Safe to driveDrivable, but watch it yourself
J1939 code SPN 704. Below — how the fault is worded at the factory by Mercedes-Benz, Meritor (ex-WABCO) and Renault Trucks, and what the second half of the code (FMI) means in the standard.
Mercedes-BenzMeritor (ex-WABCO)Renault Trucks
| FMI 4 | Digital Output 4/07 Circuit shorted to GND |
| FMI 5 | Digital Output 4/07 Open Circuit (broken wire, terminal floating) |
| FMI 3 | Failure detection 'ShortUb' at Auxiliary Output (Trailer Modulator OV) |
| FMI 4 | Failure detection 'ShortGnd' at Auxiliary Output (Trailer Modulator OV) |
| FMI 5 | Failure detection 'Interruption' at Auxiliary Output (Trailer Modulator OV) |
| FMI 13 | Failure detection 'OverEquipped' at Auxiliary Output (Trailer Modulator OV) |
| FMI 3 | Brake lights on M, Austeras on H and P, air conditioning cut-off on K, short to positive. Repair: Check the continuity and insulation of the wiring with respect to +24 V (SL2/13). |
| FMI 4 | Brake lights on M, Austeras on H and P, air conditioning cut-off on K, open circuit or short to ground. Repair: Check the continuity and insulation of the wiring with respect to ground (SL2/13). |
Safe to drive. Below — how much time you have and what happens if you keep going.
This is an electrical fault: an open, a short or a bad contact in the sensor circuit. The value it measures is most likely fine.
You can drive. But until the circuit is repaired this system runs blind and cannot warn you about a real failure.
Scanner threw several codes at once? Paste the whole list — we will show which one is the root cause and which ones simply followed.