dumbwop

My wife's check engine light came on the other day and when I hooked up my scanner it returned a P0128.

Upon quick checking, I'm guessing it could be any of these problems:

• Leaking or stuck open thermostat
• Faulty cooling fan (running too much)
• Faulty coolant temperature (ECT) sensor

I obviously checked her coolant level and its fine. My gut is telling me its a bad thermostat but I'm wondering if anyone has suggestions on a good differential diagnosis for eliminating the other possibilities? I don't feel like spending two hours pulling the serpentine belt and replacing everything and $140 on a new thermostat to still have an issue.



It certainly appears to be a straight-forward fix and ANY ideas would be appreciated before I jump into this.

Patton001

The error code means that the coolant temp is below the regulating temp of the stat. The fan could run all day but if the stat is working it will close and let the temp get to running temp. Could be the sensor I suppose. You can buy a relatively inexpensive laser temp sensor and see where the temp actually is.

If you really are running too cool your car is probably running in open loop all the time. Not the best for mileage or performance.

dumbwop

I watched the dash gauge and it rose to about 80°C after driving 30 minutes, which seemed low to me. I figured the temp wasn't coming all the way up, but the cause is the question.

It would seem that a fouled thermostat (stuck open) would be an immediate culprit, but I don't want to waste time and money replacing an unnecessary part....

Rudeney

You can get a less expensive thermostat. Most auto parts retailers (Auto Zone, Advance, Napa) sell them for under $50.