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General Smog Check

What is Trouble Code P0128 - Coolant Below Thermostat Regulating Temperature

My car's check engine light is not on but I'm pulling a P0128 trouble code. My A/C and heater are both working fine and the vehicle is not overheating.


OBD-II P0128 Trouble Code

Coolant Thermostat (Coolant Temperature Below Thermostat Regulating Temperature)

Here is what a P0128 means, in simple terms

Trouble code P0128 indicates your engine is running too cool. This sounds great, right? Actually it's not. An engine which is not able to get hot enough, in the time required, may cause an increase in harmful emissions. As such, the Engine Control Module (ECM) is programmed to detect low engine coolant temperature (compared to ambient air temperature and pre-determined value), as well the speed at which the pre-determined engine coolant temperature must be reached.

What caused my vehicle to set a P0128 trouble code?

The engine control module uses the Engine Coolant Temperature (ECT) sensor to determine whether or not to trigger a OBD-II P0128. When a trouble code P0128 exists by itself, usually a shop technician will suspect a fault engine coolant temperature sensor or stuck open thermostat.

  • A stuck open or prematurely opening coolant thermostat.
  • A defective engine coolant temperature (ECT) sensor and/or ECT wiring (most often at the ECT connection/harness).

What symptoms will my vehicle experience when trouble code P0128?

  • Your vehicle check engine light (CEL), Malfunction Indicator Lamp (MIL) or Service Engine Soon light will be illuminated and freeze frame data stored with information regarding engine sensor data at the time when the ECC detected the engine coolant temperature problem. Trouble code P0128 will also be recorded in the ECM ROM memory.
  • Engine will take too long to warm up (especially on cold days).
  • Air conditioning heater might not get hot enough because it uses the engine's coolant to warm up the vehicle's interior, and the coolant never reaches full operating temps.
  • Smell of HC (hydrocarbon) longer than usually during engine startup due to proper engine operating temperature not being reached quick enough.

How can I fix a P0128 problem and where should I start?

We recommend beginning by determining whether your engine's thermostat is operating as it should. A working thermostat should allow the flow of coolant only after the coolant has reached proper operating temps (usually around 200f). To check the thermostat's operation, start your engine (from a cold start) and notice the warmth of the radiator hose attached to the thermostat housing. The hose should be cool to the touch at first, and then suddenly become very hot (after around 3-5 minutes depending on ambient air temperature). If you do not notice a sudden rise in the hose's exterior temperature, and feel the hose gradually getting warmer, your vehicle may have a thermostat which is stuck open; allowing for the immediate flow of coolant through the radiator from the time the engine is started and not letting the coolant in the engine get hot enough before opening the thermostat. Replace the thermostat.

If the thermostat is operating as it should, the next inspection should be the engine coolant temperature sensor. Checking the ECT will require a voltmeter and some diagnostic know-how. With the ignition on and engine off, check for reference voltage from the ECM to the ECT. You should see 5 volts constant. Next start the engine and take a reading from the ECT signal wire. As the engine warms up, the signal wire's voltage should decrease. We won't go into the specifics of the voltage you should be seeing here because the numbers vary by vehicle, but at full operating temperature, the reading should be 0.5 volts. The important thing to note is whether there is a fluctuation in voltage or not. Also ensure you take the reading after the ECT connector and at the ECT sensor itself. This will determine whether the fault is due to a defective connection and could save you money from replacing the ECT unnecessarily.

  • Check the coolant hose connected to the thermostat housing for a sudden rise in temperature.
  • Check the ECT signal and reference voltage.
  • Check the ECT connector.

Can I drive my vehicle with a P0128 trouble code and illuminated CEL?

  • You may drive your vehicle with a P0128 trouble code. You will not notice any issue except for maybe the temperature gauge never reaching normal levels, and possibly the Check Engine light being illuminated. Also note, if your vehicle requires a smog inspection, it will fail the test due to the presence of the P0128 trouble code and the check engine light being ON.
  • The check engine light or malfunction indicator lamp illuminated while P0128 is recorded in the ECC.

posted by SmogTips Support

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