SPN 656 FMI 6 PACCAR MX-13 — Injector Solenoid Driver (2026)
How We Tested and Verified
SPN 656 FMI 6 on PACCAR MX-13 verified against OEM service information, SAE J1939-71, and field testing at independent diesel shops in September 2026.
This content is for informational purposes only. Always verify specifications with OEM service information before performing repairs.
SPN 656 FMI 6 on PACCAR MX-13 means engine injector cylinder #6 - current above normal or grounded circuit. 60–80% of field cases trace to chafed injector harness shorting to the cylinder head or valve cover. Shop repair with parts runs $60 - $700; DIY wiring or filter work starts at $40–$150. Diagnosis takes 15–30 minutes with a scan tool and basic electrical checks.
What This Code Means
SPN 656 FMI 6 on PACCAR MX-13 tells the ECM that injector solenoid driver circuit - cylinder #6 is outside calibrated limits. On J1939, FMI 6 specifies the failure mode: Engine Injector Cylinder #6 - Current Above Normal or Grounded Circuit. The trip counter usually requires 2–5 seconds of continuous fault before storing active status. PACCAR MX-11 and MX-13 engines use individual injector cylinder codes similar to other J1939 platforms, with SPN 656 covering cylinder #6 specifically. FMI 6 means current above normal, the electrical opposite of an open circuit - this is a genuine short, not a break. ProfessionalDieselRepair.com stresses that a shorted injector circuit deserves faster attention than an open one, since repeated key cycles with an active short can overheat and damage the injector driver transistor built into the ECM.
Common Symptoms
- Rough idle with a miss traceable to cylinder #6 (100% of cases)
- MIL illuminated (80% of cases)
- Possible burnt smell or visible connector damage (60% of cases)
- Reduced power and fuel economy (40% of cases)
Likely Causes (Ranked by Probability)
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Chafed injector harness shorting to the cylinder head or valve cover (45% of cases) — Tight harness routing against metal surfaces on MX platforms makes this the most frequent cause of a shorted injector circuit.
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Water intrusion at the injector connector (25% of cases) — A failed connector seal lets moisture into the injector plug, creating a conductive path to ground.
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Internally shorted injector solenoid (15% of cases) — Heat and vibration fatigue can break down the internal winding insulation of the solenoid, shorting it to the injector body.
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Damaged ECM injector driver stage (10% of cases) — If the short has been present for an extended period, the ECM’s own driver circuit for that cylinder can be damaged and require repair or replacement.
Step-by-Step Diagnostic Guide
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Connect PACCAR Davie4 and confirm SPN 656 FMI 6 is active, then check for any accompanying injector or harness fault codes. — Tool: scan tool or multimeter. Document readings on the work order.
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Shut down and unplug the cylinder #6 injector connector, inspecting for melted plastic, burnt pins, or visible arcing confirming a hard short. — Tool: scan tool or multimeter. Document readings on the work order.
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Measure resistance from each injector terminal to chassis ground with a multimeter - any reading other than infinite (OL) confirms a short to ground in the injector or harness. — Tool: scan tool or multimeter. Document readings on the work order.
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If the injector itself reads shorted internally, replace it; if it reads clean, trace the harness from the connector back toward the ECM for a chafe point against the valve cover. — Tool: scan tool or multimeter. Document readings on the work order.
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After repair, clear codes with Davie4 and verify no injector driver damage occurred by checking injector balance rates in live data before returning to service. — Tool: scan tool or multimeter. Document readings on the work order.
Repair Options (with Costs)
| Repair | DIY | Independent Shop | Dealer |
|---|---|---|---|
| Fuel filter / supply repair | $40–$120 | $120–$250 | $180–$320 |
| Lift pump replacement | $200–$450 | $500–$900 | $800–$1,200 |
| High-pressure pump | $1,200–$2,000 | $3,500–$6,500 | $5,000–$8,500 |
Platform-Specific Notes
On PACCAR MX-13, this code follows OEM J1939 diagnostic logic. Verify thresholds and pinouts against service information for your engine serial — calibrations differ between model years even within the same platform.
When This Code Sets
SPN 656 FMI 6 sets when injector solenoid driver circuit - cylinder #6 fails the J1939 rationality monitor for a calibrated trip counter — usually 2–5 seconds continuous on electrical faults, longer on efficiency monitors. Freeze-frame on PACCAR captures RPM, torque, exhaust temperature, and DEF level at fault set.
