Test Description

The numbers below refer to the step numbers on the diagnostic table.

1. 2. The 5-volt reference circuits for the sensors are connected together inside the PCM. Both DTCs P1635 and P1639 set at the same time, indicates a 5-volt reference circuit is shorted to a voltage.
2. 3. The 5-volt reference circuits for the sensors are connected together inside the PCM. When only P1639 is set, this indicates a short to ground on one of the 5-volt reference circuits.
3. 5. This step determines if a 5-volt reference circuit is shorted to ground.
4. 6. The 5-volt reference circuits may be shorted to another PCM circuit. The shorted circuit may not be apparent when the PCM harness connector is disconnected. Testing continuity from each 5-volt reference circuit isolates the shorted circuit.

Fig 1: DTC P1639 (1 Of 2)

Courtesy of GENERAL MOTORS CORP.

Fig 2: DTC P1639 (2 Of 2)

Courtesy of GENERAL MOTORS CORP.