OBD code PO128

I have a 2006 ford fusion and the check engine light came on with the code p0128. It says its the coolant thermostat but it never has overheated and the heat still works. The fluid levels are fine and so i dont know what to do for sure. Please help.

The code indicates that coolant temperature hasn’t reached the anticipated level for the amount of air that’s entered the engine.

A partially stuck open thermostat can cause this code.


Is the only way to fix it to replace it?

The only way to fix a “stuck open” thermostat is to replace it.

The coolant temperature sensor should also be checked. This is easy. A thermostat replacement is also easy, and sometimes done just as a matter of maintenance.


I have the same vehicle and had the same check engine light and code response when I checked it out. My thermostat was opening prematurely, thus the engine was running a bit cooler than “normal operating temperture”. If you are in Arizona like I am, that’s not too much concern since it lets the engine run cooler but should be repaired because everything is computer controlled and it could affect the overall performance.

If you are in a cold climate area, it should be addressed sooner than later. One thing that can happen if you dont address it is the backend of the engine can possibly develope rust which is not a good thing but this usually takes awhile.

I actually ran that way (not changing the thermostat) for approx 1 year with no problems or engine rust but it should be repaired. I have since had mine repaired. With the 4 cyl engine, the thermostat is not a simple replacement procedure. You have to remove a couple items above it to get to it. I paid $810.00 but had the front wheel bearings replaced too, an oil change, a coolant flush with my thermostat replaced too. I was quoted $350.00 to do it by itself by my mechanic.

Hope this helps.