06burntorange

by the way, the in dash temp gauge was reading normal temps.
So I dont where this 128 is coming from?

donbrew

If the coolant is low, the temp may read "normal". Because there is air where there should be coolant.

Just one thought.

P0128 means that the engine is not warming up fast enough, not that the temp is abnormal. It's more of a time thing.

Try searching this forum for P0128, there are at least 50 threads about it.

06burntorange

Someone please correct me if I am wrong, but if the gauge on the dash indicates normal operating temperature, and the digital readout indicates 185 to 193 deg. doesn't this mean that the thermostat is working fine?

So how could I be getting the po128 if the thermostat is good?

I just replaced the thermostat not even two months ago, therefore I believe it is good.

What else could cause this?

whopper

ARGHHHHHH!!!! the P0128 does not indicate that the thermostat is stuck open - it indicates that THE ENGINE WAS SLOW TO WARM UP!!!! Please note the difference and I'm just trying to emphasize the point, not to be mean or such.

With that said about the P0128 indicating that the engine was slow to warm up - what could cause the engine to be slow to warm up? Typically it's the thermostat is jammed open.

Why would the temperature display normal operating conditions, yet the thermostat is jammed open? Easy - note that the error is trigged when it is slow to WARM up - the CEL will light, and the gauges go inoperative. However, with the gauges inoperative, it does warm up more or less (just too slowly, and maybe a bit under the normal temperature). Once it is warmed up, and the engine is restarted, the engine warms up quick right? As it is already warmed up more or less. And when that happens, the gauges will show normal operating temps.

But why would the CEL stay on even though the gauges are working and the digital temp is in normal range? - because the CEL will only be cleared once one completes a specific number of drive cycles (stop/restarts of the engine). You can clear the CEL also by disconnecting the negative post of the battery, and reconnecting it.

Why would a recently new thermostat cause the problem? Either the new thermostat has gone defective, or when the old one was replaced it left some debris in the cooling system from the old deteriorated rubber gasket, and it is now jamming the new thermostat open. A couple people here have encountered this, and they needed to flush the system and refill.

Does that help explain?

donbrew

I wonder if saying it twice is enough? #3 it is HOW LONG IT TAKES TO WARM UP.

Oldblue

If you didn't use a GM t-stat, this could be your issue, they are revised and do not have the rubber ring like the original, they heat up at the specific rate the computer is looking for, I have three friends up here all went cheap for their t-stats, ( Motorad, Stant ) come to mind, and they all had this same issue, we changed to GM t-stat problem solved!!

06burntorange

Thank you once whopper, I am getting used to your "tone" on here, ;-)
So no worries I dont take offense. Thank you for your expertise!!!

Your explanation helps me to understand, and because there is more.

This morning the CEL light was gone, but the temp gauges were both blank.
The gauges never came back on. I parked after about 6 mile drive, after a couple of hours, went back and started, now CEL light is back.

So your explanation solves some of these mysteries.

As to the brand of thermostat, well I bought at parts store and was not GM. So okay I have to consider the thermostat as possibly bad now.

Considering my temperature related starting issues, does anyone think there could be a relationship between these symptoms?
I am wondering about the actual temperature sensor. Could it be going bad and giving the engine bad temp readings?

According to the input here, I am thinking about replacing thermostat (GM) and fluid, and temp sensor for measure. Off to find the dealer...

whopper

Cool!!! and I'd give the system a good flush (maybe without the thermostat in place) before putting in the new one - just to make sure there is no debris left in there from the original deteriorated gasket.

Let us know how it goes man. :)

donbrew

All of the "flushing" directions I've seen prominently note REMOVE THERMOSTAT and use clear water + the cleaning agent. Maybe you have seen some other directions.

My 2011 LT reaches operating temp in about 2 minutes when the ambient temp is 10F. Did you get a 190F t-stat (180F/82C is correct IIRC)? Dex-Cool is good for 150,000 miles/3 or 4 years, green/yellow stuff not so long.

Oldblue

it is most likely not the sensor, its the t-stat, either to low temp or fast opening, causing the temp sensor to tell the ECM, throw the code! there is no paper gasket material to clog the system, just the rubber ring the t-stat sits in, and a new one comes with the GM t-stat.