SPN 3362 FMI 5: Meaning and Fix
SPN 3362 FMI 5 signals an open circuit or current below normal in the DEF dosing unit input lines for bank 1. This fault commonly appears after a forced DPF regeneration when the dosing unit harness is inadvertently damaged by heat or vibration. Technicians frequently encounter this code after replacing the ECM if the connector pins were not fully seated, leading to intermittent open circuits.
Common Symptoms
- Engine Derate: ECM activates a 25% torque reduction to protect aftertreatment components from overdosing or underdosing.
- DEF Dosing Halt: Dosing unit ceases injection; DEF pump continues running but no fluid reaches the decomposition reactor.
- MIL Illumination: Red malfunction indicator lamp and amber warning lamp activate on the dashboard within one drive cycle.
- Exhaust Leak: Unmetered air from the open circuit may cause a hissing sound near the dosing unit during key-on engine-off tests.
Probable Causes
- Open Harness: Broken wire between ECM pin J2-45 and dosing unit connector due to chafing against the engine block.
- Corroded Connector: Moisture ingress at the 6-pin dosing unit connector causes resistance exceeding 10 kΩ, mimicking an open circuit.
- Failed Solenoid: Internal coil break in the air or DEF input solenoid, typically measured as infinite resistance across pins 2 and 4.
- ECM Fault: Internal driver failure on ECM output channel; less common, but verified by swapping with a known-good ECM in MAN TGS trucks.
Advanced Technical Analysis
The ECM monitors current flow through the dosing unit input line solenoid driver using a high-side MOSFET. When the commanded duty cycle produces a feedback current below 50 mA for 2.5 seconds, the microcontroller sets the FMI 5 fault. The debouncing timer prevents false triggers from transient noise during engine cranking.
Electrical analysis of the open circuit path reveals that the solenoid coil typically measures 4.5–6.0 Ω at 20°C. An infinite reading or a value above 10 Ω indicates a break. A voltage drop test across the connector while the ECM commands the solenoid on should show battery voltage; if absent, the harness or ECM driver is open.
Upon detecting the open circuit, the ECM de-energizes the DEF dosing unit and activates a safety fallback: NOx conversion efficiency is reduced, and the engine enters a soft derate (25% torque reduction) after 1 hour of operation. The DPF regeneration is inhibited to prevent thermal damage from uncontrolled DEF injection.
Long-term prevention includes installing a dielectric grease pack in the dosing unit connector and routing the harness away from exhaust heat shields. In workshop practice, this code is often misdiagnosed as a pump failure; proper procedure requires first measuring solenoid resistance at the unit before replacing any component.
Step-by-Step Troubleshooting Guide
- Resistance Check: Measure resistance between pins 2 and 4 of dosing unit connector; spec 4.5–6.0 Ω. Open reading confirms solenoid fault.
- Harness Integrity: Backprobe ECM connector J2-45; command solenoid on with diagnostic tool. Voltage should be 24V; if 0V, harness is open.
- Connector Inspection: Disconnect and inspect both harness and unit connectors for bent, corroded, or pushed-back terminals. Clean with contact cleaner.
- ECM Swap Test: If harness and solenoid test good, swap ECM with a known-good unit. If fault moves, replace ECM driver circuit.