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Heater/Overheating sensor

My 2001 Dodge Ram 1500 Magnum V6 keeps having a problem where periodically the heat gauge will inexplicably go to over heating, when the truck is not. It loves to do this when starting out on a long trip, though it only does it once. The heat gauge also likes to crawl up when driving extended periods on the road. It isn’t overheating. I don’t hear it gurgling or bubbling when I stop. In all this time, the air conditioner has worked and never warmed up. Lately, I’ve also noticed that the heater won’t work when it’s seeming to overheat. What do I tell the mechanic is wrong with it in order to get it fixed. I took it to a dealership, who only charged me $35 for other upkeep that I asked for and said they couldn’t find anything wrong.

Sounds like air in the cooling system. How’s the level in the reservoir?
If possible park the truck uphill, so the radiator cap is the highest point.
With the engine fully cooled off (several hours) remove the radiator cap and fill with coolant.
Turn the heater control to full hot and start the engine, rad cap still off.
Start the engine and idle. Bubbles might come out the radiator; and/or the level will drop.
Keep it topped off. Let it run until the top hose gets hot (thermostat opens.
Shut 'er off and replace the cap. Fill the reservoir and check it before the next 2-3 drives.

Thank you for your help. The radiator seemed like it was empty. There was a little left in the coolant jug for the reservoir after topping it off. I always thought it would overheat and stop with no coolant. Now that I think about it, they did say the coolant was low. Of course, this was a dealership and I was getting annual maintenance (the first time since 2011), and I thought they would have done it.

If it took that much coolant you really check for any leaks. With a properly working cooling system there should be about 0 loss of coolant.