Front Control Module (FCM)

B2123-IGNITION RUN CONTROL 1 CIRCUIT HIGH





For a complete wiring diagram, refer to the Wiring Information.

Theory of Operation

The family of B2100 Diagnostic Trouble Codes (DTCs) typically describes ignition performance related faults. These are often caused by a contradiction of switch status information. For example, if an Electronic Control Unit (ECU) receives a bussed message indicating ignition switch position but the state of a hardwired ignition circuit indicates another switch position a B2100 series DTC would likely result.

These DTCs can be caused by a number of conditions including:

- loose connections
- missing or blown fuses
- faulty component
- vehicle no start or long crank time

Verify the proper operation of the starting system and verify that this DTC is active before proceeding with the diagnostic test.

- When Monitored:
With the ignition off.

- Set Condition:
The Front Control Module (FCM) will set this if the Left Trailer Tow Brake Lamp Relay Control circuit is shorted to voltage for greater than 0.1 of a second.





1. CHECK FOR ACTIVE DTC
1. With the scan tool, read the active DTCs.
2. Cycle the ignition switch from off to on, leaving the ignition on for a minimum of 90 seconds.
3. With the scan tool, read the active DTCs.

Does the scan tool display this DTC as active?

Yes

- Go To 2

No

- If the DTC is stored, check for an intermittent condition. Visually inspect the related wiring harness connectors. Look for broken, bent, pushed out, or corroded terminals.
- Perform the BODY VERIFICATION TEST. Body Verification Test.

2. LEFT TRAILER TOW RELAY
1. Turn the ignition off.
2. Install a substitute relay in place of the original Left Trailer Tow Relay.
3. Cycle the ignition switch from off to on, leaving the ignition on for a minimum of 90 seconds.
4. With the scan tool, read the active FCM DTCs.

Does the scan tool display this DTC as active?

Yes

- Replace the FCM.
- Perform the BODY VERIFICATION TEST. Body Verification Test.

No

- Replace the original Left Trailer Tow Relay.
- Perform the BODY VERIFICATION TEST. Body Verification Test.