| CAUTION | If ECM replacement is instructed in following testing, always ensure ECM harness connectors and ground circuit are okay. If either are suspect, repair and repeat testing to confirm ECM malfunction. If ECM is replaced, ECM must be programmed with proper ignition key code for engine immobilizer system. For programming procedures, see COMPUTER RELEARN PROCEDURES article in GENERAL INFORMATION. |
Circuit Description
ECM monitors vehicle speed, outside air temperature and engine load, and compares that information to engine coolant temperature. If engine coolant temperature does not reach 167°F (75°C), DTC P0128 will set. Possible causes are
- Faulty thermostat.
- Cooling system not functioning properly.
- Faulty coolant temperature sensor.
- Faulty ECM.
Diagnostic Aids
Using scan tool, read freeze frame data. Freeze frame records engine conditions when malfunction is detected.