When you click on links to various merchants on this site and make a purchase, this can result in this site earning a commission. Affiliate programs and affiliations include, but are not limited to, the eBay Partner Network.
The thermostat is not “staying to far open” as you say nor does it “flip to no voltage”…I really don’t think you understand how the ECT circuit operates so if you want to educate yourself there are many YouTube videos on how to check this thermistor circuit…watch the video from the 3 minute mark…you need to observe the ECT on your scan tool and not the gauge as the DTC is set by what the ECM sees and not the gauge….I don’t know if your code reader can show you “freeze frame data” but that will show you various engine parameters when the P0128 code set…it did mention a thermostat in Diagnostic Aids but you didn’t mention anything about the cabin temp being to low…if you want to be a parts changer than go and change the thermostat…I’d buy one from a Chevy dealer if at all possible but you can buy one wherever you want…I’d buy one and before changing the new one you can test the old one in some hot water and a meat thermometer or an IR gun to see if it opens and closes properly…if it’s working properly you can return the unopened box…I will delete all posts about this dielectric grease !!
P0128 can 100% mean your coolant is not reaching operating temp and can 100% be caused by a "stuck open" thermostat.
When it was fall two years ago and the temps dropped I could get this code when I drove the car at night. If your thermostat is stuck open and the sensor is good you should still see a reading on the cluster, but it will be very low, mine was under 140 when the code set (After a certain amount of time, the car has a timer it needs to get to a certain temp by). My thermostat was bad, and since replacing it I never see low temps or this code.
When that DTC is set even with a good sensor it pegs the fan to 100%.
Update
so it is throwing multiple codes this time, p0117 and p0118
while throwing these codes and with the instrument cluster showing zero I am able to read the temp via an OBD2 reader. I’m at a loss here, I was unable to reset the codes with the cheapo obd scanner so I just disconnected the battery (used degreaser to get all the dielectric grease completely off so you can all rest easy!) and reconnected and the car is running flawlessly again. Ignition on but engine off after a drive so engine is up to temp With jumper wire across the plug reads max temp With ECT unplugged reads min temp
So you had these 2 DTC’s AFTER disconnecting and then jumping the wire together and then scanning for codes ??…when you hook everything back up and run the car try to clear them…seeing 150 C and -40 means the ECT circuit is fine.
You WILL get the circuit “high” DTC when you disconnect the sensor and circuit “low” when you jump it…I think this diagnostic may be over your head…when you say the cluster reads “0” do you mean 100 degrees ??
You WILL get the circuit “high” DTC when you disconnect the sensor and circuit “low” when you jump it…I think this diagnostic may be over your head…when you say the cluster reads “0” do you mean 100 degrees ??
yes, although the digital gauge in the touring mode of the cluster shows 0
The idea is that if the car isn't reaching temp within the time period there must be something wrong, and they assume the sensor is the most likely culprit, so they peg the fans on full. This is also the "safe" state. An engine which is "too cold" is better than one which is "too hot"
very suboptimal for a modern car with the ability to have multiple settings and configurations, dump the temp gauge to zero and set fans on max. This feels like something that would have been acceptable from OBD1 in 1995!
very suboptimal for a modern car with the ability to have multiple settings and configurations, dump the temp gauge to zero and set fans on max. This feels like something that would have been acceptable from OBD1 in 1995!
How would you know what is bad? What sensor input would you use? How would you determine to "trust" a sensor?
Software can only do so much. The car doesn't "know" the thermostat is bad. It only knows the temp from the sensor and has to decide if that is good or bad. What if the sensor is reporting bad voltage and reports low?
On the surface this seems like a stupid reaction, but if you know software its really is the only way to do things.
