Code Details
| Display code | WABCO MM-0112 SPN 791 FMI 4 |
|---|---|
| SPN | 791 |
| FMI | 4 |
| OEM code | WABCO / ZF SID 3 |
| Manufacturer | WABCO / ZF |
| System | ABS / ESC |
| Component | Left Rear WSS |
| Source address | Unknown or not applicable |
| Severity | high |
| Review status | ai source checked |
| Source confidence | high |
| Last reviewed | 2026-03-31 |
Plain-English Meaning
This WABCO ABS code flags the left-side rear axle wheel speed sensor circuit shorted to ground. FMI 4 means the signal voltage is at or near zero — the ABS ECU reads the sensor as if it is grounded rather than generating a speed signal. This prevents valid wheel speed measurement. A wire grounded against the chassis, a connector pin touching the housing, or a sensor with a failed internal element are common causes.
The WABCO MM-0112 table maps SPN 791 FMI 4 and SID 3 to Left Rear WSS / wheel-speed sensor circuit shorted to ground. This page paraphrases the factual mapping and does not reproduce WABCO troubleshooting procedures.
Common Symptoms
- ABS warning lamp active
- ABS ECU unable to read valid wheel speed from the left-side rear axle position
- WABCO TOOLBOX diagnostic will show the affected wheel and the shorted-low condition
- Related stability or traction-control indicators may also be on if equipped
Possible Causes
Possible causes may include the items below. The list is not a parts diagnosis.
- Left Rear WSS signal wire grounded against the chassis, frame rail, or brake component — common after corrosion breaks down wire insulation
- Sensor internal failure creating a short between the signal and ground circuits
- Water intrusion into the wheel-end connector, creating a conductive path to ground
- Pinched or crushed harness near the wheel end from recent brake work, tire change, or suspension repair
First Checks
- Record the SPN/FMI, SID, active status, and all active codes. ABS safety context: check for any red stop lamp or abnormal brake behavior before investigating.
- Disconnect the left-side rear axle WSS connector. With the key on, measure voltage between the signal wire and chassis ground — a reading near zero with the sensor disconnected suggests the wiring is grounded; a normal signal voltage reading with the sensor disconnected suggests the sensor itself is the fault.
- Visually trace the harness from the wheel-end to the ECU connector; pay close attention to areas where the harness contacts the suspension, frame, or brake components, especially if the vehicle has high mileage or has been through road-salt exposure.
- Check the connector shell and pins for moisture, corrosion, or spread terminals — a connector that is not fully latched can allow intermittent shorts.
- Confirm the diagnostic path with WABCO service documentation and a qualified brake technician before replacing any components.
Can I Keep Driving?
ABS and brake-system codes are safety-related. Stop safely when the red stop lamp is on, braking feels abnormal, or stability control warnings are active. Do not bypass or disable brake or stability systems.
Related Lookup Pages
Sources
- WABCO ABS and E Version Hydraulic ABS Maintenance Manual MM-0112 WABCO / ZF Commercial Vehicle Solutions · oem · accessed 2026-05-05 · confidence high
Source: WABCO / ZF Commercial Vehicle Solutions, WABCO ABS and E Version Hydraulic ABS Maintenance Manual MM-0112. This page paraphrases factual fields only and is not a substitute for the original document.
Open source - TOOLBOX PLUS Diagnostic Software ZF Commercial Vehicle Solutions · oem · accessed 2026-05-05 · confidence medium
Source: ZF Commercial Vehicle Solutions, TOOLBOX PLUS Diagnostic Software. This page paraphrases factual fields only and is not a substitute for the original document.
Open source
FAQ
How do I tell if WABCO MM-0112 SPN 791 FMI 4 is a sensor fault or a wiring fault?
Disconnect the wheel speed sensor. If the fault disappears or the voltage at the harness side returns to a normal reference level, the sensor is the likely source. If the voltage remains at zero with the sensor disconnected, the wiring has a grounded condition that needs to be traced. This simple test avoids replacing a good sensor.
Can moisture or road salt cause WABCO MM-0112 SPN 791 FMI 4 without visible wire damage?
Yes. Moisture wicking into the wheel-end connector or along the sensor wire can create a ground path without any visible insulation break. Look for discolored or wet connector terminals and signs of water tracking along the harness routing as well as obvious physical damage.
Can I keep driving with this code active?
ABS faults are safety-related. Check for red stop lamps, abnormal brake behavior, and stability control indications before continuing. Plan to have the fault diagnosed and repaired before operating in adverse conditions where ABS function could be needed.