PACCAR · Injector Solenoid Driver Circuit - Cylinder #6High

PACCAR SPN 656 FMI 6: Injector Cylinder #6 Shorted Circuit

SPN 656 FMI 6 (FMI 6) — Engine Injector Cylinder #6 - Current Above Normal or Grounded Circuit

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Quick Answer

SPN 656 FMI 6 means the cylinder #6 injector driver circuit on a PACCAR MX engine is shorted to ground or drawing excessive current. It is High severity, does not shut the engine down, but should be repaired quickly to avoid damaging the ECM injector driver stage.

Overview: PACCAR SPN 656 FMI 6

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.

Repair Snapshot

Time

2 - 4 Hours

Difficulty

Advanced

Parts Cost

$60 - $700

Dealer Labor

$650+

Tools Required

  • PACCAR Davie4
  • Multimeter
  • Diesel Laptops Diagnostic Software
  • Valve Cover Gasket

What SPN 656 FMI 6 Means

SPN 656 FMI 6 means the PACCAR MX engine ECM has detected excessive current or a short to ground in the cylinder #6 injector driver circuit. Unlike an open circuit fault, a shorted driver risks damaging the ECM's internal injector driver stage if the truck keeps getting restarted and driven with the fault active. The MIL illuminates immediately and the engine runs rough on the affected cylinder, though it typically will not shut down.

Symptoms Drivers Notice

  • Rough idle with a miss traceable to cylinder #6
  • MIL illuminated
  • Possible burnt smell or visible connector damage
  • Reduced power and fuel economy

Most Likely Root Causes

Chafed injector harness shorting to the cylinder head or valve cover

Common

Tight harness routing against metal surfaces on MX platforms makes this the most frequent cause of a shorted injector circuit.

Water intrusion at the injector connector

Common

A failed connector seal lets moisture into the injector plug, creating a conductive path to ground.

Internally shorted injector solenoid

Occasional

Heat and vibration fatigue can break down the internal winding insulation of the solenoid, shorting it to the injector body.

Damaged ECM injector driver stage

Rare

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.

Common Misdiagnoses (Don't Waste Parts)

Replacing the injector before checking for a shorted harness

Why it happens: 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.

Do this 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

Why it happens: Techs assume a burnt electrical smell is coming from somewhere else on the engine.

Do this 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

Why it happens: Once the harness or injector is fixed, techs assume the fault is fully resolved.

Do this 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.

Master Tech Commentary

Mechanic's Notes

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.

Step-by-Step Diagnostic Procedure

  1. Step 1: Connect PACCAR Davie4 and confirm SPN 656 FMI 6 is active, then check for any accompanying injector or harness fault codes.
  2. Step 2: Shut down and unplug the cylinder #6 injector connector, inspecting for melted plastic, burnt pins, or visible arcing confirming a hard short.
  3. Step 3: 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.
  4. Step 4: 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.
  5. Step 5: 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.

Professional service / OEM-level scan tools are recommended before clearing this fault for good.

Live Data & Freeze-Frame Tips

In Davie4, pull up the injector balance rate or fuel trim data for all cylinders and compare cylinder #6 against the others - a significant deviation after a repair can indicate lingering ECM driver damage even if the fault code has cleared.

Safety Warnings

  • 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.

When to Limp In vs Tow

Towing isn't usually required immediately, but continuing to run the truck with an active hard short risks ECM driver damage. If a same-day shop visit isn't possible, limit driving until repair.

Prevention Tips for Fleets & Owner-Operators

  • 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.

Related Fault Codes

SPN 656 FMI 5

Same cylinder #6 injector circuit but current below normal (open circuit) instead of shorted.

SPN 655 FMI 6

Same fault type but for the adjacent cylinder #5 injector circuit.

SPN 629 FMI 12

Controller #1 (ECM) bad intelligent device fault, worth checking if driver damage is suspected.

Frequently Asked Questions

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.

Can this code damage my ECM?

Yes, an unresolved short to ground can overheat the ECM's internal injector driver circuit for that cylinder if the truck is repeatedly started and driven with the fault active.

How do I tell if it's the injector or the harness?

Unplug the injector connector and measure resistance from each terminal to ground independently of the harness - if the short disappears with the injector disconnected, the injector itself is the problem.

Is FMI 6 more serious than FMI 5 on this SPN?

In practical terms, yes - FMI 6 (shorted/current above normal) carries more risk of ECM driver damage than FMI 5 (open circuit), which simply disables the cylinder without generating excess current.