P0126

DTC P0126

CIRCUIT DESCRIPTION
The engine coolant temperature (ECT) sensor is a variable resistor. The ECT sensor has a signal circuit and a low reference circuit. The ECT sensor measures the temperature of the engine coolant. The engine control module (ECM) supplies 5 volts to the ECT signal circuit and a ground for the ECT low reference circuit. When the ECT sensor is cold, the sensor resistance is high. When the engine coolant temperature increases, the sensor resistance decreases. With high sensor resistance, the ECM detects a high voltage on the ECT signal circuit. With lower sensor resistance, the ECM detects a lower voltage on the ECT signal circuit. If the coolant temperature does not increase normally, or does not reach the regulating temperature of the thermostat, the engine is considered not warm enough for stable low emission operation. Additionally other diagnostics that use engine coolant temperature as enabling criteria, may not run when expected. If the ECM detects that the calibrated amount of fuel has been burnt and the ECT has not reached the minimum calibrated temperature, this DTC will set. This DTC will only run once per ignition cycle within the enabling conditions.

Condition for Running the DTC
- DTCs P0016, P0112, P0113, P0116, P0117, P0118, P0201, P0202, P0203, P0204, P0261, P0264, P0267, P0270, P0234, P0299, P0300, P0301, P0302, P0303, P0304, P0335, P0336, P0500, P0602, P0642, P0643, P1293, P2146, P2147, P2148, P2149, P2150 and P2151 are not set.
- The ignition switch is ON.
- The intake air temperature (IAT) is more than 19 °F (-7 °C).
- The startup engine coolant temperature (ECT) is between -40 - 147 °F (-40 - 64 °C).
- The engine run time is longer than the calculated value by an continuous threshold depending on engine coolant temperature, intake air temperature and fuel injection quantity.
- The accumulation fuel injection quantity since engine start is less than a threshold.
- The idling time rate (idling time/engine run time) is smaller than 35%.

Condition for Setting the DTC
- The ECM detects that the maximum ECT is less than 149 °F (65 °C) for longer than 1 second.

Action Taken When the DTC Sets
- The ECM illuminates the malfunction indicator lamp (MIL) on the second consecutive driving cycle when the diagnostic runs and fails.
- The ECM records the operating conditions at the time the diagnostic fails. The ECM stores this information in the Freeze Frame/Failure Records on the second consecutive driving cycle when the diagnostic runs and fails.

Condition for Clearing the MIL/DTC
- The ECM turns OFF the MIL after 3 consecutive driving cycles when the diagnostic runs and does not fail.
- A current DTC, Last Test Failed, clears when the diagnostic runs and passes.
- A history DTC clears after 40 consecutive warm-up cycles, if no failures are reported by this or any other emission related diagnostic after the ECM turns OFF the MIL.
- Clear the MIL and the DTC with a scan tool.

Diagnostic Aids
- This DTC is designed to detect a faulty thermostat(s).
- Use the Temperature vs. Resistance table to test the ECT sensor at various temperature levels to evaluate the possibility of a skewed sensor. A skewed sensor could result in poor driveability concerns.

If an intermittent condition is suspected, the following may cause an intermittent:
- Damaged harness: Inspect the wiring harness for damage. If the harness appears to be OK, observe the "ECT Sensor" display on the scan tool with the engine running while moving connectors and the wiring harness related to the sensor. A change in the display will indicate the location of the fault.

Test Description
The number below refers to the step number on the diagnostic table.
3. This step tests for excessive resistance in the ECT sensor circuit.

DTC P0126
Schematic Reference: Engine Controls Schematics
Connector End View Reference: Engine Controls Connector End Views or Engine Control Module (ECM) Connector End Views

Step 2 - 7:

Step 8 - 11: