Test Procedure

It is normal for a Code B421 to set due to an ABS fault (codes B480 or B481). If so, diagnose the ABS codes first. See 1989 ANTI-LOCK -- CADILLAC -- ALLANTE article in BRAKES.
Checks for proper chime control.
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.