Code 42, 5TH Gear Switch Circuit (M/T Only) Grounded Circuit

The ECM supplies battery voltage to 5th gear switch. When transmission is in 5th gear (switch closed), the signal line voltage is pulled low. Code 42 will set if signal line voltage is low all the time.

1. This checks for battery voltage supplied to ECM.
2. This checks for switch being the cause of the fault.
3. This checks if ECM or circuit No. 422 is open.

Fig 1: Code 42 (M/T), Circuit Wiring Diagram & Flow Chart

Courtesy of GENERAL MOTORS CORP.