P0128: Faulty Engine Coolant Temperature

What is P0128?

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.

Common symptoms

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:

• The engine takes too long to warm up (especially after a cold start)
• Poor heater performance
• Poor fuel economy
• High idle

Can I still drive?

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:

• The check engine light may turn on
• The temperature gauge may not reach normal levels

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.

P0128 causes

Anything from irregular coolant thermostat temp (from faulty sensor readings) to low engine coolant levels can trigger a DTC P0128. 

These may include:

• A sticking or open thermostat – A thermostat helps maintain the required minimum engine temperature. It’s an important part of the engine cooling system, found inside the thermostat housing between the radiator and engine.
  When the engine is warm enough or the temperature gauge reaches the required operating temp, the thermostat lets the coolant flow through the radiator to maintain the temperature.
• A defective engine coolant temperature sensor (ECT) sensor
• Low coolant level – A coolant is an antifreeze solution that helps transfer heat and protects the engine from overheating by maintaining the engine at a normal operating temperature.
  It’s part of the car’s liquid cooling system, which comprises a:
  • Water pump: Circulates the coolant in the system
  • Radiator: Conducts heat from the coolant
  • Cooling fan: Blows cooler air through the radiator
  • Thermostat: Controls the coolant temperature

  The coolant also draws away excess heat and protects important moving components like the water pump, cylinder, head gasket, etc. 

• A faulty intake air temperature sensor (IAT sensor)
• Radiator fan malfunction
• Powertrain Control Module failure

Diagnosis

Your mechanic will do a series of checks to determine why the P0128 code showed up. 

Here’s how:

• Your mechanic will first scan your vehicle with an OBD-II scanner for all error codes. 
• If code P0128 is the only present code, your mechanic may check for an open thermostat, as thermostat malfunctions cause most cases of DTC P0128. 

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.

• If your thermostat works fine, your mechanic may check the coolant level. They’ll remove the radiator cap to check for low coolant levels. They’ll also check for coolant leaks as these may damage oxygen sensors too.
• Your mechanic can also inspect the coolant’s condition at the same time. A rusty or dirty coolant can cause the engine coolant thermostat to stick.

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.

Possible repairs for P0128

Depending on the cause of code P0128, your mechanic may do the following repairs:

• In case of a faulty thermostat, they’ll first determine what causes the malfunction and repair it accordingly. If the entire thermostat needs to be replaced, they may drain the coolant first and re-insert a new thermostat with fresh coolant.
• In case of a coolant issue, your mechanic will remove the radiator cap to examine the coolant level and condition. They may remove the drain valve and flush the coolant system if it requires a refill, then replace the drain valve and refill the tank with fresh coolant. 
• If the ECT sensor is malfunctioning, your mechanic can replace your temperature sensor. The engine coolant temperature sensor may not work in some cases due to sensor wiring and connector issues. An ECT sensor is usually replaced similarly to spark plugs. 

If the error code persists, the PCM sensor could be the culprit and require replacement.