Calling all troubleshooters ... what's going on here with the cooling system??

[R0B]

Calling all master troubleshooters ... I have a 2010 LT manual six-speed. I got a P1258 "engine coolant over-temperature protection mode" code. I look at the analog temperature gauge in the speedo cluster and it looks normal - about a third of the way between C and H (pretty much normal and where it usually is)? I scrolled the Vehicle Information Display to the "coolant temperature" screen and it's showing 221 degrees!! Noted that the radiator cooling fans were NOT on?? At that temperature shouldn't they have been on??

For the P1258 code I changed the coolant sensor, aka ECT, in the left head just behind the oil filter and nothing changed. Next recommendation was to change the thermostat. I removed the old thermostat, tested it in a pot of boiling water and it was bad refusing to open. Installed a new thermostat and it cleared the P1258 code but the coolant temperature difference between the analog and digital readings remained. Again, the digital reading climbed all the way up to 221 degrees and the radiator cooling fans remained off??

Now I'm confused as to why the analog gauge indicates "normal" while the digital one is showing 221 degrees?? I assume the digital one is right and the analog gauge is not reading correctly?? Why?? Additionally, if the coolant was actually 221 degrees why were the radiator cooling fans off and not on??

As far as I know there is only once source for temperature sensing and that is ECT, in the left head just behind the oil filter. The sensor is a bi-metal thermistor - as temperature increases resistance decreases. It has two wires - one is a ground and the other is a 5v signal from the ECM. The ECM measures the resistance, which varies with the temperature, and determines the temperature based on that.

I removed the connector from the ECT and ran a jumper from one pin to the other. This would induce 0 resistance which the ECM would interpret as a max hot condition. This should have cause the analog gauge to go full scale hot. and for the radiator cooling fans to kick on. The gauge did not go full scale hot it only went halfway between C and H?? However, the radiator cooling fans did kick on in the high speed mode.

I'm confused by all of this: Analog and digital gauges reading very differently?? Analog gauge only showing half scale in test mode when it should show full hot?? Cooling fans not coming on when the digital gauge shows 221 but come on in test mode?? I could really use some help from some of you master troubleshooters in understanding what's going on here and how to fix it. Thanks in advance for your help!!

[VChula]

I had a 2010 RS 2LT and currently have a 2014 ZL1.
Neither car ever, ever, ever went past the needle position in your picture of the analog gauge.

These normally run hotter than what you may be used to seeing in your prior cars. In the ZL1, I regularly see between 190-227 degrees on the digital display as I always encounter stop and go traffic going to and from work.
The fans seem to trigger at around 210-220 degrees.
(I also remember seeing the same temperature swings in the RS)

And like you, I have replaced the coolant temperature sensor but never saw a DTC. The digital display simply stopped indicating the coolant temperature.

[chevy pilot]

Familiar sight,analog reading is low compared to the digital.
There has never been P1258.

[ariZona28]

A while back I got stuck in traffic with my V6 car and noticed my oil temp was a bit warmer than usual (215 F or so), toggled over to coolant and found it up above 220. I think the fans don't kick on until 224 F or so. Needle was close to half, just slightly above were it normally is when my digital is reading 183 F. A/C was off. Does your temp stay near normal with the A/C on? Do the fans kick on with the A/C on? How warm was your oil temp?.