Cummins · Injector Solenoid Driver Circuit - Cylinder #1High

Cummins SPN 651 FMI 5: Injector Cylinder #1 Open Circuit

SPN 651 FMI 5 (FMI 5) — Engine Injector Cylinder #1 - Current Below Normal or Open Circuit

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Quick Answer

SPN 651 FMI 5 means the ECM sees an open circuit in the cylinder #1 injector solenoid driver and cannot fire that injector. It is a High severity fault that causes a rough-running miss and MIL, not an automatic shutdown.

Overview: Cummins SPN 651 FMI 5

SPN 651 FMI 5 is one of the most common single-cylinder fault codes techs see on Cummins ISX and ISL engines running the CM2350 or CM2250 ECM. As covered in ProfessionalDieselRepair.com's injector diagnostic guides, the number after SPN simply tells you which cylinder is affected (651 is cylinder 1, 652 is cylinder 2, and so on up through 674), while FMI 5 specifically means current below normal, which almost always points to an open circuit rather than a short. The truck will keep running because the ECM shuts fuel off to the dead cylinder and lets the others carry the load, but that means you are driving on fewer cylinders with reduced power and a rough idle until it is fixed. This code is a good example of a fault that is often blamed on the injector when the real problem is sitting in the harness.

Repair Snapshot

Time

1.5 - 3 Hours

Difficulty

Intermediate

Parts Cost

$40 - $650

Dealer Labor

$500+

Tools Required

  • Cummins INSITE
  • Multimeter
  • Diesel Laptops Diagnostic Software
  • Valve Cover Gasket Set

What SPN 651 FMI 5 Means

SPN 651 FMI 5 means the ECM has lost electrical continuity in the injector solenoid driver circuit for cylinder #1 and cannot fire that injector. This is not a shutdown code, but the ECM will cut fuel to the dead cylinder and the engine will run on the remaining cylinders with a hard miss and an active derate on some calibrations. Amber MIL comes on immediately and the fault will not clear on its own until the open circuit is repaired.

Symptoms Drivers Notice

  • Rough idle and a noticeable single-cylinder miss under load
  • Amber MIL illuminated, sometimes flashing during acceleration
  • Loss of power and reduced fuel economy
  • Cylinder #1 shows zero contribution on a cylinder cutout test

Most Likely Root Causes

Chafed or broken injector harness wire

Common

The valve cover harness runs tight against the rocker housing and vibrates against sharp edges over time, wearing through insulation and eventually breaking the conductor completely.

Corroded or backed-out harness connector pin

Common

Moisture intrusion at the injector connector corrodes the pin contact surface, raising resistance until the ECM reads it as an open circuit.

Failed injector solenoid winding

Occasional

The internal solenoid coil inside the injector can open from heat cycling and vibration fatigue, especially past 300,000 miles.

ECM injector driver output failure

Rare

An internal ECM driver stage failure will show the same fault, but only after the injector and full harness have already tested good.

Common Misdiagnoses (Don't Waste Parts)

Replacing the injector without an ohms check

Why it happens: A parts-cannon approach assumes the injector is always the failure point since it is the named component in the fault description.

Do this instead: Always ohm out the injector solenoid at the connector before pulling the valve cover. A quick resistance check takes five minutes and saves an unnecessary injector swap.

Skipping the harness wiggle test

Why it happens: Techs assume an intermittent fault is electronic noise rather than a physical wiring issue.

Do this instead: Watch live injector data on INSITE while flexing the harness by hand near the valve cover grommet - a fault that flickers on and off confirms a chafe point.

Blaming a bad ECM

Why it happens: Techs jump to the ECM when the fault won't clear after an injector swap.

Do this instead: Confirm the harness between the new injector and the ECM connector reads continuity end to end before ever suspecting the ECM driver circuit.

Master Tech Commentary

Mechanic's Notes

Shop tip from ProfessionalDieselRepair.com: don't order an injector the moment you see this code. The valve cover harness on high-mileage ISX engines chafes right where it crosses the rocker housing, and a pinhole short-to-open there mimics a dead injector perfectly. Wiggle-test the harness with the connector plugged in and watch INSITE live data for the fault to flicker before you condemn the injector itself.

