I have a 2012 malibu lt with 2.4 LE9 ecotec engine. The coolant temperature gauge on dash is showing high temperature way out of bounds. No codes and I can read the correct ect temperature via a reader.
The engine never overheated though.
It could be a radiator cap corroded and stuck shut, a bad thermostat or water pump, clogged radiator or a bad temperature sending unit.
But wouldn't all of that cause some over-heating issue? Does this then rule out everything but the bad temperature sending unit?
@barry_liu - If you already know all this then why don't you just replace the temperature sending unit?
You have dual fans which will always run together either in a series circuit at Low Speed, or in a parallel circuit at High Speed.
They are protected by fuses 17 & 18 and controlled by 3 relays in the underhood fuse box.
Make sure both fans are coming on as your engine temperature starts to rise above normal operating temperature
Great info @jack62
The engine never overheated. One time I pushed it with live data displaying and it never overheated.
The computer commands the fans to come on in Low Speed at around 220 degrees
.
The computer commands the fans to switch to High Speed at around 230 degrees
.
If one of those fuses are blown or if 1 of those 3 relays is faulty the fans won't come on at around 220 degrees. The fans won't come on until the computer commands High Speed at around 230 degrees and with one relay not working it's probable that only 1 of the fans will come on in High Speed
.
So you'll run hotter than normal
I don't know if it's the case on your car, but some (like my own) have two temperature sending units, one that controls engine operation and fans, and a second one that operates the dash gauge. With a setup like that it's possible for the engine temperature to be OK with fans operating normally while the dash gauge is indicating nonsense.
If you don't have a separate sensor for the gauge, then as @nta98 says it's likely the dash gauge itself is faulty. If it is operated by a computer module it's also possible that the module or wiring could be faulty. You'd need a professional bidirectional scan tool to test that.
I wouldnt put it past GM to have some electrical/sensor issues as time goes on. I know 2000-2006 GM SUV's & pickups had issues with the gauge cluster going bad, not sure about these in particular. Just kicking ideas around.
The 2 sensor theory could also be true. OP would have to determine which one is faulty if thats the case.
What car do you have like that?
Mine is a '97 Saab 9000. I went through this where the temperature gauge wasn't working properly but my Tech2 scan tool showed proper engine temperature, the car ran fine, good heat, and the cooling fan was working as expected. Turned out there is a separate sensor just for the dash gauge and I had to replace that. (Probably not a common setup. Saabs are weird. 
)
I can read the correct ect temperature via a reader.
Are you saying the cars computer is showing a normal temperature, while the dash is showing way overheating? I think a few people here may be misinterpreting your issue.
Id have to think that if the car isn't overheating, fans are functioning properly, and you can see a normal engine coolant temp with a scanner, then the issue is within the dash/gauge itself.
Does it just go way out of bounds as soon as you start the car when its cold?
My bet is still the temperature sending unit.