Page 961 of 5267

Theory of Operation
The engine coolant temperature sensor is a negative temperature coefficient thermistor-type sensor whose resis-
tance varies inversely with temperature. At cold temperatures the sensorresistance is high so the voltage is high.
As the coolant temperature increases the resistance decreases and the voltage becomes low.
When Monitored:
With the ignition on. Battery voltage greater than 10.4 volts.
Set Condition:
The Engine Coolant Temperature (ECT) Signal circuit voltage at the PCM goes below 0.8 of a volt for more
than 3 seconds.
Possible Causes
(K2) ECT SIGNAL CIRCUIT SHORTED TO GROUND
(K2) ECT SIGNAL CIRCUIT SHORTED TO THE (K900) SENSOR GROUND CIRCUIT
ECT SENSOR
PCM
Always perform the Pre-Diagnostic Troubleshooting procedure before proceeding. (Refer to 9 - ENGINE -
DIAGNOSIS AND TESTING).
Diagnostic Test
1.ECT SENSOR VOLTAGE BELOW 1.0 OF A VOLT
Ignition on, engine not running.
With a scan tool, read the ECT voltage.
Is the voltage below 1.0 of a volt?
Ye s>>
Go To 2
No>>
Refer to the INTERMITTENT CONDITION Symptom. (Refer to 9 - ENGINE - DIAGNOSIS AND TEST-
ING)
2.ECT SENSOR
Turn the ignition off.
Disconnect the ECT harness connector.
Ignition on, engine not running.
With a scan tool, read ECT voltage.
Is the voltage above 1.0 volt?
Ye s>>
Replace the ECT Sensor.
Perform the POWERTRAIN VERIFICATION TEST. (Refer to 9 - ENGINE - STANDARD PROCEDURE)
No>>
Go To 3
Page 962 of 5267
3.(K2) ECT SIGNAL CIRCUIT SHORTED TO THE (K900) SENSOR GROUND CIRCUIT
Turn the ignition off.
Disconnect the C1 PCM harness connector.
Measure the resistance between the (K900) Sensor ground circuit and
the (K2) ECT Signal circuit in the ECT Sensor harness connector.
Is the resistance below 5.0 ohms?
Ye s>>
Repair the short to between the (K900) Sensor ground cir-
cuit and the (K2) ECT Signal circuit.
Perform the POWERTRAIN VERIFICATION TEST. (Refer to
9 - ENGINE - STANDARD PROCEDURE)
No>>
Go To 4
4.(K2) ECT SIGNAL CIRCUIT SHORTED GROUND
Measure the resistance between ground and the (K2) ECT Signal circuit
in the ECT Sensor harness connector.
Istheresistancebelow100ohms?
Ye s>>
Repair the short to ground in the (K2) ECT Signal circuit.
Perform the POWERTRAIN VERIFICATION TEST. (Refer to
9 - ENGINE - STANDARD PROCEDURE)
No>>
Go To 5
Page 963 of 5267
5.PCM
NOTE: Before continuing, check the PCM harness connector terminals for corrosion, damage, or terminal
push out. Repair as necessary.
Using the schematics as a guide, inspect the wire harness and connectors. Pay particular attention to all Power and
Ground circuits.
Were there any problems found?
Ye s>>
Repair as necessary.
Perform the POWERTRAIN VERIFICATION TEST. (Refer to 9 - ENGINE - STANDARD PROCEDURE)
No>>
Replace and program the Powertrain Control Module per Service Information.
Perform the POWERTRAIN VERIFICATION TEST. (Refer to 9 - ENGINE - STANDARD PROCEDURE)
Page 964 of 5267
P0118-ENGINE COOLANT TEMP SENSOR VOLTAGE TOO HIGH
For a complete wiring diagramRefer to Section 8W.
Page 965 of 5267

When Monitored:
With the ignition on. Battery voltage greater than 10.4 volts.
