A3B4
ModerateA3B4 — FRM Module: Invalid Lighting Status Messages
Description
What the Code Means
Code A3B4 logs a communication fault in the lighting status messages sent by the FRM (Footwell Module). The FRM either fails to send or fails to receive valid lighting status messages — most often because of an internal fault in the module itself.
Normally the FRM controls the exterior lighting and continuously reports the current lamp status to the other modules. When that message exchange breaks down, the control modules stop getting reliable data. The FRM is the culprit in most cases, and exterior lighting control misbehaves.
Symptoms
Symptoms
- Headlights stuck on or off regardless of switch position;
- can't switch to high beam;
- loss of the tail lights or parking lights.
Causes and fixes
Diagnosis and Repair
- Run diagnostics with ISTA and read the fault codes from the FRM.
- Check the FRM for internal faults — the typical cause of code A3B4 sits inside the module itself.
- Rebuild (remanufacture) the FRM with upgraded components.
- After the repair, check that every lighting function works.
Possible secondary damage
Secondary Problems
- Driving on with a faulty FRM can lead to total lighting failure — a direct safety hazard that will also fail the annual state safety inspection.
- The battery can drain because of the faulty lighting operation.