Notes On Intermittents

If this DTC is setting intermittently along with DTC B480 or B481 then the ABS system should be diagnosed. If no other DTCs are set, symptoms of no RAP or chime operation may direct area to diagnose.

NOTE: The following numbered steps refer to circled numbers on diagnostic chart. See Fig 1 . For circuit identification, see Fig 2 .

It would be normal for DTC B421 to set if there was an ABS fault (DTCs B480 or B481), therefore, diagnose the ABS codes if set.
This checks for proper chime control.
This checks for proper RAP control.

Fig 1: DTC B421: BCM Output Driver No. 2 Failure Flow Chart

Courtesy of GENERAL MOTORS CORP.

Fig 2: DTC B421: BCM Output Driver No. 2 Failure Flow Chart Circuit

Courtesy of GENERAL MOTORS CORP.