DTC B2721: Pass-Key(R) II Detection Circuit (Out Of Range): Description

PASS-Key(R) II detection circuit is completed when PASS-Key(R) II (with embedded resistor pellet) is inserted in ignition lock cylinder. When contacts in lock cylinder mate with contacts of resistor pellet, key detection circuit through circuits No. 1073 (White/Black wire) and No. 1074 (Purple/White wire) is completed.

When ignition switch is turned from LOCK/OFF position to RUN position, BCM applies a 5-volt reference voltage to circuit No. 1073 (White/Black wire). BCM then reads resistor pellet code, and compares stored resistor pellet code with ignition key resistor pellet code to verify a correct match. If BCM determines that resistance value of key pellet is okay, BCM will use the class 2 serial data line to send an enable password to Powertrain Control Module (PCM). PCM will then energize theft deterrent relay and allow fuel delivery to engine.

DTC B2721 will set when resistance level of PASS-Key(R) II input is one of 14 valid but incorrect resistance ranges (which signals an invalid detection) during crank cycle or for one second anytime after engine has been started.

When PASS-Key(R) II resistance is out of range during crank cycle, BCM will perform the following:

Send a message to instrument panel to flash SECURITY warning indicator.
Send a message to PCM to disable theft deterrent relay and to stop fuel delivery to engine.
Disable sampling of PASS-Key(R) II resistance for a time-out period of 3 minutes.
If ignition switch is left in RUN position for at least 3 minutes, BCM will enter theft deterrent re-learn mode. See THEFT DETERRENT RELEARN under BCM PROGRAMMING .

When PASS-Key(R) II resistance is okay during crank cycle but goes out of range later in same ignition cycle, BCM will perform the following:

BCM will enter fail-enable mode.
Send a message to instrument panel to turn on SECURITY warning indicator.
During subsequent engine starts, BCM will send a message to PCM to enable theft deterrent relay and to allow fuel delivery to engine.

If conditions for malfunction no longer exist, history DTC will clear after 100 consecutive ignition cycles. DTC B2721 will also clear when conditions no longer exist or scan tool is used to clear codes.

NOTE: If DTC B2721 is a history code, problem may be intermittent. Wiggle wires while performing the following test.

NOTE: When ignition switch is turned to RUN position with BCM disconnected, PCM-related DTC P1626 will be set. After BCM repair is completed, see appropriate TESTS W/CODES article in ENGINE PERFORMANCE section:

TESTS W/CODES - Century
TESTS W/CODES - Regal