I'm new to the forum. I have a question I'm hoping someone here can help with. I have a 2005 Armada. The other day I started the car and drove about a mile when I heard an alarm and noticed that my coolant temp gauge showed very hot. I took it to the mechanic today. He said it did the same for them but he said in reality, the engine was not hot. So it's either a bad temp sensor or faulty gauge. Any opinions?

well, have you had your thermostat on the engine changed?
was the coolant low?
do you have any coolant leaks?
did your "mechanic" check for any code at the obd port?

how long have you had this car? is a "new to you car"/purchased used or have you had it for a while?

if you started the car from COLD and drove "about a mile" the engine would most likely NOT be at full operating temp, but this can change depending on how long you idled before driving off, was the car actually not at COLD (i.e. it was sitting for a few hours).

was this the first time this happened?
can you reproduce this each and every time?

if its electrical in nature, could be a faulty block temp sensor/electrical. if that is the case there could be a code at the obd port.

your "mechanic" just stating "the engine was not hot" is not a good diagnosis.

The car is new to me. It was sitting for 1.5 hours before I started it and no idling whatsoever. Literally within 3 minutes of the engine start it was on H. This is the first time it has happened. The next day I started and drove it and it did exactly the same thing. Then I tried it about 10 minutes later and the gauge never moved above half even though I drove it for 10 minutes. So no, not every time.

to me, it sounds like a faulty engine temp sensor. i think i had this happen on one of my old cars back in the 90s. the temp needle would fluctuate widely and it was determined one of the sensors was faulty. i bought the sensor and replaced it myself.

maybe try to see if there are any engine codes at the obd port. if you dont have a scanner tool, go to autozone or similar and have them test. should be a free thing and takes a minute or two.

Thanks for your responses.