Detroit Diesel · Fuel Injector Cylinder 4High

Detroit Diesel SPN 654 FMI 6: Short-to-Ground on Cylinder 4 Injector

SPN 654 FMI 6 (FMI 6) — Injector cylinder 4 driver current is above normal — FMI 6 short-to-ground fault on the solenoid drive circuit

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Quick Answer

SPN 654 FMI 6 on a Detroit Diesel means the cylinder 4 injector solenoid drive circuit has a short to ground. Measure solenoid resistance at the connector (spec: 0.3 to 0.5 Ohm) before replacing the injector.

Overview: Detroit Diesel SPN 654 FMI 6

When a Detroit Diesel DD13 or DD15 logs SPN 654 FMI 6, the ECM has detected a current-above-normal condition on the cylinder 4 injector driver — the J1939 definition of a short-to-ground fault. Unlike FMI 5 (open circuit), FMI 6 means current is flowing where it should not be, pulling the driver line to ground. At ProfessionalDieselRepair.com we see this code split roughly evenly between a failed injector solenoid (shorted winding) and a chafed harness wire inside the valve cover. The solenoid resistance test (0.3 to 0.5 Ohm on a good DD15 injector) takes under two minutes with a DVOM and immediately separates these two failure modes before any parts are purchased.

Repair Snapshot

Time

2 - 5 Hours

Difficulty

Advanced

Parts Cost

$400 - $900

Dealer Labor

$900+

Tools Required

  • DDDL (Detroit Diesel Diagnostic Link)
  • NEXIQ USB-Link 2
  • Digital Multimeter (DVOM)
  • Injector Hold-Down Torque Wrench
  • Valve Cover Gasket Set

What SPN 654 FMI 6 Means

SPN 654 FMI 6 on a Detroit Diesel DD13, DD15, or DD16 engine means the ECM detected excessive current on the cylinder 4 injector driver circuit, which is the J1939 definition of a short-to-ground fault. DDDL (Detroit Diesel Diagnostic Link) is the primary tool to confirm the fault and run injector disable tests. A shorted injector solenoid draws uncontrolled current, potentially damaging the ECM driver circuit if not addressed quickly. The cylinder 4 injector must be isolated and the solenoid resistance measured before any parts are ordered.

Symptoms Drivers Notice

  • Engine misfire on cylinder 4 with rough idle and vibration felt throughout the cab.
  • Possible protection shutdown by the ECM if the short draws enough current to trigger a driver fault.
  • White or grey smoke from unburned fuel in the cylinder 4 exhaust stream.
  • Power and torque reduction proportional to the loss of one cylinder.
  • DDDL displays SPN 654 FMI 6 as active with an injector current fault for cylinder 4.

Most Likely Root Causes

Injector solenoid winding shorted to injector body or ground

Common

The solenoid coil insulation breaks down from thermal fatigue or fuel contamination, creating a direct path from the winding to the injector barrel. Resistance drops below 0.1 Ohm.

Chafed or pinched drive harness wire in the valve cover

Common

The injector drive harness routing inside the valve cover is subject to pinch points near the rocker arms. A chafed wire can contact the valve cover or cylinder head, creating a ground fault that mimics an internal solenoid short.

Moisture or oil contamination in the injector connector

Occasional

Oil wicking into the 2-pin connector cavity creates a conductive path between the driver pins and the grounded connector shell, generating a partial short that sets FMI 6 intermittently.

Faulty ECM injector driver output for cylinder 4

Rare

A failed ECM output driver stage can generate a false short condition even when the injector and harness are good. Confirm by testing the harness and injector first before considering ECM replacement.

Common Misdiagnoses (Don't Waste Parts)

Replacing the injector without measuring solenoid resistance first

Why it happens: FMI 6 with rough idle triggers a parts-cannon injector swap, missing the cases where the short is in the harness or connector rather than the injector itself.

Do this instead: Measure resistance at the injector connector: 0.3 to 0.5 Ohm is good; below 0.1 Ohm or continuity to ground confirms solenoid failure. Check the harness before ordering the injector.

Assuming the fault is ECM-internal without testing the external circuit first

Why it happens: Experienced technicians who have seen ECM driver failures may jump to ECM diagnosis, bypassing the simpler external circuit tests.

Do this instead: Always measure injector solenoid resistance and harness isolation before considering ECM replacement. ECM driver failures are significantly less common than injector or harness failures.

Clearing the fault and road-testing without verifying repair

Why it happens: A short may not immediately damage the ECM driver, so the engine runs with the fault present. Shops under time pressure clear and road-test without confirming the current draw is normal.

Do this instead: Verify solenoid resistance is within spec after repair. Use DDDL to confirm no fault code returns after a full warm-up drive cycle.

Master Tech Commentary

Mechanic's Notes

DD13 and DD15 GHG17 injector solenoid resistance specification is 0.3 to 0.5 Ohm at 25 degrees C. A short-to-ground fault that reads below 0.1 Ohm or shows direct continuity to the injector body almost always means the solenoid winding has failed and contacted the injector barrel ground. Do not attempt to clean or repair a shorted injector — replace it. After installing the new injector, use DDDL to perform the injector calibration (fuel quantity adjustment) procedure if the DD15 calibration code is marked on the injector. The full DD15 injector R&R procedure is available at ProfessionalDieselRepair.com. Always use a NEXIQ USB-Link 2 or equivalent for DDDL connection — untested third-party adapters can corrupt the DD15 ECM.

