DTC 15

Code 15 - Coolant Temp. Sensor (Low Temp. Indicated):

Wiring Diagram for Coolant Temp. Sensor:

CODE 15 - COOLANT TEMPERATURE SENSOR CIRCUIT (LOW TEMPERATURE INDICATED)

Circuit Description:


The 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 (85~C to 95~C) the voltage will measure about 1.5 to 2.0 volts at the ECM.


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

1. Code 15 will set if:

^ Signal voltage indicates a coolant temperature less than -44~C (-47~F) for at least 3 seconds.

2. This test simulates a Code 14. If the ECM recognizes the low signal voltage, (high temperature) and the "Scan" reads 130~C, 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:

A "Scan" tool reads engine temperature in degrees centigrade. After engine is started the temperature should rise steadily to about 90~C then stabilize when thermostat opens.

An intermittent may be caused by a poor connection, rubbed through wire insulation or a wire broken inside the insulation.


Check For:


^ Poor Connection or Damaged Harness - Inspect ECM harness connectors for backed out terminals "C10" or "D12", improper mating, broken locks, improperly formed or damaged terminals, poor terminal to wire connection and damaged harness.


^ Intermittent Test - If connections and harness checks out OK, "Scan" coolant temperature while moving related connectors and wiring harness. If the failure is induced, the "coolant temperature" display will change. This may help to isolate the location of the malfunction.

^ Shifted Sensor - 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.

A faulty connection, or an open in CKT 410 or 455 will result in a Code 15.

If Code 23 or 63 is also set, check CKT 455 for faulty wiring or connections. Check terminals at sensor for good contact. Refer to "Intermittents" in Diagnosis By Symptoms - No Trouble Codes Stored.