Mercedes-Benz · Injector Cylinder #1High

Mercedes-Benz SPN 651 FMI 6: Diagnosing a Shorted Cylinder #1 Injector Circuit

SPN 651 FMI 6 (FMI 6) — Engine Injector Cylinder #01 - Current Above Normal Or Grounded Circuit

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Quick Answer

SPN 651 FMI 6 on Mercedes-Benz heavy-duty engines means cylinder #1's injector circuit is reading current above normal, consistent with a short to ground. It's a high-severity fault that shuts down that injector's driver and causes a real misfire until repaired.

Overview: Mercedes-Benz SPN 651 FMI 6

A grounded injector circuit is more urgent than a simple open circuit because it risks damaging the ECM's driver stage if left unresolved. ProfessionalDieselRepair.com treats SPN 651 FMI 6 as a fault to diagnose thoroughly and quickly - checking for melted insulation or a pinched wire near the valve cover before it has a chance to cause secondary damage to the control module itself.

Repair Snapshot

Time

1.5 - 4 Hours

Difficulty

Advanced

Parts Cost

$180 - $950

Dealer Labor

$650+

Tools Required

  • Mercedes-Benz Xentry
  • Jaltest Diagnostic Software
  • Diesel Laptops Scan Tool
  • Multimeter

What SPN 651 FMI 6 Means

SPN 651 FMI 6 means the ECM measured current above normal on cylinder #1's injector circuit, consistent with a short to ground. On Mercedes-Benz heavy-duty engines this is a high-severity fault - the ECM typically shuts down that injector's driver to protect itself, resulting in a real misfire on that cylinder until the short is repaired.

Symptoms Drivers Notice

  • Noticeable rough idle and a distinct miss under load
  • Check Engine lamp illuminated
  • Possible burnt smell or visible damage near the injector connector if the short has been present for a while
  • Reduced power and fuel economy from the ECM compensating with the remaining cylinders

Most Likely Root Causes

Chafed or pinched harness wire shorting to a grounded surface

Common

Vibration and heat cycling near the valve cover can wear through wire insulation, allowing the conductor to contact grounded engine components.

Melted or damaged connector at the injector

Occasional

Heat exposure near the injector can degrade connector plastic, creating an unintended path to ground at the terminal level.

Internal injector solenoid short

Occasional

The injector's internal winding can fail and short internally to the injector body, which is grounded to the engine.

Water or coolant intrusion into the connector

Rare

A compromised seal allowing fluid into the connector can create a temporary or permanent short-to-ground condition.

Common Misdiagnoses (Don't Waste Parts)

Replacing the injector without checking the harness for a short first

Why it happens: The fault names the injector circuit, so replacing it feels like the direct fix - a parts-cannon move that risks reinstalling a new injector into a still-shorted harness.

Do this instead: Isolate whether the short is in the harness or the injector by checking resistance to ground with the injector both connected and disconnected before replacing anything.

Not checking the ECM driver circuit after repairing the short

Why it happens: Once the visible short is found and fixed, it's easy to assume the job is done.

Do this instead: Verify the ECM can still properly drive that injector circuit after the repair - a sustained short can sometimes damage the driver stage, requiring further ECM-level diagnosis.

Master Tech Commentary

Mechanic's Notes

Shop tip: a shorted injector circuit can sometimes take out the ECM's driver stage for that cylinder if it's not caught quickly, turning a simple wiring repair into an expensive control module repair. ProfessionalDieselRepair.com always checks for a blown driver circuit on the ECM side after finding and fixing a confirmed short, rather than assuming the repair is complete once the short itself is resolved.

Step-by-Step Diagnostic Procedure

  1. Step 1: Pull the fault with Mercedes-Benz Xentry and confirm SPN 651 FMI 6 is active; check for signs of a repeat fault, since a short-to-ground condition can sometimes blow a driver circuit fuse if present long enough.
  2. Step 2: Inspect the cylinder #1 injector connector and surrounding harness closely for melted insulation, a burnt connector, or a wire pinched against the valve cover or cylinder head.
  3. Step 3: Disconnect the connector and check resistance from each injector terminal to engine ground - any continuity confirms a short, whereas a healthy circuit should show no continuity to ground.
  4. Step 4: If the short is found within the injector itself (isolated from the harness), replace that injector using the correct Mercedes-Benz procedure and calibration data entry.
  5. Step 5: If the short is in the harness, repair or replace the damaged section with appropriate heat-resistant wire and loom, then reseal any disturbed grommets.
  6. Step 6: Clear the code and verify cylinder #1 is firing correctly using a cylinder cutout or contribution test before returning the truck to service.

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

Live Data & Freeze-Frame Tips

On Xentry, run the injector circuit test for cylinder #1 and watch for the ECM flagging a repeat short-circuit condition even after a repair attempt - this indicates either the short wasn't fully resolved or the ECM driver itself sustained damage. Compare resistance-to-ground readings across all cylinders for a quick side-by-side sanity check.

Safety Warnings

  • Disconnect the battery before working on any injector harness to prevent an accidental short during repair.
  • Allow the engine to cool completely before accessing injector connectors near the valve cover.
  • Be alert for any burnt smell or visible heat damage, which indicates the short has been present for some time.

When to Limp In vs Tow

Towing generally isn't necessary since the engine will still run on the remaining cylinders, but arrange prompt service - continuing to operate with a known short-to-ground condition risks further damage to the ECM's driver circuit.

Prevention Tips for Fleets & Owner-Operators

  • Inspect injector harness routing for chafing during valve cover or injector service.
  • Keep connector seals in good condition to prevent moisture intrusion.
  • Address any burnt smell or visible connector damage immediately rather than waiting for a hard fault.

Related Fault Codes

SPN 651 FMI 5

Same cylinder #1 injector circuit flagged as current below normal (open) instead of shorted - a different electrical failure mode.

SPN 652 FMI 6

Cylinder #2 injector circuit shorted to ground - the same fault type on the adjacent cylinder, worth checking if a broader harness issue is suspected.

Frequently Asked Questions

Can I drive with SPN 651 FMI 6 active?

You can drive short distances since the engine will run on the remaining cylinders, but get it repaired promptly - an unresolved short risks further damage to the ECM's driver circuit.

How do I reset SPN 651 FMI 6 without a scan tool?

You need Xentry or a comparable diagnostic tool to clear the stored fault after confirming and repairing the short-to-ground condition.

Is this always caused by the injector itself?

Not necessarily - check the harness for chafing or a melted connector first, since these are common causes that don't require replacing the injector.

Can this fault damage my ECM?

Yes, if left unresolved for long enough, a persistent short-to-ground condition can potentially damage the ECM's driver circuit for that injector, so prompt repair matters.

Will this code cause a derate?

The ECM typically disables that specific injector's driver as a protective measure, which reduces available power similar to running on one fewer cylinder, though it may not trigger a formal derate message.

How do I know if the short is fixed and not just the code cleared?

Verify resistance to ground reads correctly (no continuity) after the repair, and confirm the cylinder fires normally with a cutout or contribution test rather than just clearing the code and hoping it doesn't return.