Hey yall, Im postin this in hopes someone has had this same problem. My coolant temp sensor recently went out. it threw PO128, so i replaced the sensor with a delco brand sensor. I then cleared the code and my gauge worked again and the fans stopped running 24/7.
It worked for a day, then the pcm stopped getting a signal from the sensor and after the second day it threw PO128 again. so i replaced it AGAIN and now its not reading the sensor again. I am getting voltage down the line to the sensor, so im stuck. could it be shorting to ground somewhere still? Thanks for any help provided.

2001 Camaro 3.8L Series 2

 

Make sure there is no corrosion on the contacts for the sensor plug and also after you have replaced the sensor have you purged the cooling system? You will get air trapped in the thermostat housing that can cause a vapor lock in the cooling system and make the sensor thing that there is no temp, or you can overheat your engine since the coolant cannot flow in a vapor locked engine. Check that stuff out and let us know.

Massey

 

So i was sitting in my car eatin lunch today and was thinkin about that sensor problem and decided to loosin that bleeder screw on the thermostat housing (after gettin it warm). It spewed air out for about 30 seconds then i ran the car and did it again and got the last of it out.
I cleared the tc and the needle on temp gauge shot up to operating temp Thanks Massey. It was in fact a vapor lock.

 

Glad I could help and good you got your car fixed. Make sure you have plenty of water in it now... The missing air was replaced with water so your rad will be a little low now. If the level of the rad drops below the upper hose mount you will prolly have to purge it again.

Massey