I have a 1995 Caprice wagon. I plugged a jumper wire to terminals A and B in the ALDL and turned the ignition to the on position but the check engine light would not flash any codes. It just stayed on. I heard the cooling fan kick on and when I pulled the wire out, both the fan and the check engine light went off. Any mechanics out there know why? Would an expensive scan tool pull the codes? It should, at the very least flash a code 12 but I get nothing.