DTC 15

Chart - Code 15 Coolant Sensor Circuit (Signal Voltage High):

Code 14 Coolant Temperature Sensor Circuit (High Temp. 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 circuit 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 terminal "C16".
Coolant temperature is one of the inputs used to control:
^ Fuel delivery
^ Engine spark timing (EST)
^ Idle Air Controller (IAC)
^ Torque Converter clutch (TCC)
^ Charcoal Canister purge (CCP)
^ Exhaust Gas Recirculation (EGR)
^ Cooling fan

TEST DESCRIPTION
Numbers below refer to circled numbers on the diagnostic chart.

1. Code 15 will set in Signal voltage indicates a coolant temperature less than -39° C (-37.5° F) for 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 circuit 410 is open. There should be 5 volts present at sensor connector if measured with a high impedance digital volt/ohm meter.

DIAGNOSTIC AIDS:
A "SCAN" tool reads engine temperature in degrees centigrade. After engine is started the temperature should rise steadily to about 95° C then stabilize when thermostat opens. Circuit 410 is routed from the ECM to a mini harness, and then to the coolant temperature sensor. A faulty connection, or an open in circuit 410 or 452 will result in a Code 15.
If Code 23 is also set, check circuit 452 for faulty wiring or connections. Check terminals at sensor for good contact. Codes 15 and 21 stored at the same time could be the result of an open circuit 452 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. A "shifted" sensor could result in poor driveability complaints.

Refer to Diagnosis by Symptom / Intermittents. - Intermittent Malfunctions