Trouble CodesGM OBD Trouble CodesGM Code 55

CODE 55 - ECM Fault

Trouble Code 55 indicates that the Analog-to-Digital (A/D) converter in the ECM is faulty. This converter reads sensors (e.g., TPS, Oxygen) and coverts their analog voltage output to a digital number for use by the ECM. If the converter fails, the ECM sets this code.

The conditions for setting this code are:

  • the A/D Converter has failed to properly read a sensor after 24 attempts.

Typical causes for this code include:

1) Defective or incorrectly inserted PROM
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.

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Chevy Trouble Codes