Code 31, Park/Neutral Switch CKT

NOTE: Complete diagnosis of Code 31 for 3.8L "E" body vehicles requires 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) "E" body vehicles. Schematics for these models are supplied for reference purposes only.

Park/neutral switch contacts are part of neutral start switch. 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:

Code 29 (3.8L) does not exist.
Circuit No. 434 indicates ground.
Transmission is in high gear.
TCC is locked (4T60 transaxle).
TPS is less than 15 percent (.94 volt) and vehicle speed is greater than 45 MPH (3T40 transaxle).
All above conditions have been met for at least 12 seconds.
NOTE: Test numbers refer to numbers on diagnostic chart.
1. This tests for a closed switch to ground in Park.
2. This tests for an open switch in Drive.
3. Be sure "Scan" tester indicates Drive, even when wiggling shifter.

Fig 1: Code 31, Schematic, Park/Neutral Switch Ckt

Fig 2: Code 31, Flow Chart, Park/Neutral Switch Ckt

Fig 3: Code 31