P0128 Ford Focus

P0128 Ford Focus

A P0128 Ford Focus trouble code is fairly common with the 2.0L engine. It’s defined as: Coolant Thermostat (Coolant Temp Below Thermostat Regulating Temperature)

The engine computer expects the engine to heat up in a predictable manner based on engine coolant temperature and ambient air temperature sensors, engine RPM, engine run time and accelerator pedal position. If the computer doesn’t see the expected engine temperature, it sets the trouble code.

Diagnosing a P0128 Ford Focus trouble code

Here are most common causes for this trouble code

1)      Faulty thermostat that is stuck in the open or closed position
2)      Faulty water pump that isn’t circulating coolant properly
3)     Air in the cooling system that prevents the engine coolant temperature sensor from obtaining an accurate reading.
4)      Faulty engine coolant temperature sensor that’s sending bad information

To determine whether the engine coolant temperature sensor is working properly, check the sensor resistance and voltage, or read the reported reading from a scan tool equipped with live data. Then compare the readings to an actual temperature check with a contact thermometer or non contact infra red thermometer. Check the engine temperature right at the thermostat housing. If the temps are close, the sensor is ok.

If the engine coolant temperature sensor shows a reading that’s much different than the intake manifold, you may have a thermostat or air pocket problem. If the trouble code showed up after a recent coolant flush or coolant leak refill, try bleeding air out of the system.

If the thermometer test shows a temp that’s lower than normal operating temperature 190°F – 210°F, chances are the thermostat is stuck in the full open position and is overcooling the engine.

P0128 is usually caused by a bad thermostat

You can conduct all the testing listed above, but chances are the thermostat

Engine Thermostat

has failed in the open position or is getting stuck in a full or partially opened position. Your best bet is to replace the thermostat.

Best thermostat for Ford Focus

I recommend replacing the thermostat with either a genuine  Ford or STANT brand.

I’ve personally had very bad luck with the MotoRad Fail-Safe thermostat. The Fail-Safe thermostat is designed to lock in the full open position in the event of an overheating condition. But my experience is that the thermostats result in a false positive far too often. I notice that the heating doesn’t get hot enough and then discover that the thermostat is locked open. At first I thought that maybe the engine did have an overheating event. So I replace the MotoRad product with another one and watched the temp gauge. No signs of overheating, yet it failed open again. I then replaced the Fail-Safe unit with a STANT thermostat and haven’t had a problem since then.

©, 2018 Rick Muscoplat