When you first start your car, it’s supposed to run for a few minutes before the coolant circulates to warm up. If the car fails to warm up, you’ll get this code.

What Does P0128 – “Coolant Thermostat Temperature Below Regulating Temperature” Mean?

The engine in your car doesn’t like being too cold. It also really dislikes being too hot, so some sensors constantly monitor the coolant’s temperature to keep your car at the temperature where it’s happiest. If the PCM, the engine’s brain, notices that one of the sensors detects a temperature outside the range of efficiency; it generates a code to let you know.

The P0128 code is the code you get when the PCM detects that the engine isn’t warming up fast enough. This is good news since an engine running cold is much safer than an engine running too hot. Worst case scenario, you miss out on a couple of MPGs.

How Serious Is Code P0128?

If your engine is running a little cold, chances are there is nothing to worry about, and it can be fixed when it’s convenient for you.

The only thing that you may have to worry about is that a few states require your car to have no stored fault codes; otherwise, it will fail a smog inspection.

However, it is possible to receive this code when a sensor has failed. If that happens, your car can be overheating without the sensor reading the temperature correctly.

Stop Driving

• If you smell burning coolant
• You notice steam or other signs of overheating

Related Sensors

• Coolant temperature sensor

Symptoms

• Check engine light is illuminated
• Fuel mileage is lower than normal
• The temperature gauge reads low even when driving at speed
• The engine idles higher than normal and doesn’t warm-up

Causes

There are four main causes of the P0128 code.

1. The coolant level is low
2. There is an issue with the thermostat
3. The coolant temperature sensor is faulty
4. The cooling fan is stuck on (this is rare)

Solutions

The solution for a P0128 DTC is often very straightforward. Follow the next steps, and your problem will likely be solved.

1. Check the coolant level. While it’s one of the less common issues, it’s a very easy thing to fix. Make sure to check your user’s manual so you can add the correct coolant.
2. Check the thermostat. We have an entire write-up about the process here.
3. Visually inspect the temperature sensor for damage. Focus on the wires leading to the sensor. If the car spends a lot of time outside, rodents can chew on wires and create all manner of issue.
4. Replace the temperature sensor. They are often inexpensive and can be replaced using simple hand tools. Make sure to top off the coolant if you replace the sensor since some will usually leak out.
5. Test the fan using a multimeter or use a friend to visually inspect the fan as the engine runs. It Still Runs has an excellent web page about checking the radiator fan.

If the problem persists after you have completed all the steps, you may have a bad PCM/ECU or need to have your computer reprogrammed. It’s a fairly rare issue, but the only way to solve it is to bring the car to a shop that can flash the computer. You will have to call ahead to your mechanic to see if they have the necessary equipment, and you can expect to pay around $200.