Last weekend I changed out my thermostat from the stock to my 180 drilled. Last year I used this t'stat without problems but changed it out for the winter (I live in the far north). About 3 days later my ses light came on, p0128 thermostat malfunction, I reset the code and it never came back. I have been wondering why the code was set so soon after the swap?
The new pcm I have comming from intense will take care of this but why no problems last year and so soon this?

I had the cooling system flushed last fall and the coolant level is good.

This is a DTC that only sets after two or three (can't remember which) driving cyles where the PCM detects an issue. So if you're too cold one time, then OK the next time, the counter resets. This is why most people won't see this code for several days or even a week after installing a colder thermostat.

Most people are actually fine with a 180, but a 160 will trip this code on new Bonnevilles pretty quickly.

Hope this helps.

Scott

I've got a 180 in mine and just recently with the weather cooling off I've been getting this code. I haven't gotten an Intense PCM yet, would getting one of those take care of this? What are the chances that the coolant temp sensor has actually gone bad. The guage registers between 150 and 200 depending on time of day, so I'm presuming it'* working. Any ideas on how to keep this code from popping up?