2004 Grand Prix L26 P0128

**Cougar281** · 11-30-2009

My friend's GP started throwing a P0128 last week, some time after a coolant flush at one of the Valvoline shops (car has 80k or so); Searching here didn't turn up much. According to the snapshot data I gathered with my Tech2, since I cleared the code initially, the car has been driven 133 miles since the first failure, has driven 99 miles since the last failure, and has had four fails, three pass and 7 not run. One person posted in response to another person's P0128 that the thermostat was bad and stuck open, which would make sense; If the thermostat is stuck open or part way open, it would cause the coolant to not heat up as fast as it should tripping the code, but the next person posted "You are so wrong you're right.

"

. I'm confused. So does he mean he's right that it's the thermostat, or or not? Thoughts?

**OH4CompG** · 11-30-2009

Watch the coolant temp as you drive. If it doesn't appear to heat up as quickly as normal then yes it the thermostat sticking open. Cheap replacement. Now's a good time to throw a 180 in there.

**Cougar281** · 11-30-2009

Originally Posted by OH4CompG

Watch the coolant temp as you drive. If it doesn't appear to heat up as quickly as normal then yes it the thermostat sticking open. Cheap replacement. Now's a good time to throw a 180 in there.

On that note, the other thing I should be able to look for is the temp getting up to operating (200ish?), then dropping a few degrees, and back up as the thermostat opens and closes, right?

**Cougar281** · 12-03-2009

Ok, here's an update: I had him do some logging, and got some decent data sets. Not sure what the outside temp was, but the IAT was between 45* & 60*, and while on the highway, the ECT got up to 185 or so and pretty much stayed in that area, until he stopped and got onto slower roads, then it finally got over 190. On another log, when the IAT was in the high 30's, low 40's, ECT hit a max of 178. Since the car is stock, I'd assume it's got a 195* T-Stat, and as such, it should typically run somewhere between 185 & 205 or so while cruising. Correct me if I'm wrong, but isn't the idea of a thermostat to keep the engine in the neighborhood of the thermostat temp regardless of OAT? Here is a zip file with the logs if anyone has EFILive or wants to download it to look at them and give input.