Possible Causes

Code 21 will set if there is a poor connection due to loose mounting, dirt, or corrosion at safing sensor mounting surface. Code 21 will also set if there is an open or damaged wire between terminal No. 16 of diagnostic monitor connector and safing sensor or an open or loose wire inside of safing sensor.

1. Verify Condition  - Turn ignition on. Check for fault codes. If Code 21 is flashing, go to step 2). If Code 21 is not flashing, check connectors, wiring, splices and crimps for an intermittent problem. DO NOT  proceed with diagnostic test unless code is flashing. Doing so will result in needless replacement of system components.
2. Check Safing Sensor Ground  - Deactivate air bag system. See COMPLETE SYSTEM DEACTIVATION  under DISABLING & ACTIVATING AIR BAG SYSTEM. Disconnect negative battery cable. Measure resistance between terminals No. 3 (Black/White wire) and No. 16 (Dark Blue/White wire) of diagnostic monitor connector. If resistance is more than 2 ohms, go to step 3). If resistance is less than 2 ohms, replace diagnostic monitor.
3. Check Safing Sensor Circuit  - Disconnect safing sensor. Check resistance between circuit No. 613 (Dark Blue/White wire) of safing sensor connector and safing sensor housing. See Figure . If resistance is more than 2 ohms, go to step 4). If resistance is less than 2 ohms, locate and repair high resistance or open in circuit No. 613 (Dark Blue/White wire). Reconnect safing and diagnostic monitor connectors.
4. Check Safing Sensor  - Remove safing sensor from vehicle. Clean safing sensor mounting surfaces. Install safing sensor. Check resistance between ground and circuit No. 613 (Dark Blue/White wire) of safing sensor connector. If resistance is less than 2 ohms, cleaning sensor mounting surfaces has corrected the problem. Reconnect safing and diagnostic monitor connectors. If resistance is more than 2 ohms, replace safing sensor.