Cadillac: service engine light soon, exorbitant fees, climate control

Question
My 1990 Eldorado's service engine light soon came on today.  I noticed that the RPM's stopped showing up when I scrolled through the instrument panel.  It remained at "0", but while driving it started to show the rpm's intermittently.  The car's engine ran smoothly and showed no signs of any malfunction of the engine idle or engine running while driving at speeds up 45mph locally.  I hate taking it to the dealer and get charged exorbitant fees if the car does not need service as I am now retired and living off pensions.

Answer
Perform this test:

1. Ignition ON. Engine OFF.

2. Simultaneously push and hold the OFF and WARMER buttons on the climate control panel for 3 seconds and release them

3. Diagnostic codes will appear in the form of LETTERS and NUMBERS and will stop at .7.0

4. Let me know EXACTLY what displays on the screen.

5. Turn ignition OFF.