Code Details
| Display code | WABCO RSSplus SID 3 FMI 4 |
|---|---|
| SPN | Not applicable or not verified |
| FMI | 4 |
| OEM code | WABCO / ZF SID 3 FMI 4 |
| Manufacturer | WABCO / ZF |
| System | Trailer ABS / RSSplus |
| Component | Wheel Sensor c |
| Source address | Unknown or not applicable |
| Severity | high |
| Review status | ai source checked |
| Source confidence | high |
| Last reviewed | 2026-04-09 |
Plain-English Meaning
The signal or supply wire for sensor C is being pulled to ground potential rather than its expected level. This is typically caused by a wire that has chafed through its insulation and is contacting bare metal on the trailer frame or axle, or by a damaged sensor with an internal ground short.
WABCO MM-0888 FMI 4 maps to a circuit voltage below the lower detection threshold for Wheel Sensor c. Unlike FMI 1 (signal too low from a weak sensor), FMI 4 is a hard electrical fault — the circuit is actively shorted to chassis ground rather than just producing a weak signal.
Common Symptoms
- Trailer ABS lamp on, typically persistent
- Sensor C reads near-zero or zero voltage in WABCO PC diagnostics
- ABS inhibited on the affected wheel position — a hard ground short is a definitive fault condition
Possible Causes
Possible causes may include the items below. The list is not a parts diagnosis.
- Sensor signal wire chafed against trailer frame, axle housing, or suspension component — most common cause
- Corroded connector where corrosion bridges the signal pin to a grounded shell or adjacent ground pin
- Sensor internally shorted — ground fault inside the sensor body
First Checks
- Disconnect the sensor C harness at the ECU connector and measure resistance to chassis ground on the signal wire. Near-zero ohms confirms a hard short.
- Walk the sensor harness from the ECU to the wheel end, checking for chafing at frame crossmembers, suspension pivot points, and anywhere the harness is not protected by conduit.
- Inspect the wheel-end connector for corrosion bridges or moisture that could create a ground path.
- If the harness tests clean, disconnect the sensor itself and measure resistance from the sensor terminals to the sensor body/ground — a fault internal to the sensor confirms sensor replacement.
Can I Keep Driving?
Trailer ABS and brake-related codes should be handled conservatively. Stop safely for brake warnings, red stop lamps, abnormal braking, wheel-end concerns, or any severe warning condition, and follow fleet or OEM guidance.
Related Lookup Pages
Sources
- WABCO RSSplus Trailer ABS Maintenance Manual MM-0888 WABCO / ZF Commercial Vehicle Solutions · oem · accessed 2026-05-05 · confidence high
Source: WABCO / ZF Commercial Vehicle Solutions, WABCO RSSplus Trailer ABS Maintenance Manual MM-0888. 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 the short to ground is in the wire or in the sensor for WABCO RSSplus SID 3 FMI 4?
Disconnect the sensor at the wheel-end connector and measure resistance from the signal wire to ground at the harness (ECU side) — if still near zero, the fault is in the harness. If the fault clears with the sensor disconnected, the short is inside the sensor. This two-point test narrows the fault location before disassembly.
Does a short to ground on one sensor affect other sensors in the RSSplus system?
Short-to-ground faults are typically isolated to the affected sensor circuit. The ECU monitors each sensor channel independently. However, if there is a common wiring run where the ground fault could affect a harness shared by multiple sensors, it is worth verifying the other sensor readings are normal after the repair.
Can a cracked sensor housing cause FMI 4?
Yes. If a cracked sensor housing allows moisture intrusion, corrosion can develop inside the sensor body and create a ground short. Visible cracks in the sensor tip or body, or heavy corrosion on the sensor terminals, support replacing the sensor rather than attempting a repair.