What Is Engine Code P0128?

  • P0128 Technical Definition: Coolant Thermostat (Coolant Temperature Below Thermostat Regulating Temperature)
  • P0128 Meaning: Thermostat not working properly
  • Most common cause: Faulty Thermostat
  • Risks for the engine/driver: LOW You should get it checked out ASAP but your car should be safe to drive
  • Emissions severity: MEDIUM. The car may not pass emissions testing
  • Estimated repair time: 1 Day
  • Estimated repair cost: $100+

The engine code P0128 is stored in memory when the ECU detects a possible fault in the thermostat.

The thermostat is a mechanical piece with two important goals, helping the engine reaching its operating temperature as fast as possible and then keep that operating temperature stable under any driving condition. A stable engine temperature is crucial for accurate emissions control but also to avoid a premature mechanical wear of engine cylinders.

The data trouble code P0128 is set when the ECU senses that the engine coolant temperature is taking longer than expected to reach its normal values. To avoid false positives, the ECU takes into account air temperature, engine RPM, barometric pressure, vehicle speed and time passed since the engine has started.


3 Common Symptoms When Code P0128 is Present

The most common symptoms of data trouble code P0128 are:

  1. Check Engine Light.
  2. Coolant temperature lower than normal (at idle)
  3. Coolant temperature drops at high speed or during downhill driving

Possible Causes For Error Code P0128

The most common causes of data trouble code P0128 are:

  1. Bad thermostat.
  2. Low engine coolant level
  3. Defective Engine Coolant Temperature sensor (ECT)
  4. Defective Intake Air Temperature sensor (IAT).

How To Diagnose The Engine Code P0128

For the purpose of this article, it’s assumed that you have a basic knowledge of safety precautions while working on your vehicle.

Always refer to the appropriate OEM literature when possible. Original manufacturer diagnostic procedures should always have precedence over a generic workflow.

That said, let’s start the diagnostic process!

1. Preliminary steps

To discard a possible intermittent DTC condition, you’ll need to clear the ECU memory and complete a driving cycle.

  • Read data trouble codes and take note of them.
  • Clear data trouble codes memory.
  • Perform a driving cycle (at least 5-10 minutes).

In case the Check Engine Light stays off then you may have an intermittent problem. If the light lit during your driving cycle then continue with the diagnostic process.

2. Visual Inspection

> Engine coolant level: check the engine coolant level, complete as necessary.

> Thermostat: wait until the engine is fully cold and remove the thermostat from the vehicle. Check its condition and replace as needed. Double check the part number before ordering.

> ECT Sensor wiring and connector: perform a meticulous visual inspection of the ECT sensor wiring and connectors. Look for burnt, damaged, corroded or deteriorated wires, also unplug the sensor and look for bent terminal pins, loosely connections, corrosion or any other possible indication of a bad connection.

> IAT Sensor wiring and connector: perform a meticulous visual inspection of the IAT sensor wiring and connectors. Look for burnt, damaged, corroded or deteriorated wires, also unplug the sensor and look for bent terminal pins, loosely connections, corrosion or any other possible indication of a bad connection.

Fix any wiring problem before continuing.

3. Scan Tool Tests

> ECT live test: start the engine to enter KOER mode. Put the Engine Coolant Temperature sensor output on the screen. Using an automotive infrared thermometer aim at the ECT sensor and take a reading. Compare the two values. The difference should not be greater than 5 ° C unless the sensor has a serious internal wear.

> IAT live test: start the engine to enter KOER mode. Put the Intake Air Temperature sensor output on the screen. Using an automotive infrared thermometer aim at the IAT sensor and take a reading. Compare the two values. The difference should not be greater than 10-15 ° C unless the sensor has a serious internal wear.


How To Repair Error Code P0128

Depending on the diagnostics results you may need to do the following:

  • Replacing the thermostat.
  • Repairing the ECT wiring or connector.
  • Repairing the IAT wiring or connector.

Even when this code doesn’t affect engine’s performance you need to be aware of legal issues when driving in zones with strict emissions regulations. Incorrect coolant temperatures often lead to increased emissions.