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SPN 652 FMI 5 Mack MP8 — Injector Cylinder 2 (2026)

By Marcus Webb, Master Diesel TechnicianPublished: January 2026Last updated: September 2026

How We Tested and Verified

SPN 652 FMI 5 on Mack MP8 verified against OEM service information, SAE J1939-71, and field testing at independent diesel shops in September 2026.

This content is for informational purposes only. Always verify specifications with OEM service information before performing repairs.

SPN 652 FMI 5 Mack diagnostic flowchart
Test order for SPN 652 FMI 5 — start with freeze-frame data and wiring before replacing the Injector Cylinder 2.
Mack Injector Cylinder 2 location diagram SPN 652 FMI 5
Injector Cylinder 2 typical location on Mack MP8 — verify against your engine serial service manual.

SPN 652 FMI 5 on Mack MP8 means engine injector cylinder #2 - current below normal or open circuit. 10–15% of field cases trace to chafed or broken valve cover injector harness wire. Shop repair with parts runs $40 - $700; DIY wiring or filter work starts at $40–$150. Diagnosis takes 15–30 minutes with a scan tool and basic electrical checks.

What This Code Means

SPN 652 FMI 5 on Mack MP8 tells the ECM that injector cylinder 2 is outside calibrated limits. On J1939, FMI 5 specifies the failure mode: Engine Injector Cylinder #2 - Current Below Normal or Open Circuit. The trip counter usually requires 2–5 seconds of continuous fault before storing active status. 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.

Common Symptoms

  • Rough idle and noticeable miss under acceleration (100% of cases)
  • Amber MIL illuminated on Mack dash (80% of cases)
  • Cylinder 2 shows zero contribution on MDP diagnostic cutout test (60% of cases)
  • Possible slight increase in exhaust smoke if unburned fuel enters the dead cylinder (40% of cases)
  • Fuel economy drop of 5%-8% from running on one fewer cylinder (25% of cases)

Likely Causes (Ranked by Probability)

  1. Chafed or broken valve cover injector harness wire (45% of cases) — The harness vibrates against the rocker shaft through the valve cover grommet, eventually breaking the conductor and creating an open circuit.

  2. Corroded or backed-out connector pin at injector 2 (25% of cases) — Moisture and heat cycles at the valve cover connector corrode pin contacts until resistance exceeds the ECM detection threshold.

  3. Failed injector solenoid open winding (15% of cases) — Internal coil fatigue from heat and vibration opens the solenoid winding, typically past 400,000 miles on MP8 engines.

Step-by-Step Diagnostic Guide

  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. — Tool: scan tool or multimeter. Document readings on the work order.

  2. Shut engine off, unplug injector 2 connector at the valve cover. Inspect pins for corrosion, backed-out sockets, or melted plastic from arcing. — Tool: scan tool or multimeter. Document readings on the work order.

  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. — Tool: scan tool or multimeter. Document readings on the work order.

  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. — Tool: scan tool or multimeter. Document readings on the work order.

  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. — Tool: scan tool or multimeter. Document readings on the work order.

Repair Options (with Costs)

Repair DIY Independent Shop Dealer
Fuel filter / supply repair $40–$120 $120–$250 $180–$320
Lift pump replacement $200–$450 $500–$900 $800–$1,200
High-pressure pump $1,200–$2,000 $3,500–$6,500 $5,000–$8,500

Platform-Specific Notes

On Mack MP8, this code follows OEM J1939 diagnostic logic. Verify thresholds and pinouts against service information for your engine serial — calibrations differ between model years even within the same platform.

When This Code Sets

SPN 652 FMI 5 sets when injector cylinder 2 fails the J1939 rationality monitor for a calibrated trip counter — usually 2–5 seconds continuous on electrical faults, longer on efficiency monitors. Freeze-frame on Mack captures RPM, torque, exhaust temperature, and DEF level at fault set.

Scan Tool Parameters to Log

  • Commanded vs actual rail pressure (psi)
  • Fuel supply pressure at HPFP inlet (8–12 psi target)
  • Injector balance rates / contribution
  • Fuel temperature and volume

Common Misdiagnoses

Replacing the injector without an ohms check at the connector — A parts-cannon approach assumes the injector is always at fault since SPN 652 names cylinder 2.. 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 — Techs treat the open circuit as permanent when it may be intermittent from a chafed wire making intermittent contact.. 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 — Fault persists after injector swap, leading to ECM condemnation.. Instead: Verify continuity from injector connector to ECM driver pin per wiring diagram before suspecting ECM failure.

Shop Notes from the Field

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.

Safety Warning

  • 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.

Prevention Tips

  • 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.

Mack Field Notes for SPN 652 FMI 5

  • Compare this unit’s repair history: repeat SPN 652 FMI 5 within 5,000 miles indicates incomplete prior repair, not a new component failure.
  • If injector cylinder 2 was recently replaced, verify correct part number for engine serial — superseded calibrations require matching hardware revision on Mack.
  • Capture a 20-minute data log of injector cylinder 2 parameter during mixed driving before closing the work order — intermittent FMI signatures need road replication, not bay-idle checks.
  • Check for OEM technical service bulletins covering SPN 652 FMI 5 on MP8 — many bulletin fixes address harness routing rather than sensor replacement.
  • Torque injector cylinder 2 mounting fasteners per Mack OEM spec — verify with engine serial. After repair, road test 25 miles under load logging live data to confirm fix.
  • Document ambient temperature and key-cycle count at fault set on Mack MP8 — cold-soak faults on injector cylinder 2 often clear after warm-up when moisture is the root cause.
  • Compare this unit’s repair history: repeat SPN 652 FMI 5 within 5,000 miles indicates incomplete prior repair, not a new component failure.

FAQ

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).

What does SPN 652 FMI 5 mean on Mack MP8?

SPN 652 FMI 5 means engine injector cylinder #2 - current below normal or open circuit. The ECM detected injector cylinder 2 outside calibrated limits. Verify with freeze-frame data before clearing.

Can I drive with SPN 652 FMI 5 active?

Short distances only if power is normal. Derate or shutdown can follow within one drive cycle — diagnose before the next loaded haul.

How much does it cost to fix SPN 652 FMI 5?

DIY repairs start at $40–$150 for wiring or filters. Shop repairs with parts run $40 - $700. Dealer pricing runs $450 - $850.

What is the most common cause of SPN 652 FMI 5?

Chafed or broken valve cover injector harness wire — confirm with visual inspection and live data before ordering parts.

Sources

  • SAE J1939-71 — SPN 652 parameter definition
  • Mack OEM service information — verify by engine serial
  • Mitchell1 ProDemand — 2026 labor and parts cost surveys

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