Dear Car Talk:

I have a 2005 Buick LeSabre. The only thing I've had to repair so far are the rear shocks. But one morning I started the car, and the needle on the temperature gauge was a bit past the center, which isn't normal. I took the car to a mechanic, and he said the problem might be something in the radiator, so he backflushed the system. This made the needle on the temperature gauge register a bit below the center of the gauge. After six days, the needle went past the center of the gauge again. I took it to another shop, and they checked the temperature, which was between 180 and 190. They replaced the thermostat. All was well for 10 days, but then the temperature gauge went into the red, "overheating" zone, which it had never done before.

I was told that the 2005 Buick LeSabres were having trouble with the temperature gauge: The gauge shows that the car is overheating, but it's actually not. Can you figure out the problem? My wife is afraid to ride in the car. -- Thomas

The first question to answer is, Is the car actually overheating, or does the gauge just say it's overheating?

When you went to one of the mechanics, he measured the temperature, presumably with his own gauge, and got a reading of 180-190 F. That's perfect.

Since the gauge went into the red zone after that, I would do that test again. We have a pyrometer at the shop, which is an infrared temperature gauge that we can point at any of the coolant hoses. It'll tell us the temperature of the coolant, independent of what your gauge is saying.

If the engine actually is overheating, then you've got a serious problem. It could be anything from a bad water pump or a leaky head gasket to a cracked cylinder head. Let's hope the coolant reads 190 again, which means the engine is just fine.

If the engine is running at its proper temperature, then the problem is in the system that monitors and reports the temperature. It could be a bad gauge, as you suggest. But before you rip out the dashboard and pay for a new gauge or instrument cluster, Istart by replacing the temperature sending unit (TSU).

The TSU reads the coolant temperature in the engine and sends that info to the gauge. The TSU is a $40-$50 part, and no big deal to install. If that fixes it, you're all done and your wife will ride with you to the rock-climbing gym again. If a new temperature sending unit doesn't fix it, then go ahead and replace the gauge.

But if you've been kind to strangers and flossed regularly, all you'll need is a temperature-sending unit, Thomas. Good luck.