P0128 Engine Code Meaning and How to Fix

Interviewer: Do you know what a P0128 engine code means and how to fix it?

CowboyBenWade: It means your engine didn't get hot enough or it didn't get hot enough fast enough. It can be caused by leaking coolant, a faulty thermostat, or an engine temperature sensor that isn't working. In other words, your engine is running too cold.

Interviewer: My house has a thermostat, but I didn't know my car's engine needed one.

CowboyBenWade: That's right. Engines are cold when they start, and then they heat up to a designated temperature. The radiator, water pump, coolant, and fans work to keep the engine at a constant operating temperature. We do not want the engine to get too hot or it will overheat - that will blow head gaskets, burn valves, and wreak all sorts of havoc. Inversely, if the temperature stays too low, the engine will wear excessively, you will get water built up in the crankcase due to condensation not burning off, and your car will have less power. An engine that is running at too low a temperature is not as serious as an engine that is overheating.

Your engine must warm up quickly when you start it, and then it must maintain that temperature - the thermostat is responsible for this. Engine thermostats in older cars were surprisingly simple. It was just a heat activated value that opened at a certain temperature to let coolant pass into the engine. The idea is that when you start the engine, you don't pump cold coolant through it immediately - wait to do that until it reaches operating temperature. Then the cold water will flow when needed to to maintain that temperature.

In modern cars, computers control thermostats. Electronics activate and deactivate coolant flow through the engine to keep it at the right temperature based on sensor values. So if you get a P0128, it could be the thermostat itself, or it could be the temperature sensor.

Unless your engine is overheating, it's safe to drive your vehicle. Most cars have a temperature gauge on the dashboard; check this and see if the car is in the expected range after driving it for a while. If it stays running too cold, that means the thermostat is open all the time and the cooling system is cooling the engine too much.

The P0128 activates at a certain thresholds. To be specific, your car's manufacturer designed the engine to run between X and Y degrees. If you start the car, and the temperature doesn't get into this range fast enough, it can throw the code. If you're driving it and it doesn't stay in this range or it dips out of the range, it will throw the code.

Just because your car has a P0128, it doesn't mean anything is wrong with it - it just means that there was some abnormality in the temperature range of the engine. Most often this is due to faulty sensors or mechanical problems but not always. Try resetting the error codes, and if the code comes back, you've got a problem.

Engines often throw multiple, related DTC codes. The P0118 means there is problem with the temperature sensor itself. Whereas, P0128 just means the temperature wasn't in range. So pay attention to the P0118 because that tells you exactly where the problem is. Replacing the temperature sensor should fix both trouble codes. If you have the P0128 without the P0118, it's more likely that the thermostat is bad.

Temperature sensors are cheap, and thermostats are pretty cheap too. It's not a major repair, and labor costs for this problem are low. Depending on what needs to be replaced, you're looking at several hundred dollars maximum.

You can even do this repair yourself. Replacing sensors is easy; Replacing a thermostat is a little bigger job because you must drain the coolant.