P06BC — Cylinder 4 Glow Plug Circuit Range/Performance

Diagnostic algorithm

1. Retrieve freeze frame, pending codes, and confirm P06BC is current. Clear codes and attempt to re-create following OEM test conditions (cold start/pre-heat).
2. Visually inspect wiring, connectors, and ground for cylinder 4 glow plug. Repair obvious damage before further testing.
3. Check battery state-of-charge and charging system; charge or replace battery if low. Re-test if battery condition was poor.
4. With ignition off, remove glow plug from cylinder 4 and measure DC resistance of the glow plug; compare to spec. If out of range, replace the glow plug.
5. With glow plug connected (or careful backprobe), turn key to glow/preheat and measure voltage at the glow plug terminal. Note voltage and compare to battery. Repeat measurement while cranking. Significant drop indicates wiring/connection issue.
6. Verify continuity between ECM/relay output and the glow plug connector and check for shorts to ground or to battery. Repair wiring faults as needed.
7. If available, measure current draw of the glow plug during preheat with an appropriate clamp meter. Compare with known-good cylinder or specification.
8. Swap the cylinder 4 glow plug with another cylinder’s plug and re-scan. If the code follows the plug, replace the plug; if the code stays on cylinder 4, focus on wiring/driver/ECM.
9. Test glow relay/fuse and inspect relay control signal from ECM. Replace relay/fuse as needed.
10. If wiring, relay, fuses, and glow plug are good, investigate glow driver/ECM output. Verify ECM grounds and power feeds; consult manufacturer repair information before ECM replacement.
11. Safety note: avoid shorting terminals, allow hot components to cool, disconnect battery when performing removal/repair of wiring or glow plugs where required by procedure.