Hello,
My wife has a 1999 Sable GS with automatic transmission. It is the 4-door
The heater will not blow hot air.
It seems that the gage does not work either, no matter how long the car runs it stays on C.

I am guessing that there is a thermostat issue, but I do not know how to change it.
Are there any guides online that show how to work on this?

Thanks,
Rod

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Good first idea. Cheap and most likely guilty. Feel all the coolant hoses for similar temp. All Sables are 4-door, just to let you know. Keep me updated.

Thanks! I will check that this evening and let you know.
What is odd about the situation is the temperature gage inside the car.
Sometimes it works, sometimes it does not. We drove to her parents this weekend, about a 3 hour drive. On the way there the gage worked and the car ran in the normal range. On the way back, it did not work and the needle did not move off of the C or below.

Just wanted to mention that...

Thanks,
Rod

More than likely a faulty coolant temp sensor. Easy to replace. If you still have no heat try a coolant system flush. BE sure all hoses are the same temp when the car is at operating temp, especially the two heater hoses that enter the pass. side firewall.