Dear Car Talk:

My wife, Mary, has a 2010 Chevy Cobalt. Recently, she noticed that the fuel gauge indicated more gas than there could have been in the tank. The following week, she took the car to the local dealer. After more than an hour, Mary was given an explanation that she didn't understand: She was instructed to keep the tank full, and told that the problem might resolve itself. Until this situation arose, the service people had always been very helpful. Mary returned home disheartened, and with an inaccurate fuel gauge. There was no charge for the "help." Any suggestions? -- James



Well, that was nice of them not to charge her. But the advice she got was worth exactly what she paid for it.

It sounds like she's got a bad sending unit. There's a sensor in the tank that floats up and down along with the level of gasoline. It sends a signal to the fuel gauge, telling the gauge what position it's in, and therefore how much fuel is in the tank. It sounds like Mary's sending unit no longer works.

Maybe the dealer thinks the float is stuck, and by filling the tank frequently, Mary might somehow help unstick it. We sometimes do see sending units that fail on and off for a bit when they're on their last legs. But eventually they always seem to conk out completely. So if she gets relief, my guess is it probably won't be long-lived.

The dealer may have just sensed that Mary was feeling financially vulnerable when she came in. He may have noticed the $400 worth of Bull Mastiff Chow she had just purchased in the back seat, and didn't think she could stomach the estimate for a new sending unit the same day. Unfortunately, making that repair involves a fair amount of labor, because it requires removing the gas tank.

But unless she's comfortable keeping track of her mileage, filling the tank based on how many miles she's driven, always driving with a fully charged cellphone and never lending the car to anybody, she probably should get it fixed.

I'd suggest that you go back, James, and ask for a more detailed diagnosis. If they're sure it's the sending unit, and Mary plans to keep the car for a while, it's probably worth spending a few hundred bucks to fix it.