Powertrain Controls - ECM/PCM

DTC 15 Engine Coolant Temperature Sensor Circuit (Low Temp):

Engine Coolant Temperature (ECT) Sensor Schematic:

CIRCUIT DESCRIPTION
The Engine Coolant Temperature (ECT) sensor uses a thermistor to control the signal voltage to the Powertrain Control Module (PCM). The PCM applies a voltage on CKT 410 to the sensor. When the engine is cold, the sensor (thermistor) resistance is high, therefore the PCM will see high signal voltage.
As the engine warms, the sensor resistance becomes less and the voltage drops. At normal engine operating temperature 88 to 106°C (190 to 222°F) the voltage will measure about 0.5 to 1.1 volts at the PCM.

DIAGNOSTIC TROUBLE CODE (DTC) 15 WILL SET WHEN
The signal voltage indicates engine coolant temperature is less than -33°C (-27°F). Engine must be running longer than 30 seconds before this test will begin.

ACTION TAKEN (PCM will default to)
The PCM will use a default value of 87°C (189°F) for fuel delivery calculations. The cooling fans will be enabled and the Malfunction Indicator Lamp (MIL) will be illuminated.

DTC 15 WILL CLEAR WHEN
A current DTC 15 will clear when the low temperature failure is no longer detected [temperature rises above -33°C (-27°F) threshold].

DTC CHART TEST DESCRIPTION
Number(s) below refer to circled number(s) on the diagnostic chart.
1. This is to verify that the temperature fault is not intermittent.
2. This test simulates a DTC 14. If the PCM recognizes the low signal voltage, (high temperature) and Tech 1 scan tool reads 130°C (266° F) or above, the PCM 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 DVM.

DIAGNOSTIC AIDS
A Tech 1 scan tool reads engine temperature in degrees centigrade and degrees fahrenheit. After engine is started the temperature should rise steadily between 88°C (106°F) then stabilize when thermostat opens. The fan will turn "ON" at 106°C (222°F).

If DTC 21 is also set, check CKT 452 for faulty wiring or connections. Check terminals at sensor for good contact.
Refer to Intermittents under Computers and Control Systems/System Diagnosis/Diagnosis By Symptom. - Intermittent Malfunctions