Mack SPN 654 FMI 5: Injector Cylinder #4 Open Circuit
SPN 654 FMI 5 (FMI 5) — Engine Injector Cylinder #4 - Current Below Normal or Open Circuit
Quick Answer
SPN 654 FMI 5 means the Mack ECM sees an open circuit in the cylinder #4 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: Mack SPN 654 FMI 5
Mack MP7, MP8, and MP10 engines use the same J1939 injector cylinder numbering as other heavy-duty platforms, with SPN 654 flagging cylinder #4 specifically. FMI 5 points to current below normal, the classic signature of an open circuit rather than a short. As ProfessionalDieselRepair.com covers in its injector diagnostic content, most of these codes trace back to harness damage rather than an actual dead injector, making a thorough electrical check well worth the extra fifteen minutes before ordering parts.
Repair Snapshot
Time
1.5 - 3 Hours
Difficulty
Intermediate
Parts Cost
$50 - $600
Dealer Labor
$500+
Tools Required
- Mack Premium Tech Tool (PTT)
- Multimeter
- Jaltest Diagnostic Tool
- Diesel Laptops Diagnostic Software
What SPN 654 FMI 5 Means
SPN 654 FMI 5 means the Mack MP engine ECM has detected an open circuit in the cylinder #4 injector driver and cannot fire that injector. The engine keeps running on the remaining cylinders, but with a rough idle and noticeable power loss, and the amber warning lamp comes on right away. This is not a shutdown fault, but continued operation with a dead cylinder can dilute engine oil with unburned fuel over time.
Symptoms Drivers Notice
- Rough idle with a miss traceable to cylinder #4
- Amber warning lamp 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
CommonThe harness routing near rocker arm hardware wears through insulation over time from engine vibration, eventually breaking the conductor.
Corroded or loose injector connector
CommonMoisture intrusion or an improperly seated locking tab creates a high-resistance or open contact point 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.
ECM injector driver stage failure
RareAn internal ECM fault on that 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 the failure point since it's the component named in the fault description.
Do this instead: Ohm out the injector and check the harness back to the ECM before committing to an injector replacement.
Not re-checking harness clips after valve cover service
Why it happens: Techs focus on gasket sealing and torque specs but skip re-securing loose harness routing.
Do this instead: Always re-clip and inspect harness routing any time a valve cover is removed for service.
Assuming a rough idle always means a fuel system problem
Why it happens: Rough idle symptoms are often blamed on fuel filters or injectors generally without pulling codes first.
Do this instead: Scan for active codes before chasing fuel delivery components - a single dead cylinder from an electrical fault produces very similar symptoms.
Master Tech Commentary
Mechanic's Notes
Shop tip: ProfessionalDieselRepair.com sees Mack MP engines throw this fault most often after valve cover gasket service where the harness wasn't re-clipped properly. A loose harness that's free to vibrate against the rocker cover will chafe through in a matter of months, so always re-secure every clip when buttoning up a valve cover.
Step-by-Step Diagnostic Procedure
- Step 1: Connect Mack Premium Tech Tool (PTT) and confirm SPN 654 FMI 5, then run a cylinder cutout test to verify cylinder #4 shows zero contribution.
- Step 2: Shut down and unplug the cylinder #4 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 Mack MP engine 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 with PTT, and re-run the cutout test to confirm cylinder #4 is firing correctly.
Live Data & Freeze-Frame Tips
On PTT, run the cylinder cutout test and confirm cylinder #4 shows no RPM contribution. Watching injector driver status while wiggling the harness near known chafe points can help catch an intermittent open circuit before it becomes a permanent fault.
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 gasket or rocker 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 653 FMI 5
Same fault type but for the adjacent cylinder #3 injector circuit.
SPN 654 FMI 6
Current above normal (shorted) on the same cylinder #4 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 654 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 654 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 Mack 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 resistance spec for a Mack MP injector solenoid?
It varies by engine family and injector type, so always confirm the exact spec in Mack PTT or the service manual for your specific MP7, MP8, or MP10 application.
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 okay to keep driving with a dead cylinder for a few days?
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.