Hope someone can help me... for nearly the past year I have fought a never ending battle with my 2008 Mazda 3 and the thermostat system. When the issue first appeared a year ago, my husband and I replaced the thermostat. After clearing the codes and driving it around, it was fine. About a month later, the check engine light came on again and the codes read the same as before. P0128 and P0126 were the codes, I believe. Took it to our local mechanic and they said it needed a new O2 sensor. We paid them to do the work and they replaced it. Another month goes by with it running fine, then the codes popped up yet again. Took it back to the same mechanic since they said they "fixed it" and they discovered that they had put the wrong part in. Apparently they had been sent the incorrect part when they ordered it and didn't think to check it. So this time they replaced it with the correct O2 sensor and things were fine and dandy. Now, about 6 months has passed and my check engine light came on. Same code is showing. P0128. My thermostat gauge on the dash reads fine. It's right in the middle between hot and cold. Any ideas what's going on? Do we just have a crappy mechanic who isn't fixing it correctly? Thanks in advance! About ready to just get rid of this car...

 

A couple of thoughts:

• The wrong heat range thermostat may have been put in.
• The coolant temperature sensor may be faulty.

 

Hmm. I can see either of those being the issue. How would we be able to tell if the wrong heat range thermostat was used? Wouldn't the check engine light constantly be coming on if that was the case?

 

No, the CEL will only illuminate when the coolant temperature is out of the expected range for current conditions.

 

Did you fit a Genuine Mazda Thermostat?

Did your mechanic fit a Genuine Mazda O2 sensor?

 

Good questions. I'm honestly not sure. Thermostat we got from Pep Boys... so maybe not?

 

More like "definitely not."