93 cadillac deville strange code

I started my 93 Cadillac Deville and the heater came on full blast, haet and blower (no defroster front or rear) and the electronic gas guage and temperature contols both have a small letter c appearing in the wiondow. The check engine soon light comes on after a few seconds. I can not access the system for any codes, it appears non responsive. The car starts and seems to run fine and all other controls other than the heater controls and gas guage are functional. Any ideas?

Thanks,
Phil

  pfaust