SPN 2798 FMI 4: Meaning and Fix
This fault indicates the ECM detected voltage below normal or a short to low on the injector group 2 common supply or return circuit. In practice, this code commonly appears after a forced DPF regeneration when wiring harnesses near the exhaust manifold become heat-damaged, causing intermittent shorts that ground the injector group.
Common Symptoms
- Rough Idle: Engine runs unevenly at idle due to multiple injectors in group 2 not firing correctly.
- Misfire Under Load: Noticeable power loss and misfire when engine is under heavy load or acceleration.
- Hard Start: Extended cranking time as the ECM struggles to synchronize fuel delivery from the affected injector group.
- Diagnostic Lamp On: Amber or red warning lamp illuminates on the dash; fault logged in ECM memory.
Probable Causes
- Harness Chafing: Wires shorting to ground where harness rubs against engine block or exhaust manifold.
- Injector Solenoid Failure: Internal short to ground within one or more injectors in group 2.
- Connector Corrosion: Moisture ingress at the ECM or injector connectors causing low-resistance path to ground.
- ECM Driver Damage: Failed internal output driver for injector group 2, often due to previous overcurrent event.
Advanced Technical Analysis
The ECM continuously monitors the voltage on the injector group 2 common supply line during the injection dwell period. When the voltage falls below a calibrated threshold (typically < 2.0 V for 12V systems) for more than 50 ms, it sets FMI 4. The ECM uses a pull-up resistor network to detect shorts; a persistent low voltage indicates a hard short to ground.
Electrical analysis reveals that the injector group 2 circuit is shared among 2-4 injectors depending on engine configuration (e.g., Bosch common-rail systems). A single faulty injector solenoid with 0.5 ohms or less resistance can pull the entire group voltage down. Technicians should measure resistance between the injector group pin and engine ground; values below 1 kohm indicate a short.
When this fault is active, the ECM deactivates the entire injector group 2 as a safety fallback. This triggers an immediate torque derate of up to 50% to prevent engine damage from incomplete combustion. The ECM also disables cylinder cut-out tests for that group, forcing a manual electrical inspection before any further diagnostics can proceed.
Long-term prevention requires inspecting wiring harness routing after any engine work. In one documented MAN D2676 case, a technician found the harness was pinched under a valve cover gasket after a cylinder head replacement. Using Deutsch DT-series connectors with silicone seals and applying dielectric grease on injector terminals reduces recurrence of moisture-related shorts.
Step-by-Step Troubleshooting Guide
- Visual Inspection: Inspect injector group 2 harness for chafing near exhaust manifold and engine block grounding points.
- Resistance Check: Measure resistance from group 2 pin to ground; should be > 1 MΩ. Low reading indicates short.
- Injector Isolation: Disconnect each injector in group 2 one by one; monitor resistance change to isolate faulty unit.
- ECM Output Test: Use breakout box to command injector group 2; verify voltage at ECM pin rises above 10V during pulse.
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