Circuit Description
The engine coolant temperature (ECT) sensor is a thermistor. A thermistor is a resistor whose value varies with temperature. The ECT sensors resistance is high when the coolant temperature is cold, and the ECT sensors resistance is low when the coolant temperature is warm. The ECT sensor is wired in series with a fixed resistor in the powertrain control module (PCM). The PCM applies 5 volts to the ECT sensor. The PCM monitors the voltage across the ECT sensor and converts the voltage into a temperature reading. The PCM will receive a high voltage signal when the coolant temperature is cold. The PCM will receive a low voltage signal when the coolant temperature is warm. If the ECT sensor voltage does not indicate that the engine coolant temperature has increased sufficiently during engine operation, this DTC sets.
The following table illustrates the difference between temperature, resistance, and voltage:
Temperature | ECT Resistance | ECT Signal Voltage |
|---|---|---|
Cold | High | High |
Warm | Low | Low |
DTC Descriptor
This diagnostic procedure supports the following DTC:
DTC P1126 Engine Coolant Temperature (ECT) Sensor Performance in Cold Temperature
Conditions for Running the DTC
| • | DTCs P0030, P0050, P0107, P0108, P0112, P0113, P0117, P0118, P0125, P0135, P0155, P0171, P0172, P0174, P0175, P0201-P0206, P0300-P0306, P0335, P0401, P0403, P0404, P0443, P0496, P0506, P0507, P0641, P1128, P1129, P2227, P2228, P2229, P2413, P2414, P2415, P2646, P2647, P2648, and P2649 are not set. |
| • | The engine has been OFF for more than 6 consecutive hours. |
| • | The engine is started. |
| • | DTC P1126 runs once per drive cycle once the above conditions are met. |
Conditions for Setting the DTC
The difference in temperature between the intake air temperature (IAT) sensor and the engine coolant temperature (ECT) sensor is more than 26°C (79°F).
Action Taken When the DTC Sets
| • | The PCM illuminates the malfunction indicator lamp (MIL) on the second consecutive ignition cycle that the diagnostic runs and fails. |
| • | The PCM records the operating conditions at the time the diagnostic fails. The first time the diagnostic fails, the PCM stores this information in the Failure Records. If the diagnostic reports a failure on the second consecutive ignition cycle, the PCM records the operating conditions at the time of the failure. The PCM writes the conditions to the Freeze Frame and updates the Failure Records. |
Conditions for Clearing the MIL/DTC
| • | The PCM turns OFF the malfunction indicator lamp (MIL) after 3 consecutive ignition cycles that the diagnostic runs and does not fail. |
| • | A last test failed, or current DTC, clears when the diagnostic runs and does not fail. |
| • | A history DTC clears after 40 consecutive warm-up cycles, if no failures are reported by this or any other emission related diagnostic. |
| • | Use a scan tool in order to clear the MIL and the DTC. |
Diagnostic Aids
Check for the following conditions:
| • | Check for the proper operation of the engines cooling system, including the correct coolant level. |
| • | A faulty thermostat that stays open slightly can cause DTC P1126 to set in cold weather when the vehicle is started and let sit while warming up. Replace any suspect thermostat. |
| • | Use the Temperature vs Resistance table in order to evaluate the possibility of a shifted ECT sensor. A shifted sensor could result in DTC P1126. Refer to Temperature Versus Resistance - Engine Coolant Temperature Sensor . |
| • | Check for a faulty electrical connection to the PCM. |
An intermittent malfunction may be caused by a fault in the ECT sensor electrical circuit. Inspect the wiring harness and components for an intermittent condition. Refer to Intermittent Conditions .
Repair any electrical circuit faults that were found. Refer to Wiring Repairs in Wiring Systems.
If DTC P1126 cannot be duplicated, the information included in the Freeze Frame data can be useful in determining the vehicle operating conditions when the DTC was first set.
Test Description
The numbers below refer to the step numbers on the diagnostic table.
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The Diagnostic System Check-Vehicle prompts the technician to complete some basic checks and store the Freeze Frame data on the scan tool if applicable. This creates an electronic copy of the data taken when the fault occurred. The information is then stored in the scan tool for later reference.
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This step checks for a faulty ECT sensor or ECT sensor circuit.
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This step checks the integrity of the ECT sensor signal circuit.
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This step checks the integrity of the ECT sensor signal and ground circuits.
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This step checks the integrity of the ECT sensor ground circuit.
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After replacing the PCM a new minimum throttle position and idle speed must also be established.
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This step determines if DTC P1126 is the result of a hard failure or an intermittent condition.
Step | Action | Values | Yes | No |
|---|---|---|---|---|
Schematic Reference: Engine Controls Schematics Connector End View Reference: Powertrain Control Module Connector End Views or Engine Controls Connector End Views | ||||
Did you perform the Diagnostic System Check - Vehicle? | -- | Go to Step 2 | Go to Diagnostic System Check - Vehicle in Vehicle DTC Information | |
2 | Check for DTC P0117 or DTC P0118 with a scan tool. Is DTC P0117 or DTC P0118 set? | -- | Go to Step 3 | |
Is the ECT sensor value displayed near the actual coolant temperature? | -- | Go to Step 11 | Go to Step 4 | |
Does the engine coolant temperature equal the specified value? | -40°C (-40°F) | Go to Step 5 | Go to Step 8 | |
Is the engine coolant temperature more than the specified value? | 168°C (334°F) | Go to Step 11 | Go to Step 6 | |
Is the engine coolant temperature more than the specified value? | 168°C (334°F) | Go to Step 7 | Go to Step 9 | |
7 | Repair the open or high resistance in the ECT sensor ground circuit. Refer to Wiring Repairs in Wiring Systems. Did you complete the repair? | -- | Go to Step 15 | Go to Step 9 |
8 |
Did you find and correct the condition? | -- | Go to Step 15 | Go to Step 10 |
9 |
Did you find and correct the condition? | -- | Go to Step 15 | Go to Step 10 |
Did you complete the replacement? | -- | Go to Step 15 | -- | |
11 | Observe the ECT sensor parameter on the scan tool. Is the temperature more than 80°C (176°F)? | -- | Go to Step 12 | Go to Step 13 |
Did the ECT sensor parameter indicate that the coolant temperature has decreased more than the specified value? | 2°C (3.6°F) | Go to Diagnostic Aids | Go to Step 14 | |
13 |
Did the ECT sensor parameter indicate that the coolant temperature has increased more than the specified value? | 2°C (3.6°F) | Go to Diagnostic Aids | Go to Step 14 |
14 | Replace the ECT sensor. Refer to Engine Coolant Temperature Sensor Replacement . Did you complete the replacement? | -- | Go to Step 15 | -- |
15 |
Does the DTC run and pass? | -- | Go to Step 16 | Go to Step 2 |
16 | Observe the Capture Info with a scan tool. Are there any DTCs that have not been diagnosed? | -- | Go to Diagnostic Trouble Code (DTC) List - Vehicle in Vehicle DTC Information | System OK |
