Circuit Description
The engine coolant temperature (ECT) sensor contains a semiconductor device which changes the resistance based on the temperature (a thermistor). The ECT sensor is mounted in bank one cylinder head near the center of the engine. The ECT sensor has a signal circuit and a ground circuit. The powertrain control module (PCM) applies a voltage (about 5 volts) on the signal circuit to the sensor. The PCM monitors the changes in this voltage caused by changes in the resistance of the sensor in order to determine the coolant temperature.
When the coolant is cold, the sensor (thermistor) resistance is high, and the PCMs signal voltage is only pulled down a small amount through the sensor to ground. Therefore, the PCM senses a high signal voltage (low temperature). When the coolant is warm, the sensor resistance is low, and the signal voltage is pulled down a greater amount. This causes the PCM to senses a low signal voltage (high temperature). At normal operating temperature, the voltage should measure about 1.5-2.0 volts at the PCM.
When the PCM senses a signal voltage higher than the normal operating range of the ECT sensor, this DTC will set which is a type C DTC.
Conditions for Running the DTC
Engine operates longer than 60 seconds
Conditions for Setting the DTC
| • | The PCM indicates engine coolant temperature is less than -35°C (-31°F) |
| • | Above condition present for less than a second |
Action Taken When the DTC Sets
| • | The PCM stores the DTC information into memory when the diagnostic runs and fails. |
| • | The malfunction indicator lamp (MIL) will not illuminate. |
| • | The PCM records the operating conditions at the time the diagnostic fails. The PCM stores this information in the Failure Records. |
Conditions for Clearing the MIL/DTC
| • | A History DTC will clear after 40 consecutive warm-up cycles, if no failures are reported by this or any other non-emission related diagnostic. |
| • | A last test failed (Current DTC) will clear when the diagnostic runs and does not fail. |
| • | The PCM battery voltage is interrupted. |
| • | Use a scan tool in order to clear the MIL/DTC. |
Diagnostic Aids
| • | Inspect the harness routing for a potential short to ground in the 5 volt reference circuit. |
| • | The scan tool displays the engine temperature in degrees centigrade. After the engine is started, the temperature should rise steadily to about 90°C (194°F) then stabilize when the thermostat opens. |
| • | An intermittent DTC can be caused by electromagnetic interference (EMI). Inspect related circuits for being too close to secondary ignition wires and the alternator. |
| • | If the engine has been allowed to sit overnight, the engine coolant temperature and intake air temperature values should display within a few degrees of each other. |
| • | Use the Temperature vs. Resistance Value scale to test the coolant sensor at various temperature levels in order to evaluate the possibility of a skewed or mis-scaled sensor. A skewed sensor could result in poor driveability complaints. Refer to Temperature Versus Resistance . |
Test Description
The numbers below refer to the step numbers on the diagnostic table.
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If DTC P0118 failed this ignition, this indicates a hard failure is present. When a hard failure is present, both the hard and intermittent DTCs set.
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When moving related connectors, visually and physically inspect connectors for the following conditions:
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When moving the related wiring harnesses, visually and physically inspect the wiring for the following:
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Using the Freeze Frame and Failure Records data may aid in locating an intermittent condition. If you cannot duplicate the DTC, the information included in the Freeze Frame and Failure Records data can help determine how many miles since the DTC set. The Fail Counter and Pass Counter can also help determine how many ignition cycles the diagnostic reported a pass and a fail. Operate the vehicle within the same Freeze Frame conditions (RPM, load, vehicle speed, temperature etc.) that you observed. This will isolate when the DTC failed. For any test that requires probing the PCM or component harness connectors, use the Connector Test Adapter Kit. Using this kit prevents any damage to the harness connector terminals.
| • | Poor mating of the connector halves or a terminal not fully seated in the connector body (backed out) |
| • | Improperly formed or damaged terminals. Carefully reformed or replaced all connector terminals in the related circuits in order to insure proper terminal contact tension |
| • | Poor terminal to wire connection. Inspect for poor crimps, crimping over wire insulation rather than the wire |
| • | Dirt or corrosion on the terminals. Inspect the connector seals for being there and for being damaged |
| • | Wire insulation that is rubbed through, causing an intermittent short |
| • | Wiring broken inside the insulation |
Step | Action | Values | Yes | No |
|---|---|---|---|---|
1 | Did you perform the Powertrain On-Board Diagnostic (OBD) System Check? | -- | ||
Did DTC P0118 Fail This Ignition cycle? | -- | Go to DTC P0118 Engine Coolant Temperature (ECT) Sensor Circuit High Voltage | ||
Does the sensor value change abruptly while a related connector is being moved? | -- | |||
Observe the affected sensor value on the scan tool while moving the related wiring harnesses. Does the sensor value change abruptly while moving the related electrical harnesses? | -- | |||
Does the scan tool indicate that this diagnostic failed this ignition? | -- | Go to Diagnostic Aids | ||
6 | Repair the damage connectors/terminals . Refer to Wiring Repairs in Wiring Systems. Is the action complete? | -- | -- | |
7 | Repair the faulty wiring. Refer to Wiring Repairs in Wiring Systems. Is the action complete? | -- | -- | |
8 |
Is the action complete? | -- | -- | |
9 |
Does the scan tool indicate that this test ran and passed? | -- | ||
10 | Select the Capture Info option and the Review Info option using the scan tool. Does the scan tool display any DTCs that you have not diagnosed? | -- | Go to the applicable DTC table | System OK |