Set Condition:
The Engine Coolant Temperature (ECT) sensor circuit voltage at the PCM is greater than 4.94 volts for more
than 3.0 seconds.
Possible Causes
(K2) ECT SIGNAL CIRCUIT SHORTED TO BATTERY VOLTAGE
(K2) ECT SIGNAL CIRCUIT OPEN
(K900) SENSOR GROUND CIRCUIT OPEN
ECT SENSOR
PCM
Always perform the Pre-Diagnostic Troubleshooting procedure before proceeding. (Refer to 9 - ENGINE -
DIAGNOSIS AND TESTING).
Diagnostic Test
1.ECT SENSOR VOLTAGE ABOVE 4.94 VOLTS
Ignition on, engine not running.
With a scan tool, read the ECT voltage.
Is the voltage above 4.94 volts?
Ye s>>
Go To 2
No>>
Refer to the INTERMITTENT CONDITION Symptom. (Refer to 9 - ENGINE - DIAGNOSIS AND TEST-
ING)
2.ECT SENSOR
Turn the ignition off.
Disconnect the ECT harness connector.
Connect a jumper wire between the (K2) ECT Signal circuit and the
(K900) Sensor ground circuit in the ECT harness connector.
Ignition on, engine not running.
With a scan tool, read ECT voltage.
Is the voltage below 1.0 volt with the jumper wire installed?
Ye s>>
Replace the ECT Sensor.
Perform the POWERTRAIN VERIFICATION TEST. (Refer to
9 - ENGINE - STANDARD PROCEDURE)
No>>
Go To 3
NOTE: Remove the jumper wire before continuing.
Page 966 of 5267
3.(K2) ECT SIGNAL CIRCUIT SHORTED TO BATTERY VOLTAGE
Turn the ignition off.
Disconnect the C1 PCM harness connector.
Ignition on, engine not running.
Measure the voltage on the (K2) ECT Signal circuit in the ECT Sensor
harness connector.
Is the voltage above 0 volts?
Ye s>>
Repair the short to battery voltage in the (K2) ECT Signal
circuit.
Perform the POWERTRAIN VERIFICATION TEST. (Refer to
9 - ENGINE - STANDARD PROCEDURE)
No>>
Go To 4
4.(K2) ECT SIGNAL CIRCUIT OPEN
Turn the ignition off.
Measure the resistance of the (K2) ECT Signal circuit from the ECT
Sensor harness connector to the PCM harness connector.
Is the resistance below 5.0 ohms?
Ye s>>
Go To 5
No>>
Repair the open in the (K2) ECT Signal circuit.
Perform the POWERTRAIN VERIFICATION TEST. (Refer to
9 - ENGINE - STANDARD PROCEDURE)
Page 967 of 5267
5.(K900) SENSOR GROUND CIRCUIT OPEN
Measure the resistance of the (K900) Sensor ground circuit from the
ECT Sensor harness connector to the PCM harness connector.
Is the resistance below 5.0 ohms?
Ye s>>
Go To 6
No>>
Repair the open in the (K900) Sensor ground circuit.
Perform the POWERTRAIN VERIFICATION TEST. (Refer to
9 - ENGINE - STANDARD PROCEDURE)
6.PCM
NOTE: Before continuing, check the PCM harness connector terminals for corrosion, damage, or terminal
push out. Repair as necessary.
Using the schematics as a guide, inspect the wire harness and connectors. Pay particular attention to all Power and
Ground circuits.
Were there any problems found?
Ye s>>
Repair as necessary.
Perform the POWERTRAIN VERIFICATION TEST. (Refer to 9 - ENGINE - STANDARD PROCEDURE)
No>>
Replace and program the Powertrain Control Module per Service Information.
Perform the POWERTRAIN VERIFICATION TEST. (Refer to 9 - ENGINE - STANDARD PROCEDURE)
Page 968 of 5267
P0121-TP SENSOR VOLTAGE DOES NOT AGREE WITH MAP
For a complete wiring diagramRefer to Section 8W.