Mercedes-Benz SPN 657 FMI 5: Injector Cylinder #7 Open Circuit
SPN 657 FMI 5 (FMI 5) — Engine Injector Cylinder #7 - Current Below Normal or Open Circuit
Quick Answer
SPN 657 FMI 5 means the Mercedes-Benz ECM sees an open circuit in the cylinder #7 injector driver and cannot fire that injector. It's a High severity fault causing a rough idle and power loss, but it does not shut the engine down.
Overview: Mercedes-Benz SPN 657 FMI 5
Mercedes-Benz OM460 and OM470 engines, commonly found in European-spec Actros and Axor platforms as well as some North American applications, use the same J1939 injector cylinder numbering convention as other heavy-duty engines, with SPN 657 covering cylinder #7. FMI 5 points to current below normal, the classic electrical signature of an open circuit rather than a short. ProfessionalDieselRepair.com covers this fault often because harness chafing at the valve cover is such a consistent root cause across virtually every OEM platform using individual cylinder injector codes.
Repair Snapshot
Time
1.5 - 3 Hours
Difficulty
Intermediate
Parts Cost
$60 - $650
Dealer Labor
$550+
Tools Required
- Jaltest Diagnostic Tool
- Multimeter
- Diesel Laptops Diagnostic Software
- Valve Cover Gasket Set
What SPN 657 FMI 5 Means
SPN 657 FMI 5 means the Mercedes-Benz OM engine ECM has detected an open circuit in the cylinder #7 injector driver and cannot fire that injector. The engine keeps running on the remaining cylinders with a rough idle and reduced power, and the MIL illuminates immediately. This is not a shutdown fault, but continued operation with a disabled cylinder risks fuel dilution of engine oil over time.
Symptoms Drivers Notice
- Rough idle with a miss traceable to cylinder #7
- MIL illuminated
- Reduced power and fuel economy
- Possible white smoke from unburned fuel at the affected cylinder
Most Likely Root Causes
Chafed valve cover harness wire
CommonVibration wears through insulation where the harness routes near rocker arm hardware, eventually breaking the conductor completely.
Corroded or loose injector connector
CommonMoisture intrusion or an improperly seated locking tab creates a high-resistance or open contact the ECM reads as a dead circuit.
Failed injector solenoid winding
OccasionalInternal solenoid coil failure from heat and vibration fatigue can open the circuit after extended service life.
ECM injector driver stage failure
RareAn internal ECM fault on that specific driver channel would show the same code, but only after the injector and harness both test clean.
Common Misdiagnoses (Don't Waste Parts)
Ordering a replacement injector before checking harness continuity
Why it happens: A parts-cannon shortcut assumes the injector is automatically the failure point since it's named in the fault description.
Do this instead: Ohm out the injector at the connector and check harness continuity back to the ECM before committing to an injector replacement.
Assuming rough idle always means a fuel filter or fuel quality issue
Why it happens: Rough idle symptoms get blamed on fuel delivery generally without pulling codes first.
Do this instead: Scan for active codes before chasing fuel delivery components, since a single dead cylinder produces very similar symptoms.
Not checking the locking tab engagement on the injector connector
Why it happens: A quick visual check can miss a connector that appears plugged in but isn't fully locked.
Do this instead: Physically confirm the connector cannot be pulled apart by hand before proceeding to more invasive diagnostics.
Master Tech Commentary
Mechanic's Notes
Shop tip from ProfessionalDieselRepair.com: on Mercedes OM460 and OM470 engines, valve cover harness chafing near the rocker shaft brackets is the most frequent cause of a single-cylinder injector fault. Always ohm out the injector at the connector before pulling the valve cover to inspect the harness - it saves an unnecessary injector return in most cases.
Step-by-Step Diagnostic Procedure
- Step 1: Connect a Mercedes-Benz compatible diagnostic tool and confirm SPN 657 FMI 5, then run a cylinder cutout test to verify cylinder #7 contributes nothing.
- Step 2: Shut down and unplug the cylinder #7 injector connector, checking for corrosion, bent pins, or a loose locking tab.
- Step 3: Measure resistance across the injector solenoid terminals with a multimeter and compare to the OM engine family spec.
- Step 4: If the injector reads good, trace the harness back toward the ECM for chafing near the valve cover or rocker assembly.
- Step 5: Repair or replace the faulty component, clear the code, and re-run the cutout test to confirm cylinder #7 is firing correctly before road testing.
Live Data & Freeze-Frame Tips
Run the cylinder cutout test on your diagnostic tool and confirm cylinder #7 shows zero RPM contribution. Watching injector driver status while wiggling the harness near known chafe points can help catch an intermittent open circuit before it becomes permanent.
Safety Warnings
- Never probe injector connectors with the engine running due to high transient voltages during firing.
- Allow the engine to cool before working near the valve cover.
- Disconnect batteries before disturbing injector wiring.
- Wear safety glasses when cleaning connector pins with contact cleaner.
When to Limp In vs Tow
Towing is generally not needed since the truck will still drive with reduced power. Consider towing if additional misfire or fuel pressure codes appear alongside it, suggesting a broader fuel system problem.
Prevention Tips for Fleets & Owner-Operators
- Inspect valve cover harnesses for chafing at every major PM interval.
- Re-secure harness clips any time the valve cover is removed for service.
- Use dielectric grease on injector connectors during reassembly.
- Address rough idle complaints early before they progress to a hard fault code.
Related Fault Codes
SPN 656 FMI 6
Same fault type but shorted rather than open, and for the adjacent cylinder #6 injector circuit.
SPN 657 FMI 6
Current above normal (shorted) on the same cylinder #7 injector circuit rather than open.
SPN 629 FMI 12
Controller #1 (ECM) bad intelligent device fault, worth ruling out if the harness and injector both test clean.
Frequently Asked Questions
Can I drive with SPN 657 FMI 5 active?
Yes, the truck will drive with a rough idle and reduced power since the ECM has disabled that cylinder's injector. It's not a shutdown code, but repair is recommended soon.
How do I reset SPN 657 FMI 5 without a scanner?
A battery disconnect may clear it temporarily, but the fault returns on the next drive cycle if the open circuit hasn't actually been repaired.
Does this code trigger a derate?
Some calibrations apply a mild power reduction with a disabled cylinder, but it's not the aggressive emissions inducement derate seen with aftertreatment faults.
What's the difference between FMI 5 and FMI 6 here?
FMI 5 means current below normal, typically an open circuit or broken wire. FMI 6 means current above normal, typically a short to power or ground. Both affect the same circuit but represent opposite electrical failure modes.
Can a bad ground cause this code?
Yes, since injector solenoids are grounded through the ECM driver circuit, a corroded or broken ground path reads identically to an open supply wire on a resistance test.
Is it safe to drive a few days with a dead cylinder?
Short-term driving is generally fine, but extended operation with a disabled cylinder risks fuel washing down the cylinder wall and diluting engine oil, so timely repair is worthwhile.