Please help with overcooling issue

Welcome to my world Chevy468. I have 2 of these cars, a 2002 GTP and my 04 Comp G. Prior to me tearing my 04 apart it was throwing the same p0128 code you are seeing. Now it was doing exactly as yours....... sortof. Mine refused to exceed 200 degrees without sitting and holding it at about 3000 rpm in park for a couple minutes. Otherwise my fans never cycled, nor did the engine ever actually even get warm. It blew good heat but about 1/4 mark on the gauge, that's it. Upon tearing it down it needed LIM gaskets of course but had a 195 tstat installed. I am not reassembling that car stock so I really will never know with it, and it's getting a 180 tstat.

The 2002 however is stock, does not need LIM gaskets ( replaced and leakage free for about 1.5 years now) and does the same damn thing as the 2004. It never has tossed a code, but verified by my scan tool and so forth it has trouble exceeding around 80 degrees C under normal driving ( or 176 F). Odd that's exactly the same as the 2004.............. And the 2002 has done this from day one of ownership as well.

So do I have 2 seperate GTP's of different ages with bad 195 degree t-stats? Maybe? However if you verified your t-stats operation and still no sucess...... Well I don't know. The 2002 went through last winter just fine and never gave any issue, but the engine never even remotely hits the 195 degree setpoint, just like my other 2004.

It is cool here too, approaching around 32F or less at night and it compounds the issue. I know I am of no help, but I have double your issue!

Just replaced the gaskets, no change. I appreciate all the help guys, thanks

Badass, that's odd. I have never heard of these issues before, and the fact I can't figure it out is killing me. Especially since it should be so simple. At least I'm not alone here.

Well I ended up replacing the coolant temp sensor with a Delco unit, fixed the issue. I find it odd that 3 different shops swore the sensor was reading correctly, but at any rate that was the problem. Thanks for all the help everyone.

Glad you got it fixed (lets hope its permanent this time!) but, if you don't mind me asking, since you were swapping parts, wouldn't it have been cheaper and easier to replace the sensor first, and then all the other gaskets?

Just wondering - no hate. I guess the lesson to learn is to not trust most shops!

EDIT: Did you notice any difference in performance due to the low coolant temperature? Or was the temperature reaching normal and the sensor just not reading it correctly?

Thanks.

The sensor was actually the first thing I replaced. It seems I got a bad one right off the shelf. Kinda ticks me off, that damn thing caused me much aggravation. And about $50 in thermostats. lol The intake gaskets needed to be done anyway, I was just hoping they'd find something while they were in there.
Well, the car was actually reaching normal temp, the sensor was just inaccurate. That said, I do notice a little more power now that the sensor is reading correctly, and my mileage has gone up almost 2 mpg.