I recently displayed the fault codes on my 1988 Eldorado using the DIC (per instructions elsewhere on this site) and was puzzled by the last code. It reads: ECM ?

I simply shut off the key and did not clear any codes until the work is actually performed on the repairs.

Any ideas?

"ECM?" is not a code. When it says "ECM?" it's asking you if you want to access the menu of stuff you can do with the ECM (Engine Control Module). You can have it clear the ECM codes, you can view ECM data and even override systems. I'm not sure of the extent of the privileges it gives you on the 88 model though.

Once ECM? is reached, you can hit the hi fan button to respond as "yes" or the low fan button to say "no" prior poster is correct. You can monitor ECM parameters but you need the factory manual to understand what you're seeing. This car has the capability to scan the ECM, BCM functions to address any fault codes you have. The service manual is the best money spent regarding diagnosing and/or repairing this car.