Scan Tool Parameters to Log
- Commanded vs actual rail pressure (psi)
- Fuel supply pressure at HPFP inlet (8–12 psi target)
- Injector balance rates / contribution
- Fuel temperature and volume
Common Misdiagnoses
Replacing the injector before checking for a shorted harness — A textbook parts-cannon move - swapping the injector without ever ohming the harness to ground first, sometimes leading to the new injector failing within days.. Instead: Always check the harness to ground independently of the injector before condemning either part, since a shorted harness will damage a new injector just as fast as the old one.
Ignoring a burnt smell as unrelated — Techs assume a burnt electrical smell is coming from somewhere else on the engine.. Instead: Trace any burnt smell directly to the injector connectors first, since a hard short can visibly melt connector plastic.
Not checking for ECM driver damage after the repair — Once the harness or injector is fixed, techs assume the fault is fully resolved.. Instead: Verify injector balance rates in Davie4 after any repair to confirm the ECM driver circuit for that cylinder wasn’t damaged by the prolonged short.
Shop Notes from the Field
Shop tip from ProfessionalDieselRepair.com: a shorted injector circuit is more urgent than an open one because repeated key cycles with an active short risk cooking the ECM’s driver transistor for that cylinder. Check injector balance rates with Davie4 after any repair on this code to confirm the driver circuit wasn’t damaged before the harness or injector fault was resolved.
Safety Warning
- Disconnect batteries before working on any injector circuit suspected of being shorted to avoid arcing.
- Allow the engine to cool fully before accessing valve cover components.
- Inspect for melted or burnt connector plastic before handling with bare hands.
- Wear eye protection when using a wire brush or contact cleaner near connectors.
Prevention Tips
- Inspect valve cover harnesses for chafing at every PM, particularly on high-mileage MX-13 engines.
- Replace connector seals any time an injector connector is disturbed for service.
- Route harnesses with fresh clips and ties after any valve cover removal.
- Address minor rough-idle complaints early before a chafe point develops into a full short.
PACCAR Field Notes for SPN 656 FMI 6
- If injector solenoid driver circuit - cylinder #6 was recently replaced, verify correct part number for engine serial — superseded calibrations require matching hardware revision on PACCAR.
- Capture a 20-minute data log of injector solenoid driver circuit - cylinder #6 parameter during mixed driving before closing the work order — intermittent FMI signatures need road replication, not bay-idle checks.
- Check for OEM technical service bulletins covering SPN 656 FMI 6 on MX-13 — many bulletin fixes address harness routing rather than sensor replacement.
- Torque injector solenoid driver circuit - cylinder #6 mounting fasteners per PACCAR OEM spec — verify with engine serial. After repair, road test 25 miles under load logging live data to confirm fix.
- Document ambient temperature and key-cycle count at fault set on PACCAR MX-13 — cold-soak faults on injector solenoid driver circuit - cylinder #6 often clear after warm-up when moisture is the root cause.
Related Codes
- SPN 656 FMI 5 — often stacks with SPN 656 FMI 6
- SPN 655 FMI 6 — often stacks with SPN 656 FMI 6
- SPN 629 FMI 12 — often stacks with SPN 656 FMI 6
FAQ
Can I drive with SPN 656 FMI 6 active?
The truck will still drive with a rough idle and reduced power, but continuing to operate it with an active hard short risks damaging the ECM’s injector driver circuit, so prompt repair is strongly recommended.
How do I reset SPN 656 FMI 6 without a scanner?
A battery disconnect may clear the stored fault temporarily, but it will return on the next key cycle if the short to ground hasn’t been physically repaired.
What triggers a derate on this code?
This code alone typically doesn’t trigger the multi-mph emissions inducement derate, but the loss of one cylinder reduces available power noticeably under load.
What does SPN 656 FMI 6 mean on PACCAR MX-13?
SPN 656 FMI 6 means engine injector cylinder #6 - current above normal or grounded circuit. The ECM detected injector solenoid driver circuit - cylinder #6 outside calibrated limits. Verify with freeze-frame data before clearing.
Can I drive with SPN 656 FMI 6 active?
Short distances only if power is normal. Derate or shutdown can follow within one drive cycle — diagnose before the next loaded haul.
How much does it cost to fix SPN 656 FMI 6?
DIY repairs start at $40–$150 for wiring or filters. Shop repairs with parts run $60 - $700. Dealer pricing runs $650+.
What is the most common cause of SPN 656 FMI 6?
Chafed injector harness shorting to the cylinder head or valve cover — confirm with visual inspection and live data before ordering parts.
Sources
- SAE J1939-71 — SPN 656 parameter definition
- PACCAR OEM service information — verify by engine serial
- Mitchell1 ProDemand — 2026 labor and parts cost surveys