Trouble Code 55 indicates that the Analog- to-Digital (A/D) converter in the ECM timed out. The ECM checks this by initiating a conversion and timing how long it takes to complete. If the converter, for whatever reason, fails to signal the End-Of-Conversion (EOC) before a timeout counter runs down, the ECM assumes it is bad and sets this code.
Possible causes for Code 55 are limited to:
1. Defective or incorrectly inserted MEM-CAL
2. Defective ECM
If replacing the ECM, be sure to transfer both the PROM and the CALPACK from the old unit. If the old-ECM also indicated PROM failure (Code 51) or a bad CALPACK (Code 52), be sure to verify these components are fully operational in the new unit. If they are not, they too will have to be replaced.
like dan said look for shorted wires, chafed to ground or each other first
1998 Bonneville SSE
2007 Monte Carlo LT-KYB, Clarion, Polk audio, Kicker
1999 Yukon SLE-KYB, Air lift, Reese dual-cam, Tekonsha
1987 Firebird-355/th400-nothing stock