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Code 51, No A/C Signal

The A/C amplifier supplies a signal to the ECM when A/C is selected. This signal allows the ECM to adjust engine speed to compensate for additional load on engine. Code 51 will set if this signal is not present.

NOTE: Test numbers refer to test numbers on diagnostic chart.
  1. This determines that a signal is being sent from the A/C switch.
  2. This determines that A/C amplifier is sending a signal to the ECM.
Fig 1: Code 51, Circuit Wiring Diagram & Flow Chart
G121650Courtesy of GENERAL MOTORS CORP.
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