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Engine code P0128 means “Thermostat Rationality/Engine Coolant Temperature Below Thermostat Regulating Temperature.” It shows up when the coolant temp sensor detects a coolant temp below the predicted engine coolant temperature.
Since P0128 is a generic engine code, it indicates coolant temp issues in all OBD-II equipped cars manufactured after 1996. But specific repairs may depend on the car’s make and model.
Your engine coolant should reach a defined thermostat regulating temperature within a specific amount of time after you start the engine. If it doesn’t, the Engine Control Module (ECM) or Powertrain Control Module (PCM) generates the P0128 error code.
Your car must reach the correct engine operating temperature so that the fuel system can enter a closed-loop and oxygen sensors can ensure proper air to fuel ratio for better car performance. Ideally, the operating coolant temperature should be around 160°F (71°C) within 15 minutes of starting your car. If the coolant temperature takes more time to reach this value, or if the temp gauge reads abnormal engine temperature levels, it would cause a P0128 engine code.
The most common symptom of a P0128 error code is an illuminated malfunction indicator lamp (MIL) or check engine light.
Here are some other symptoms you should look out for:
You can drive your car with the trouble code P0128, as this error code isn’t critical and it doesn’t require immediate repair.
However, you may face the following issues if you don’t get it fixed:
Moreover, prolonged coolant temperature issues may affect fuel economy and emissions or cause engine damage. The extent of damage usually depends on what causes DTC P0128. For example, almost all coolant thermostat issues can affect how well your engine functions.
The best way to understand what caused the code would be to call a professional mechanic. Let’s see how a mechanic may diagnose your P0128 issue.
Anything from irregular coolant thermostat temp (from faulty sensor readings) to low engine coolant levels can trigger a DTC P0128.
These may include:
The coolant also draws away excess heat and protects important moving components like the water pump, cylinder, head gasket, etc.
Your mechanic will do a series of checks to determine why the P0128 code showed up.
To check your thermostat, the mechanic will place their hand on the radiator hose after you start your car. Ideally, the hose should heat up when the hot coolant passes through it. If the radiator hose doesn’t warm up or takes too long, you may have a faulty thermostat.
Finally, they may check if the coolant temperature sensor works properly with a multimeter. Ideally, the ohm reading should change with temperature. If it doesn’t, it may indicate a coolant temperature sensor problem.
Depending on the cause of code P0128, your mechanic may do the following repairs:
If the error code persists, the PCM sensor could be the culprit and require replacement.
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