Diagnostic Instructions
| • | Perform the Diagnostic System Check - Vehicle prior to using this diagnostic procedure. |
| • | Review Strategy Based Diagnosis for an overview of the diagnostic approach. |
| • | Diagnostic Procedure Instructions provides an overview of each diagnostic category. |
DTC Descriptor
DTC P0128: Engine Coolant Temperature (ECT) below thermostat Regulating TemperatureTypical Scan Tool Data
Circuit | Short to Ground | Open | Short to Voltage |
|---|---|---|---|
Operating Conditions: ECT Sensor Parameter Normal Range: -32 to +130°C (-26 to +275°F) | |||
ECT Signal | 140°C (284°F) | -40°C (-40°F) | -40°C (-40°F) |
ECT Low Reference | -- | -40°C (-40°F) | -40°C (-40°F)¹ |
¹Internal ECM damage may occur if shorted to B+ | |||
Circuit/System 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.
Conditions for Running the DTC
| • | DTCs P0016, P0112, P0018, P0113, P0117, P0126, P0201, P0202, P0203, P0204, P0205, P0206, P0234, P0299, P0335, P0336, P0500, P0602, P0642, P0643, P0652, P0653, P1293, P2146, P2147, P2148, P2149, P2150, P2151, P2161, P2164, P2167, P2170, P2173, P2176 and P268A are not set. |
| • | The ignition switch is ON for longer than 2 seconds. |
| • | The intake air temperature (IAT) is more than 19°C (-7°F). |
| • | The start-up ECT is between -40 to +64°C (-40 to +147°F). |
| • | The engine run time is longer than the calculated value by a continuous threshold depending on engine coolant temperature, intake air temperature, and fuel injection quantity. |
| • | The idling time rate, idling time/engine run time, is smaller than 35 percent. |
Conditions for Setting the DTC
The ECM detects that the maximum ECT is less than 72°C (162°F) for longer than 1 second.
Action Taken When the DTC Sets
DTC P0128 is a Type B DTC.
Conditions for Clearing the DTC
DTC P0128 is a Type B DTC.
Diagnostic Aids
| • | Inspect for any recent engine mechanical repairs. An incorrectly installed ECT sensor will cause this DTC to set. |
| • | A skewed sensor could result in poor driveability conditions. Refer to Temperature Versus Resistance . |
Reference Information
Schematic Reference
Connector End View Reference
Electrical Information Reference
DTC Type Reference
Powertrain Diagnostic Trouble Code (DTC) Type Definitions
Scan Tool Reference
Circuit/System Verification
Verify the integrity of the cooling system and fuel rail pressure sensor by inspecting for the following conditions:
| • | Damaged components |
| • | Loose or improper installation |
Circuit/System Testing
Important: You must perform the Circuit/System Verification before proceeding with Circuit/System Testing.
- Ignition OFF, disconnect the harness connector at the ECT sensor.
- Ignition OFF, test for less than 5 ohms of resistance between the low reference circuit terminal A and ground.
- Ignition ON, verify the scan tool ECT parameter is less than -39°C (-40°F).
- Install a 3A fused jumper wire between the signal circuit terminal A and the low reference circuit terminal B. Verify the scan tool ECT parameter is greater than 149°C (300°F).
- If all circuits test normal, test or replace the ECT sensor.
| ⇒ | If greater than the specified range, test the low reference circuit for an open/high resistance. If the circuit tests normal, replace the control module. |
| ⇒ | If greater than the specified range, test the signal circuit terminal B for a short to ground. If the circuit tests normal, replace the control module. |
| ⇒ | If less than the specified range, test the signal circuit for a short to voltage or an open/high resistance. If the circuit tests normal, replace the control module. |
Component Testing
Measure and record the resistance of the ECT sensor at various ambient temperatures and compare those measurements to the Temperature vs. Resistance Table. Refer to Temperature Versus Resistance .
Repair Instructions
Perform the Diagnostic Repair Verification after completing the diagnostic procedure.
| • | Control Module References for engine control module (ECM) replacement, setup, and programming |
