Circuit Description
The engine coolant temperature (ECT) sensor uses a thermistor to control the signal voltage to the engine control module (ECM). The ECM supplies a 5-volt reference and a ground to the sensor. When the engine coolant is cold, the resistance is high, therefore the ECT signal voltage will be high. If the engine coolant is warm, the sensor resistance becomes less, therefore the IAT signal voltage drops. At normal engine operating temperature, the voltage will be between 1.5-2 volts at the ECT signal terminal.
The ECT sensor is used to control following items:
| • | Fuel delivery |
| • | Ignition |
| • | Evaporative emission (EVAP) canister purge valve |
| • | Idle air control (IAC) valve |
| • | Electric cooling fan |
Conditions for Setting the DTC
| • | The ECT is less than 149°C (300°F). |
| • | The engine run time is greater than 60 seconds. |
Action Taken When the DTC Sets
| • | The malfunction indicator lamp (MIL) will illuminate. |
| • | The ECM will record operating conditions at the time the diagnostic fails. This information will be stored in the Freeze Frame and Failure Records buffers. |
| • | A history DTC is stored. |
| • | Both cooling fans turn ON. |
| • | The ECM will default to 20°C (68°F) for engine coolant temperature for the first 60 seconds of engine run time and then 92°C (198°F). The scan tool will not show the defaulted value. |
Conditions for Clearing the MIL/DTC
| • | The MIL will turn OFF after 4 consecutive ignition cycles in which the diagnostic runs without a fault. |
| • | A history DTC will clear after 40 consecutive warm-up cycles without a fault. |
| • | DTCs can be cleared by using the scan tool. |
| • | Disconnect the ECM battery feed for more than 10 seconds. |
Diagnostic Aids
After engine start the ECT should rise steadily to 90°C (194°F) then stabilize when the thermostat opens.
Use the Temperature vs. Resistance Values table to evaluate the possibility of a skewed sensor. Refer to Temperature Versus Resistance .
Test Description
The numbers below refer to the step numbers on the diagnostic table.
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The On-Board Diagnostic System Check prompts the technician to complete some basic checks and store the Freeze Frame and Failure Records data on the scan tool if applicable. This creates an electronic copy of the data taken when the malfunction occurred. The information is then stored on the scan tool for later reference.
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This step simulates a DTC P0118 condition. If the ECM senses the change, then the ECM and the ECT wiring are OK.
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The replacement ECM must be reprogrammed. Refer to the latest Techline procedure for ECM reprogramming.
Step | Action | Values | Yes | No |
|---|---|---|---|---|
Perform an On-Board Diagnostic System Check. Was the check performed? | -- | Go to Step 2 | ||
2 |
Is the engine coolant temperature (ECT) sensor value greater than the specified value? | 130°C (266°F) | Go to Step 4 | Go to Step 3 |
3 |
Is the ECT sensor value greater than the specified value? | 130°C (266°F) | Go to Step 4 | Go to Diagnostic Aids |
Is the ECT sensor value less than the specified value? | -30°C (-22°F) | Go to Step 6 | Go to Step 5 | |
5 | Inspect the ECT sensor signal circuit at terminal 1 for a short to ground and repair as needed. Did you find and correct the condition? | -- | Go to Step 8 | Go to Step 7 |
6 | Replace the ECT sensor. Refer to Engine Coolant Temperature Sensor Replacement Did you complete the replacement? | -- | Go to Step 8 | -- |
Did you complete the replacement? | -- | Go to Step 8 | -- | |
8 |
Does the scan tool indicate that this diagnostic has run and passed? | -- | Go to Step 9 | Go to Step 2 |
9 | Are any DTCs displayed that have not been diagnosed? | -- | Go to applicable DTC table | System OK |
