Code Details
| Display code | WABCO RSSplus SID 251 FMI 2 |
|---|---|
| SPN | Not applicable or not verified |
| FMI | 2 |
| OEM code | WABCO / ZF SID 251 FMI 2 |
| Manufacturer | WABCO / ZF |
| System | Trailer ABS / RSSplus |
| Component | Trailer ABS power supply |
| 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 power coming to the RSSplus ECU from the trailer's ABS power circuit is not stable. Rather than a constant dropout (which would produce a different code) or an overvoltage (FMI 3), FMI 2 on the power supply means the voltage is varying erratically. This is often associated with a high-resistance connection that causes voltage to fluctuate under varying load conditions.
WABCO MM-0888 SID 251 FMI 2 maps to an erratic or intermittent power supply condition. The ECU monitors supply voltage continuously; irregular fluctuations that the ECU cannot characterize as a stable overvoltage or undervoltage trigger FMI 2. This is distinct from FMI 3 (voltage consistently above normal) and from the power-off / power-on cycle that would set a different condition.
Common Symptoms
- Trailer ABS lamp may flicker or behave inconsistently — correlating with power fluctuations
- SID 251 FMI 2 active in WABCO PC diagnostics
- ECU may reset or show intermittent communication issues during voltage fluctuations
Possible Causes
Possible causes may include the items below. The list is not a parts diagnosis.
- High-resistance connection in the ABS power supply path — most common cause of erratic voltage
- Corroded or damaged 7-way connector blue pin or its mating terminal in the tractor socket
- Loose power supply connection at the RSSplus ECU itself
- Intermittent contact in an inline fuse holder or relay in the ABS power circuit
First Checks
- Measure voltage at the RSSplus ECU power input terminal while the trailer is connected to a tractor and the ABS system is powered — compare to voltage at the 7-way connector to find voltage drop in the wiring.
- Inspect the 7-way trailer connector ABS circuit for corrosion, damage, or loose pin seating.
- Inspect any fuse holders or relay sockets in the trailer ABS power circuit — these can develop intermittent contact over time.
- Check the ABS power circuit wiring for loose ring terminals at the ECU mounting point and at any splices or junction blocks.
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 much voltage drop is acceptable on the trailer ABS power circuit?
WABCO specifications typically require the ABS ECU to receive close to the system voltage (12 V or 24 V depending on the trailer) with minimal drop. Excessive voltage drop — generally more than 0.5 V at operating current — is enough to affect ECU operation. Measure at the ECU terminals, not just at the connector, to catch drop in the wiring run.
Can a bad trailer ground cause SID 251 FMI 2?
Yes. The ECU measures supply voltage relative to its ground reference. A high-resistance ground connection causes the reference to shift, which can appear as fluctuating supply voltage even if the positive circuit is clean. Always check both supply and return (ground) quality when diagnosing power supply issues.
Will SID 251 FMI 2 clear after fixing the power connection?
Active power supply codes typically clear once the ECU receives stable voltage across a defined operating period. Some conditions may require a WABCO PC diagnostic reset to confirm the fault has cleared after repair.