HT4100 4.1, 4.5, 4.9 Discussion, C bieng displayed in Cadillac Engine Technical Discussion; 1992 4.9 a "C" is being displayed on both my climate center and fuel center and my check engine light ...
A "C" display in both the CCP and FDC indicates a break in the circuit between the BCM/Programmer/CCP - FDC.
First locate the programmer. It's about the size of a paperback book, mounted on the vertical under the dash. You'll have to remove the insulation cover on the bottom of the dash to see it. It's easy to spot, because it will have a mass of wires and vacuum hoses running to it. Disconnect the electrical connector from the programmer and turn on the key. If the CCP/FDC displays return to normal, replace the programmer.
If both displays still show the "C," disconnect the connector on the back of the CCP. If the FDC display returns to normal, replace the CCP. If not, try connecting the CCP and disconnecting the FDC, then check the CCP display.
If none of this produces a change, there is a short or bad connection between the BCM and programmer, or the BCM is faulty.