Code 31, Park/Neutral Switch Circuit - 3.3L & 3.8L

NOTE: Complete diagnosis of Code 31 for 3.8L (VIN L) C and E body vehicles requires the use of a GM Tech 1 "Scan" tester which is capable of indicating status of all 4 park/neutral switch positions. This chart does not apply to 3.8L (VIN L) C and E body vehicles.

Fig 1: Code 31 Park/Neutral Switch Circuit Diagram (3.3L A & N Bodies)

Courtesy of GENERAL MOTORS CORP.

Fig 2: Code 31 Park/Neutral Switch Circuit Diagram (3.8L H Bodies)

Courtesy of GENERAL MOTORS CORP.

Fig 3: Code 31 Park/Neutral Switch Circuit Diagram (3.8L W Bodies)

Courtesy of GENERAL MOTORS CORP.

The park/neutral switch contacts are part of the neutral start switch. The contacts close to ground in Park or Neutral and open in Drive. Code 31 will set if park/neutral signal circuit indicates an open for 3-4 consecutive starts or if conditions occur as follows:

No Code 38 (3.3L) or 29 (3.8L) exists
Circuit No. 434 indicates ground
Transmission is in high gear
TCC is locked on 4T60 trans. only
Vehicle speed is greater than 45 MPH and TPS is less than 15 percent (.94 volt) on 3T40 trans. only
All above conditions have been met for at least 12 seconds
NOTE: Test numbers refer to test numbers on diagnostic chart.
1. This tests for a closed switch to ground in Park.
2. This test for an open switch in Drive.
3. Be sure "Scan" tester indicates Drive, even when wiggling shifter.

Fig 4: Code 31 Troubleshooting Diagram (3.3L & 3.8L)

Courtesy of GENERAL MOTORS CORP.