Buick Reatta - Used $20.00 of gas in 20 miles. I'm thinking bad temp sensor. Suggestions.?

The engine temperature sensor has absolutely nothing to do with fuel. The engine temperature sensor registers the temperature of the coolant in the hottest part of the engine which are the cylinder heads. If the check engine light is on, more than likely the up-stream oxygen sensor- (before the catalytic converter) or down stream oxygen sensor-(after the catalytic converter MAY meed to be replaced. Don't be surprised if the catalytic converter needs to be replaced. A Buick or Chevrolet Dealer will be able to explain which part/s are causing the issue.

Emissions parts generally need to be replaced long before the age of this car.

Was black sooty smoke coming out of your exhaust pipe?

That's like 2.5 mpg!

Is your Check Engine light on??

Does your car have a strong gasoline odor around it?

Do you see a trail of liquid on the ground behind your car?

Is there a strong, sweet smell that burns your eyes while the car is running?

Fuel leak or gas thief. No modern car gets 2 MPG, not even a Ferrari or Lamborghini.

I'm thinking you should actually diagnose the problem instead of just throwing random parts at it. Live data from a scan tool would tell you what you need to know.

....so, at current pump prices you're getting about 2 mpg? I don't think so. Look for a leak.

Look for a leak. If the car is consuming that much gas, the engine would barely run.

Trade it in for a two litre diesel. Ample power but 60+ mpg.

Someone stealing gas from your car?

The temperature sensor has nothing to do with it!

You have a leak in the furl line or tank.