digital Volt Meter (DVM).

Diagnostic Aids:


Check harness routing for a potential short to ground in CKT 410.
Circuit is routed from the ECM to the 10-way INJ/ECM connector and then to the Engine Coolant Temperature (ECT) sensor.

After engine is started the temperature should rise steadily to about 95~C (203~F), then stabilize when thermostat opens. CKT 410 is routed from the ECM to a mini harness, and then to engine coolant temperature sensor.

A faulty connection, or an open in CKT 410 or 470 will result in a DTC 15.

DTC 15 and 21 stored at the same could be the result of an open CKT 470, which would also turn the temperature warning indicator "ON".

The "Temperature To Resistance Value" scale may be used to test the coolant sensor at various temperature levels to evaluate the possibility of a "shifted" (mis-scaled) sensor, which may result in driveability complaints. - Intermittent Malfunctions