Step-by-Step Diagnostic Procedure

  1. Step 1: Connect DDDL (Detroit Diesel Diagnostic Link) to the 9-pin J1939 port on the Freightliner or other Detroit-powered vehicle. Confirm SPN 654 FMI 6 is active. Run the DDDL injector disable test to command cylinder 4 off — if the code becomes inactive when cylinder 4 is disabled, the fault follows the injector circuit rather than being an ECM internal fault.
  2. Step 2: With the key off and engine off for 3 minutes to allow rail pressure to drop, access the cylinder 4 injector connector under the valve cover (DD15: right-side valve cover, cylinder 4 is the fourth from the front). Unplug the 2-pin connector. Measure solenoid resistance between the two injector pins: expected value for DD13/DD15 is 0.3 to 0.5 Ohm at room temperature. A reading below 0.1 Ohm or direct continuity to ground from either pin indicates an internal short in the solenoid.
  3. Step 3: With the injector connector unplugged, measure each solenoid pin to chassis ground using the DVOM in ohms mode. Both pins should read above 10 MOhm to chassis. A reading below 100 Ohm on either pin to chassis indicates the injector solenoid has shorted internally to the injector body or the drive harness has a ground fault.
  4. Step 4: If the injector solenoid resistance is in spec (0.3 to 0.5 Ohm) and neither pin shows a ground path, the fault is in the harness. Inspect the valve cover harness run for chafing, heat damage near the rocker assembly, or moisture ingress in the connector. Use a NEXIQ USB-Link 2 adapter with DDDL to perform a live current waveform capture if available to confirm the short location.
  5. Step 5: Replace the injector if the solenoid is confirmed shorted. Replace or repair the harness segment if the solenoid is good and the fault is wiring. After repair, reconnect DDDL and run the injector disable test again to confirm the fault is resolved and all six cylinders contribute normally.

Live Data & Freeze-Frame Tips

In DDDL, navigate to the injector current feedback parameters if visible on the connected ECM software version. Cylinder 4 with FMI 6 active will show an overcurrent reading or a driver protection flag. The DDDL injector disable test allows commanding each cylinder off in sequence — disabling cylinder 4 while monitoring the current fault parameter should reduce the fault indication if the fault is external to the ECM. Use a NEXIQ USB-Link 2 for DDDL connection to ensure the full diagnostic capability is available.

Safety Warnings

  • Relieve fuel rail pressure before disconnecting any injector — high-pressure common rail systems operate at up to 2,000 bar and pose a severe injection injury risk.
  • Allow the engine to cool for at least 30 minutes before removing the valve cover to prevent burns from residual heat.
  • Wear safety glasses when working near high-pressure fuel lines — even a microscopic pinhole in a HPCR line can cause a high-pressure fuel jet capable of penetrating skin.
  • Ground the vehicle before handling ECM connectors to prevent electrostatic damage to the driver circuits.

When to Limp In vs Tow

If the ECM has entered a protection shutdown or if the engine is running on fewer than four cylinders with rough operation that threatens safe driving, tow immediately. A shorted injector driver has the potential to damage the ECM if the protective circuitry fails, making prompt repair economically important as well as operationally necessary.

Prevention Tips for Fleets & Owner-Operators

  • Inspect the valve cover harness for chafing and pinch points at every 150,000-mile PM interval.
  • Replace injector connector seals whenever the valve cover is removed to prevent oil contamination.
  • Use DDDL to monitor injector balance rates at PM intervals — declining cylinder 4 contribution can predict solenoid degradation before an FMI 6 fault occurs.
  • Use only OEM Detroit Diesel or approved replacement injectors to ensure correct solenoid resistance and duty-cycle ratings.

Related Fault Codes

SPN 654 FMI 5

FMI 5 on cylinder 4 indicates an open circuit (opposite of FMI 6). Both affect cylinder 4 injector operation but require different diagnostic approaches.

SPN 653 FMI 6

SPN 653 FMI 6 is cylinder 3 with the same short-to-ground pattern. Multiple adjacent cylinders showing FMI 6 may indicate a shared harness fault.

SPN 652 FMI 6

SPN 652 cylinder 2 FMI 6 — if cylinders 2, 3, and 4 all show FMI 6, suspect the shared section of the injector drive harness rather than individual injector failures.

Frequently Asked Questions

What does Detroit Diesel SPN 654 FMI 6 mean?

SPN 654 FMI 6 means the cylinder 4 injector driver circuit has detected excessive current — a short-to-ground fault. This can be an internal injector solenoid failure or a shorted wire in the drive harness.

What is the injector solenoid resistance specification for a DD15?

The cylinder 4 injector solenoid on a Detroit Diesel DD13 or DD15 should read 0.3 to 0.5 Ohm at room temperature (25 degrees C). Below 0.1 Ohm or continuity to chassis ground confirms a shorted solenoid.

Can I drive with Detroit Diesel SPN 654 FMI 6 active?

Briefly at reduced loads, but not long-term. The short draws excessive current from the ECM driver stage, which can damage the ECM if the protective circuit fails. Diagnose and repair promptly.

How do I tell if the injector or the harness is shorted?

Disconnect the injector connector and measure Pin A and Pin B to chassis ground (both should be above 10 MOhm). If a short exists with the connector unplugged, the harness is the fault. If the short only appears with the sensor plugged in, the injector solenoid is shorted internally.

Which diagnostic tools diagnose SPN 654 on a Detroit Diesel?

DDDL (Detroit Diesel Diagnostic Link) is the factory tool and provides injector disable tests, current feedback, and calibration routines. A NEXIQ USB-Link 2 J1939 adapter provides the required interface. Third-party adapters may not support full bi-directional injector testing on DD15 GHG17 engines.

How much does fixing SPN 654 FMI 6 cost on a DD15?

A replacement DD15 injector costs $400 to $900 depending on supplier and calibration code. Dealer labor for valve cover removal and injector replacement typically runs $700 to $1,200.