Mack · Injector Cylinder 2High

Mack SPN 652 FMI 5: Injector Cylinder 2 Current Below Normal

SPN 652 FMI 5 (FMI 5) — Engine Injector Cylinder #2 - Current Below Normal or Open Circuit

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Quick Answer

SPN 652 FMI 5 on Mack means the cylinder 2 injector circuit is open — no current flow. Ohm test the injector solenoid and inspect the valve cover harness before replacing the injector.

Overview: Mack SPN 652 FMI 5

SPN 652 targets the cylinder 2 injector circuit on Mack MP7, MP8, and MP10 engines. FMI 5 means current below normal, almost always an open circuit rather than a mechanical injector failure. ProfessionalDieselRepair.com guides Mack techs to test the valve cover harness first — chafe at the rocker housing is the leading cause on high-mileage MP8 trucks.

Repair Snapshot

Time

1.5 - 3 Hours

Difficulty

Intermediate

Parts Cost

$40 - $700

Dealer Labor

$450 - $850

Tools Required

  • Jaltest Diagnostic System
  • Texa
  • Multimeter
  • Valve Cover Gasket Set

What SPN 652 FMI 5 Means

SPN 652 FMI 5 on Mack MP7, MP8, and MP10 engines means the ECM has lost electrical continuity in the cylinder 2 injector solenoid driver circuit. The ECM cuts fuel to cylinder 2 and the engine runs with a single-cylinder miss. FMI 5 indicates current below normal — an open circuit from a broken wire, corroded connector, or failed injector solenoid winding. The amber MIL illuminates immediately and the fault persists until the open circuit is repaired.

Symptoms Drivers Notice

  • Rough idle and noticeable miss under acceleration
  • Amber MIL illuminated on Mack dash
  • Cylinder 2 shows zero contribution on MDP diagnostic cutout test
  • Possible slight increase in exhaust smoke if unburned fuel enters the dead cylinder
  • Fuel economy drop of 5%-8% from running on one fewer cylinder

Most Likely Root Causes

Chafed or broken valve cover injector harness wire

Very Common

The harness vibrates against the rocker shaft through the valve cover grommet, eventually breaking the conductor and creating an open circuit.

Corroded or backed-out connector pin at injector 2

Common

Moisture and heat cycles at the valve cover connector corrode pin contacts until resistance exceeds the ECM detection threshold.

Failed injector solenoid open winding

Occasional

Internal coil fatigue from heat and vibration opens the solenoid winding, typically past 400,000 miles on MP8 engines.

Common Misdiagnoses (Don't Waste Parts)

Replacing the injector without an ohms check at the connector

Why it happens: A parts-cannon approach assumes the injector is always at fault since SPN 652 names cylinder 2.

Do this instead: Ohm the injector at the connector before removing the valve cover. Five minutes of electrical testing saves an unnecessary injector pull.

Skipping the harness wiggle test during live data monitoring

Why it happens: Techs treat the open circuit as permanent when it may be intermittent from a chafed wire making intermittent contact.

Do this instead: Flex the harness by hand near the valve cover grommet while watching Jaltest live data — a flickering fault confirms wiring, not injector.

Blaming the ECM without end-to-end harness continuity test

Why it happens: Fault persists after injector swap, leading to ECM condemnation.

Do this instead: Verify continuity from injector connector to ECM driver pin per wiring diagram before suspecting ECM failure.

Master Tech Commentary

Mechanic's Notes

ProfessionalDieselRepair.com sees Mack SPN 652 FMI 5 most often from valve cover harness chafe, not failed injectors. Wiggle the harness at the rocker housing with Jaltest live data before pulling the injector. Parts-cannon injector orders on open-circuit codes waste $500 when a $30 harness repair fixes it.

Step-by-Step Diagnostic Procedure

  1. Step 1: Connect Mack Diagnostic Protocol (MDP) via Jaltest or Texa and confirm SPN 652 FMI 5. Run cylinder cutout test — cylinder 2 should show no RPM drop when disabled, confirming the injector is not firing.
  2. Step 2: Shut engine off, unplug injector 2 connector at the valve cover. Inspect pins for corrosion, backed-out sockets, or melted plastic from arcing.
  3. Step 3: Measure resistance across the two injector solenoid terminals — Mack MP8 unit injectors typically read 0.5-3.0 ohms. Infinite (OL) resistance confirms open winding inside the injector.
  4. Step 4: If injector ohms good, check harness continuity from the connector to the ECM injector driver pin using a wiring diagram. Mack MP8 valve cover harnesses fail at the grommet where the loom crosses the rocker shaft.
  5. Step 5: Clear codes in Jaltest after repair, restart, and re-run cutout test. Cylinder 2 RPM drop should match other cylinders within 20 RPM. Wiggle-test the harness during live monitoring to confirm no intermittent open.

Live Data & Freeze-Frame Tips

In Jaltest, run cylinder cutout and watch RPM drop per cylinder. Cylinder 2 with FMI 5 shows zero drop. Monitor injector driver activation voltage — a healthy circuit shows square-wave pulses; an open circuit shows no driver activity on cylinder 2.

Safety Warnings

  • Do not probe injector connectors with the engine running.
  • Disconnect batteries before valve cover service.
  • Allow engine to cool — valve cover and exhaust manifold are adjacent on MP8.
  • Wear safety glasses when cleaning corroded connector pins.

When to Limp In vs Tow

Single-cylinder FMI 5 rarely requires towing. Tow if severe derate develops or multiple cylinder injector codes appear together.

Prevention Tips for Fleets & Owner-Operators

  • Inspect valve cover harness routing at every PM — replace grommets showing wear.
  • Use dielectric grease on injector connectors during reassembly.
  • Address intermittent rough idle before it becomes a hard open-circuit fault.
  • Replace valve cover gasket and harness clips together on MP8 injector service.

Related Fault Codes

SPN 651 FMI 5 — Cylinder 1 Injector

Cylinder 1 open circuit — shared harness chafe point if both appear together.

SPN 652 FMI 6 — Injector Solenoid Driver Circuit - Cylinder #2

Cylinder 2 short circuit — opposite electrical failure on the same circuit.

SPN 653 FMI 5 — Injector Solenoid Driver Circuit - Cylinder #3

Cylinder 3 open circuit — adjacent cylinder, check for pattern indicating harness damage.

Frequently Asked Questions

What does Mack SPN 652 FMI 5 mean?

It means the ECM sees an open circuit in the cylinder 2 injector solenoid driver — current below normal. The injector cannot fire until the circuit is repaired.

Can I drive with SPN 652 FMI 5 active?

Yes, the truck runs on remaining cylinders with reduced power and rough idle. It is not a shutdown code, but repair promptly to avoid fuel washing and oil dilution.

What is normal injector resistance on Mack MP8?

Measure 0.5-3.0 ohms across the solenoid terminals. Infinite resistance means open winding. Below 0.3 ohms means short (FMI 6 territory).

How do I reset SPN 652 FMI 5 without a scanner?

You cannot reliably clear it without Jaltest or Texa. Battery disconnect may temporarily clear the MIL, but the fault returns on the next drive cycle if the open circuit remains.

Does this code cause derate on Mack?

Most MP8 calibrations apply mild power reduction with a single disabled cylinder, but not the aggressive aftertreatment-style derate.

Which scan tools read Mack injector live data?

Jaltest and Texa provide cylinder cutout and injector driver data on MP7, MP8, and MP10 engines. Mack MDP is the OEM interface through dealer tools.

More Mack codes on this system

All Mack fault codes

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