Diagnostic Aids

An intermittent problem can be caused by misrouted harness, rubbed-through wire insulation or broken wire inside insulation. If other DTCs are set that share the same ground and/or 5-volt reference circuit, check for faulty connections or wiring. PCM 5-volt reference circuits are internally connected within PCM. If it is determined the DTC is intermittent, performing DTC P1121 diagnostics may isolate the cause of fault.

Fig 1: DTC P0121 Schematic - Fleetwood

Courtesy of GENERAL MOTORS CORP.

Fig 2: DTC P0121 Schematic - Except Fleetwood

Courtesy of GENERAL MOTORS CORP.