Check engine light just came on the other day, got it read and shows P0128. Coolant Thermostat (coolant temperature below thermostat regulating temperature).

Any suggestions for what might be wrong?

Temperature gauge still reads normal temps and the interior heat is working fine as well.

Thanks

 

In my case it was the thermostat. The first time it happened, we found the cracked radiator. Replaced it and thought we were good. About a month later, started throwing the code again. Replaced the thermostat and it is gone. H3 showed no signs other then the code.

 

Mine did the same thing, I replaced thermostat code went away. However, a week later chk engine light did not come back but digital display says check system been going on and off for a year. Only when temp outside drops below 75.

 

Thanks, how easy/difficult would it be to replace the thermostat myself?

 

I had a mechanic do it, however I did watch him and he took off driver side tire and wheel well skirt to get to it. Took about an hour.

 

I to paid the dealership (used my GM points to get it for free). There is a thread that ended on 12/30/11 that explains where it is and how to replace it. I don't know how to insert the link or I would for you. Either look back by dates or search on thermostat.

 

https://www.hummerforums.com/forum/h...2006-h3-26441/