SPN 3362 FMI 31: Meaning and Fix
SPN 3362 FMI 31 indicates a condition exists on the diesel exhaust fluid (DEF) dosing unit input lines for bank 1. This includes the air supply line from the air tank and the DEF supply line from the tank. Technicians frequently encounter this fault after a failed forced DPF regeneration where the dosing unit was unable to prime due to restricted lines. The ECM monitors pressure and flow integrity; a sustained anomaly triggers this code.
Common Symptoms
- Dosing Inactive: ECM disables DEF dosing, causing increased NOx emissions and potential derate.
- Reduced Engine Power: Torque derate of up to 25% is applied to protect aftertreatment components.
- Warning Lamp Active: Amber malfunction indicator lamp (MIL) illuminates on the dashboard.
- Failed Regeneration: DPF regeneration aborts or fails to initiate due to dosing unit fault.
Probable Causes
- Line Obstruction: Crystallized DEF or debris blocking the input lines from tank to dosing unit.
- Air Supply Failure: Compressed air line from air tank kinked, leaking, or blocked.
- Dosing Unit Malfunction: Internal valve or heater failure within the dosing unit causing line pressure loss.
- Wiring Harness Damage: Chafed or corroded connector pins at the dosing unit causing intermittent contact.
Advanced Technical Analysis
The ECM monitors a dedicated pressure sensor at the dosing unit inlet. Under normal operation, a 5V reference signal returns a voltage proportional to line pressure. If the signal remains outside a calibrated window (typically 0.5–4.5V) for more than 500 ms, FMI 31 triggers. This logic prevents false positives from transient pressure spikes during priming cycles.
A debouncing timer of 2 seconds is applied before the fault becomes active. The ECM samples the pressure signal at 100 Hz. If the condition persists after the timer expires, the fault is latched. This prevents nuisance codes during cold starts when DEF viscosity is high, but still catches genuine line blockages or air leaks.
Upon activation, the ECM sets a torque derate of 15% immediately, increasing to 25% after 30 minutes of operation. Additionally, the DEF dosing is permanently disabled until the fault is cleared. The ECM also disables active DPF regeneration to prevent thermal damage from incomplete DEF injection.
Long-term prevention requires inspecting the DEF line heater resistance (typically 2.5–3.5 ohms) and air line check valve function. Real-world cases show that replacing the dosing unit without clearing crystallized DEF from the supply line leads to repeat failures within 50 hours. Always flush lines with hot water before component replacement.
Step-by-Step Troubleshooting Guide
- Visual Inspection: Check DEF and air lines for kinks, cracks, or crystallization. Repair or replace as needed.
- Pressure Test: Connect a pressure gauge to the dosing unit inlet; verify 5–8 bar air pressure and 3–5 bar DEF pressure.
- Electrical Check: Measure resistance between dosing unit connector pins; ensure continuity and no shorts to ground.
- Clear Faults: After repairs, perform a key cycle and use diagnostic tool to clear active and inactive fault codes.
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