Code Details
| Display code | WABCO MM-0112 SPN 791 FMI 1 |
|---|---|
| SPN | 791 |
| FMI | 1 |
| 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 signal as below the expected amplitude — FMI 1 means the signal is valid in form but too weak for reliable operation. The most common cause is an air gap between the sensor tip and the tone ring that has grown beyond specification: from a worn or loose sensor mounting, bearing end play that pushes the ring away from the sensor, or a sensor that was reinstalled after service without verifying the gap.
The WABCO MM-0112 table maps SPN 791 FMI 1 and SID 3 to Left Rear WSS / air gap or low wheel-speed sensor signal. This page paraphrases the factual mapping and does not reproduce WABCO troubleshooting procedures.
Common Symptoms
- ABS warning lamp active
- Low-amplitude or weak signal at the left-side rear axle wheel channel — WABCO diagnostic software may show sub-specification signal levels
- Code may be more noticeable at low wheel speeds when signal frequency and amplitude are both reduced
- May accompany bearing play, loose sensor mounting, or post-service installation concerns
Possible Causes
Possible causes may include the items below. The list is not a parts diagnosis.
- Air gap between sensor tip and tone ring beyond specification — from worn sensor mounting, incorrect installation, or bearing play pushing ring away
- Wheel bearing with excessive end play, which allows the tone ring to move away from the sensor under dynamic load
- Sensor installed to the wrong depth or without the sensor tip fully seated against the tone ring at initial installation
- Tone ring surface oxidation or heavy rust reducing signal amplitude without mechanically damaged teeth
First Checks
- Record the code, active status, and note whether this appeared after recent wheel-end service.
- Check the sensor-to-tone ring air gap using a feeler gauge per WABCO specification. An excessive gap is often the sole cause of FMI 1.
- Inspect the sensor mounting point for wear, thread damage, or missing retainer hardware that would allow the sensor to back out over time.
- Check wheel bearing end play; a bearing with excessive play causes the tone ring to move off-center relative to the sensor face, increasing effective gap.
- Measure sensor output voltage across its terminals to confirm the sensor element is producing a signal — a weak but present signal combined with a confirmed large gap supports a gap adjustment rather than sensor replacement.
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
Is a large sensor air gap dangerous on a WABCO ABS system?
Yes. A gap beyond specification reduces the signal the ECU receives from that wheel. If the signal drops below the ECU's minimum readable level, the wheel speed data for that channel becomes unreliable. ABS modulation depends on accurate individual wheel speeds, so a weak signal can degrade ABS braking performance at that wheel.
Can I adjust the sensor gap, or does it need to be replaced?
On most WABCO installations, the sensor is a fixed-gap design where correct gap is achieved by fully seating the sensor against the tone ring and relying on the ring's rotation to establish the operating gap automatically. If the sensor tip is worn, the mounting hardware is damaged, or the sensor has backed out, replacement and correct reinstallation is the appropriate repair rather than shimming.
Does WABCO MM-0112 SPN 791 FMI 1 point to a bearing replacement?
Bearing end play is one possible cause of excessive effective air gap, but it is not the only one. Check the sensor mounting and air gap first before evaluating the bearing — if the gap is correct and the sensor is properly seated, then evaluate bearing play with a dial indicator.