Thread: P0128 engine coolant too low, stuck thermostat?

My check engine light came on my 2002 grand prix gt. My coolant levels are all fine. When I plugged in my obd2 scanner I got P0128 "coolant thermostat (coolant temp below thermostat regulating temperature)" I have noticed that since it has been getting colder out (i live in WI) my car seems to be not warming up to the normal temperature that it usually runs. Does this most likely sound like a stuck thermostat or could it be a faulty ECT sensor?

I would love some input. If it is the thermostat anyone have a right up on it? any tips for doing it? Thanks guys !

Ok that's what I thought. I had a 1998 bonneville that I tore apart because I had a faulty upper intake manifold which eventually destroyed my rear main bearing. Anyway my question is do you guys prefer using the oem 195 degree tstat or should I go with the 180? I was thinking the 180 but I would love the input.

Stick with the 195. I noticed my 98 bonnie didn't like the 180. Seemed to suck down the gas more than normal.

I'd also check the coolant sensor. It may be gummed up.
Flush with Prestone All make All Model.

But yes...if it has the original intake gaskets/upper manifold, you need to do the swap before its too late.
Refill with Prestone All Make All Model.

you'd get more with a 195 though.

After sitting for more then a year, I got tired of looking at my wife's old car. So I fired it up, cleaned it out, added some air to the tires, changed the oil then found out there was a recall for that engine fire thingy. I got that taken care of two days ago. I noticed while driving it that the temp never came up much off of 160* but the coolant is full, and hot air came out of the vents fairly soon.
Today on my way into work it threw the P0128 code. When I looked it up I wasn't surprised. Since it seems to heat up reasonably quick, I'm thinking it's a faulty coolant temperature sensor. I was looking around the thermostat housing, where I'd expect to find the coolant temp sensor, and I didn't spot it. Can someone tell me where they hid this thing? Also, is there a way to easily test this sensor? Am I going to lose coolant the same as if I pull out the thermostat? And of course, besides decrease fuel economy, what am I going to screw up by driving it tomorrow?

I don't know what year/make/model you're referring to, but if it's an N/A (non-supercharged) 97-03 GP GT then the coolant temp. sensor is near the thermostat housing, underneath it in the LIM. There is a wired connection to it. You will loose a bit of coolant but not much when changing it.

Not sure how to test it. I'm sure a quick search will bring that up.

Shouldn't mess anything else up other than the MPG if you drive it.

Yes sorry my mistake, it's an L36 in a 2001 Grand Prix GT.

mine had the same thing it was the t-stat but i got a newb question for you guys the tempeture gauge on the dash where does it get the temp reading from? the radiator? infront of the t-stat? after it leaves the engine? Id assume its after it runs through the system but idk