Coolant temperature fluctuation ?

The reason for the question is I am curious if the readings I see are a precursor to thermostat failure. Or, do all of the thermostat failures we see have a root cause other than the actual thermostat?

My previous 2017 Platinum had a gauge display which I really liked. One of the gauges was coolant temperature which would climb normally during warmup until it reached 210* and stay there. Looking back, was that accurate? I am beginning to doubt it was. I convinced my dealer to change the thermostat in my 2020 because I saw it dropping to the vicinity of 180* and told him at the time the 2017 always ran at 210 and he said that is too hot. I never looked for the digital display on that 2017 because I was content 210 was proper.

My Corvette similarly climbs to 220* and stays there. When I looked a a different digital display, the temperature is in the mid 190s, sometimes to 200.

We cannot be content with the gauges that are normally displayed because the engineers seem to be giving us a display to be content.

The question is, what is the normal actual operating temperature range?

I do not have any codes set during these fluctuations, so the control modules must be content with the coolant range of temperatures during operation.

Took my XT5 to the dealer on Feb 7 complaining that the Coolant Temperature warms to 190* and then drops to 176*. Dealer would not do anything because there are no codes. Service department did not know the conditions to set PO128 for thermostat. Pushed Service Writer for better definition and he gave me the number for GM Customer Service.

Contacted the number provided and they transferred me to Cadillac Customer Care who opened a case on Feb 8. It took until Feb 23 for a Senior Advisor called back. He agreed to contact another dealer to find the answer to the question about PO128.

On Feb 28, the Senior Adviser reported the normal operating range is 195* - 205*. With mine running at 176* he suggested and setup an appointment with a different dealer. I asked which dealer he asked about the operating range and he said it was my original dealer.

That same day, I called the Service Writer and advised him that I have an appointment at a different dealer and asked him why someone at his dealership told the Senior Advisor the normal range is around 195* and they would not do anything about it. He gave the expected excuse that Cadillac will not pay them if there are no codes and perhaps a second opinion is worthwhile. Told him if the other dealer takes care of me, that will be my dealer and I will not be back for sales or service.

In less than 1/2 hour he called back and said he talked with a more senior technician who told him they should change the thermostat. He said they would like to make another attempt to repair and would order a new thermostat that same day so it is available when I return. I agreed to give them one more chance.

Called March 10 because I had not heard anything and found the Service Writer was on vacation, but the part was there. Made an appointment for March 15.

March 15, talked with the Service Writer first and asked why I was not called when the part arrived, he said he was on vacation. Told him I should have been called if he was there or not and asked when the part arrived. He never did tell me how long the part was there.

The thermostat was changed that day in 1 1/2 hours. The Coolant Temperature now rises to 194* in half the time and half the distance than before. It will drop to 192* for a couple seconds, then rise to 194* quickly. The temperature will rise to slightly above 200* if stopped, but that is OK. The thermostat is now operating as I expect it should.

It is unfortunate dealerships hire Service Writers who do not hesitate to lie, even when Cadillac is paying for the work.