Step-by-Step Diagnostic Procedure

  1. Step 1: Pull the code with Cummins INSITE and confirm SPN 651 FMI 5 is active, then run a cylinder cutout test to verify cylinder #1 contributes nothing to RPM.
  2. Step 2: Shut the engine off, unplug the injector #1 harness connector at the valve cover, and inspect the pins for corrosion, melted plastic, or backed-out sockets.
  3. Step 3: With the connector unplugged, measure resistance across the two injector solenoid terminals with a multimeter - most Cummins ISX/ISL CM2350 injector solenoids read between 0.5 and 3 ohms. Infinite (OL) resistance confirms an open winding inside the injector.
  4. Step 4: If the injector reads good, check the harness itself for continuity from the injector connector back to the ECM injector driver pin, and inspect the valve cover grommet for chafing where the harness passes through.
  5. Step 5: Clear the code with INSITE, restart, and re-run the cutout test to confirm cylinder #1 is back online before returning the truck to service.

Live Data & Freeze-Frame Tips

With Cummins INSITE connected, run the cylinder cutout test and watch RPM drop on each cylinder individually - cylinder #1 should show zero or near-zero drop if the injector isn't firing. Also monitor the injector driver voltage waveform if your scan tool supports it; a clean square wave that suddenly drops to zero volts confirms an open circuit downstream of the driver rather than an ECM internal fault.

Safety Warnings

  • Never probe injector connectors with the engine running - injector solenoid circuits can carry high transient voltages during firing events.
  • Let the engine cool before working near the valve cover to avoid burns from exhaust manifold proximity.
  • Disconnect batteries before disturbing high-pressure fuel lines or injector wiring harnesses.
  • Wear safety glasses when cleaning connector pins with electrical contact cleaner.

When to Limp In vs Tow

This code alone rarely requires a tow since the truck will still drive, just with reduced power and a rough idle. Tow only if the derate becomes severe enough that the truck cannot maintain highway speed, or if additional codes for fuel rail pressure or multiple cylinder misfires appear alongside it.

Prevention Tips for Fleets & Owner-Operators

  • Inspect valve cover harnesses for chafing at every PM interval, especially where the loom crosses sharp rocker housing edges.
  • Use dielectric grease on injector connectors when reassembling after any valve cover service.
  • Route harnesses with new zip ties or clips any time you remove and reinstall a valve cover.
  • Address any intermittent misfire complaints early before they become a hard fault code.

Related Fault Codes

SPN 652 FMI 5

Same fault type but for cylinder #2 - check for a shared harness chafe point if both appear together.

SPN 651 FMI 6

Current above normal (shorted) rather than open circuit, but affects the same cylinder #1 injector circuit.

SPN 651 FMI 7

Mechanical system not responding, indicating the injector itself is not moving fuel even with good electrical signals.

Frequently Asked Questions

Can I drive with SPN 651 FMI 5 active?

Yes, the truck will drive, but you'll have a rough idle and reduced power because the ECM has shut fuel off to that cylinder. It's not a shutdown code, but running on fewer cylinders long-term can stress the remaining injectors and increase fuel dilution risk.

How do I reset SPN 651 FMI 5 without a scanner?

You can't reliably clear it without a scan tool like Cummins INSITE, and even if you disconnect the batteries to clear it temporarily, the fault will come right back on the next drive cycle if the open circuit isn't repaired.

Does this code trigger a derate?

Some CM2350 calibrations will apply a mild power derate when a single injector is disabled, but it is not the aggressive multi-mph crawl derate associated with aftertreatment inducement codes.

What's the difference between FMI 5 and FMI 6 on this SPN?

FMI 5 means current below normal, typically an open circuit or broken wire. FMI 6 means current above normal, typically a short to power or ground inside the injector or harness. Both point to the same circuit but opposite electrical failure modes.

Can a bad ground cause SPN 651 FMI 5?

Yes, injector solenoids are switched to ground by the ECM driver, so a corroded or broken ground path on that circuit reads identically to an open supply-side wire on a resistance test.

Is it safe to just unplug the bad injector and keep driving?

Physically unplugging a working injector is not recommended, but if the injector is already electrically dead, the ECM has already stopped firing it. The bigger risk is ignoring the fault long enough that fuel washes down the cylinder wall and dilutes the oil.