Hello Scotty, I have a 2006 Chevrolet Silverado 5.3L automatic trans. Bought the truck used in 2012, Have had a PO128 code since I have had it off and on, changed 3 thermostats, finally put the OEM GM thermostat in, it finally stated reading right on the temp gauge, but only in summer time, as soon as it get around 60 degrees, it doesn't warm up fast enough and of course here comes the PO128 code. Like I said changed out thermostats, antifreeze(Dextron), checked to make sure cooling fans weren't running(they weren't), I could find the problem easier if it was overheating, not underheating. Truck runs like a top, has almost 200,000 miles on it. Change oil in it every 4000 miles. The only thing I can think of is reflashing the PCM, dealer wants 120 bucks to do that. I have looked everywhere for advice on a underheating motor, its like hens teeth. Maybe you can help?
Either the sensor is reporting the wrong information or the computer itself needs reprogramming it could be either
Thanks, I have a new OEM temp sensor in it too, only does this in cold weather, not warm, never get the PO128 code in warm to hot weather. Thought about cutting thin sheet metal to cover up half of the radiator in winter, but might try the reprogramming first. Have a son who lives in Austin, watches your You Tube channel all the time too. Thanks