Weird trouble code on 2005 Grand Prix
 

Last year when it got cold the check engine light came on in my wife's 05 Grand Prix GT. I scanned it and it came up a P0128 which indicates that the coolant temperature is staying too low. After doing a little research I found out that GM put this code in place to indicate a faulty thermostat. I cleared the code and it returned several times. The car ran fine no matter if the light was on or not. After becoming annoyed by the stupid light, I finally gave in and replaced the thermostat. Problem solved. No more light......until now. As soon as it got cold this year, the light returned. Same code. I find it very hard to believe that a new thermostat went bad in less than a year. Has anyone else had this happen? My neighbor a few doors down has a car just like ours. He has the same problem every winter. His probelm (like ours) started after having the cooling system flushed one year. He has resigned to replacing the thermostat every winter and just living with it. I think that is just silly. Any thoughts?

I would say you have a bad stat. I ran into a run of bad Stant thermostats last year had like 4 or 5 new thermostats fail within the 1 year warranty they where either opening to early or blowing apart. We would take them out in pieces. I haven't had any problems with them in the last year or so now. If you have the 3.4 V6 thats alittle hard to get to or just want a good quality t stat get the stainless steel version its twice the money and lifetime warranteed. I have never had to warranty one of these.

i ran into this on my wifes 01 impala 3.4. had the same thing happen to me more than once. this was before i worked at a dealership, i went to my local dealer, got an oem thermostat and temp sensor and the code never returned. had the car for roughly 3 years after this before it was wrecked. quick question for you, the sensor is reading cold temp or flow to early, is it possible in your case that the sensor is feeding the computer the wrong temp? how does your temp guage look? i am not sure which fixed my issue, i just didn't wanna do a thermostat again on a 3.4. not a lot of fun.

Mine is the 3.8. The t-stat is easy to change so that's not really the issue. The gauge reads normal. I really haven't noticed the car taking any longer than normal to get warmed up, so that's one of the reasons I'm a little puzzled by this code. The gas milage has not changed at all either. I would assume that if a car was running too cool all the time, as the code idicates, the fuel milage would suffer. I don't remember what brand the t-stat was that I put in last year. I'm pretty sure it came from AutoZone, which is not my normal parts supplier, but I did the job on a Sunday and they were the only one open. If I have to do it again, I will get a new stat from the dealer or NAPA probably. I'm wondering if I might should replace the sensor too?? Seems odd that I didn't have any issues all year long though, until now.

Google search or ask the dealer, there may be a computer flash for a erroneous code. But you may have put in a cheap t/stat. If you do it again I would use a GM part. Tom

I finally got to drive the car this morning from a cold start. It was about 31 degrees this morning when I left. I drove the car for about 25 minutes and after a few minutes I had nice warm air coming out of the heater vents. Even after 25 minutes of driving, however, the temp gauge had never moved from it's resting position. I popped the hood when I stopped and felt the hoses and they were for sure warm to the touch and they had pressure on them so I think the thermostat is working. I think maybe the temp sensor is screwed up. Seems odd that it only acts up when it's cold out though. I'm thinking I'll just replace the sensor and the stat both just to be on the safe side. The stat is cheap and easy to do....I'll have to check on the sensor, but I can't imagine it being a high dollar item. Thanks for the posts guys! As always, this forum is a lifsaver.

I would concur with the temp sending unit is not within tolerance, not way off just off far enough that it's getting the SES light on intermitently. There are resistance specifications for sending units (usually in a shop manual) and before just replacing it you could take an ohm meter and test it to be sure your not just throwing parts at a problem. That is how I'd isolate the problem as far as a sending unit. Testing is cheap as opposed to chasing down a problem by throwing parts at it and possibly missing the root cause of the problem, your call. Don't use Teflon tape when and if you end up replacing a sending unit as sometimes it interferes with the unit grounding properly to the block, if it doesn't ground through wiring. I use red RTV as a thread sealant when I replace them.

In Texas?
After 10 minutes driving you wouldnt be able to touch the hoses.

get an IR temp gun and probe back from tstat housing to radiator.
If you dont get at least 200 right at the housing,somethings amiss
Ive had a good IR temp gun for several years and dont know how I did without.

Cammer, i agree.
Find myself check'n all kinds of things never thought about b4.
Different parts of engine, trans, rear of course.
Stuff in house.
Windows vs walls vs outer doors, furnass ducts, etc.
The cats while fighting and later after they relax. Be suprised bty.
Wood stove.
Ice cubes and how cold beer is b4 opening...:D

works to check for a clogged cat too(converter that is)
checked the dogs temp the other day when she was sick.

when I first bought it,main use was to tune the kids nitro cars.
Cylinder head temp is high when its too lean.
I check wheel bearings,brake discs(for excessive drag).
Dog loves to chase the laser too. She knows the sound of it from 2 rooms away.