I have a Mazda 6 and recently purchased the OBD 2 module and saw the check engine light with the PS0128 Error Coolant Thermostat (Coolant Temperature Below Thermostat Regulating Temperature).

I cleared the error to see if it recurs and several days later it has not.

Should it recur if it was not just a random false positive or does my clearing it clear it for good?

Should I get my car checked out? If so what should I ask them to check.

ps: The engine temperature gauge is fine

3 Answers 3


Clearing out the error shouldn't clear it out for good. If the problem persists you should see the error code come back again.

You may have already checked it out, but here is some info on your error code: OBD-II-Code-P0128


As Tim suggests, clearing the code doesn't solve the problem if it's not a random positive.

The engine temperature gauge is fine

Based on the information that you've provided so far (and using Tim's link as a reference), it seems likely that you have either a bad engine coolant temperature sensor. The implication from your PS is that the car is, in fact, reaching operating temperature just fine (so the thermostat must be working properly).

If you had said that the car was fine at highway speeds but overheating in city driving (or other abnormal temperature ranges), I'd be inclined to suggest that the thermostat is the problem.

All that said, it is worth noting the caveat on RepairPal:

An engine coolant temperature sensor code is not always caused by a defective sensor. It can be triggered by a defective thermostat, defective wiring, rusty coolant, or an overheating engine.

So, after all of the above, it's not impossible that your coolant is just full of goop.


I'm leaning towards a bad thermostat. They gunk up and fail over time (usually on the "too cold" side). My experience with factory coolant temp gauges is that they're pretty much useless. My experiences show that the coolant temp can be a good 20 degrees (F) off of normal and the factory gauge will still sit in almost exactly the same spot as if the temperature was normal.

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