My 91' Deville fuel mileage message center has been working poorly for 4 or 5 years now. It will start scrolling thru fuel economy, average fuel economy, distance till empty, etc. all on it's own. That never really bothered me.

It just started having issues with the fuel gauge. The display for fuel economy, distance till empty, etc is blank and a letter "d" is displayed where the number of gallons left would normally be. After awhile of driving the gauge starts working again. Any ideas?

Oh, also I forgot to mention that the climate control seems to work OK other than when I turn off the car and then restart it the next day, the climate control sometimes resets to "OFF" with the temperature set to 75 degrees.

I'm having the same sort of problem...'91 sedan deville 4.9 litre...the letter 'd' refers to default, meaning when there is a problem somewhere in the circuit the fan will run non-stop at 75 degrees so that the windows wont fog up in cold or warm weather...You should have a trouble code displayed in your diagnostic system which may help you find the problem...Refer to my recent post 'ECM Code'...

It seems like the place to start will be to just check connectors and grounds for corrosion. I haven't found any posts with my exact issues.