Engine light on in 2003 PT Cruiser and code P0128 shows up what does this mean?

P0128 is an OBD-II diagnostic trouble code that indicates a malfunction in the engine coolant temperature (ECT) sensor circuit. The ECT sensor is responsible for providing the engine control module (ECM) with information about the temperature of the engine coolant. This information is used by the ECM to control the engine's fuel mixture, ignition timing, and other engine functions.

Potential causes of P0128:

- Faulty ECT sensor

- Damaged or corroded wiring in the ECT sensor circuit

- Faulty ECM

- Low engine coolant level

- Engine coolant temperature sensor harness is open or shorted

- Engine coolant temperature sensor circuit poor electrical connection

Possible symptoms:

- Engine light will be illuminated

- Engine may run rough or stall

- Engine may overheat

- Reduced fuel economy

Diagnostic and Repair

1. Check the engine coolant level. If the coolant level is low, add coolant and see if the code goes away.

2. Inspect the ECT sensor and its wiring. Look for any damage or corrosion. If the sensor or wiring is damaged, replace it.

3. Test the ECT sensor. You can do this using a multimeter or an OBD-II scanner. The sensor should read between 100 and 250 ohms at room temperature. If the sensor is out of spec, replace it.

4. Check the ECM. If the ECT sensor and its wiring are good, the problem may be with the ECM. You will need to have a dealer or a qualified mechanic diagnose the ECM.

Additional Notes

- P0128 can also be caused by a faulty thermostat or a coolant leak. If you are having this code and you have not yet found the cause, you may want to have the thermostat and cooling system checked.

- If you have a code reader, you can monitor the ECT data to see if it appears to be reasonable. The ECT should increase as the engine warms up and decrease as the engine cools down.