SPN 4335 FMI 31: Meaning and Fix
SPN 4335 FMI 31 indicates the engine ECM has detected a continuous air assist absolute pressure condition outside the expected range for aftertreatment 1 SCR dosing. This code commonly appears after a forced DPF regeneration when residual soot clogs the air assist nozzle, or when a technician inadvertently leaves the air supply valve closed after service. The ECM monitors pressure via a dedicated sensor on the dosing unit.
Common Symptoms
- Derated Power: ECM reduces engine torque by up to 40% to protect SCR components from incomplete reagent atomization.
- Malfunction Lamp: Amber warning lamp illuminates on dashboard, often accompanied by a stop engine lamp if pressure drops critically.
- Poor Atomization: Visible white reagent mist is replaced by liquid streaks on the mixer, leading to deposit formation.
- Frequent Regen: DPF regeneration cycles become more frequent due to unburned reagent coating the diesel oxidation catalyst.
Probable Causes
- Blocked Air Line: Carbon deposits or moisture ice block the compressed air supply line between the chassis tank and dosing unit.
- Failed Pressure Sensor: Internal diaphragm rupture or signal drift causes the sensor to report a constant value outside the valid range.
- Air Supply Valve: Solenoid valve stuck closed or partially open due to coil failure or debris, preventing correct pressure regulation.
- ECM Calibration: Outdated software in the engine control module misinterprets sensor readings, triggering false condition exists faults.
Advanced Technical Analysis
The ECM evaluates the air assist absolute pressure signal from a 0.5–4.5 V ratiometric sensor. At key-on, the microcontroller checks the plausibility against atmospheric pressure. If the signal deviates by more than ±15 kPa for 10 seconds continuously, the FMI 31 condition exists flag is set. The debouncing timer prevents transient spikes from triggering a fault, but a sustained offset confirms a hardware issue.
Electrical analysis of the sensor circuit reveals that a short to ground pulls the signal below 0.25 V, while an open line floats near 5 V. The ECM’s internal pull-up resistor creates a diagnostic voltage of 4.8 V when the circuit is broken. Technicians frequently encounter this after replacing the ECM without properly reseating the sensor connector, causing intermittent contact that eventually sets FMI 31.
Once the fault is active, the ECM enters a safety fallback: it disables SCR dosing entirely and sets a torque derate of 25% after 30 minutes of operation. If the condition persists beyond one hour, the derate increases to 50%. This protects the SCR catalyst from thermal damage caused by raw urea impinging on the substrate, which can cause irreversible deactivation.
Long-term prevention requires verifying the air assist line is routed away from heat sources to prevent carbonization. In real-world workshops, this fault frequently follows a DPF replacement where the air line was disconnected and reconnected with debris inside. A full pneumatic flush and sensor recalibration using a J1939 diagnostic tool with manufacturer-specific data links is the only reliable fix.
Step-by-Step Troubleshooting Guide
- Visual Inspection: Check air assist lines for kinks, cracks, or ice blockage; verify the air supply valve is open and pressurized.
- Sensor Test: Measure sensor output voltage at idle; should be 1.0–1.5 V at atmospheric pressure (approx. 100 kPa).
- Pneumatic Flush: Disconnect air line at dosing unit, apply 800 kPa shop air for 30 seconds to clear carbon debris.
- ECM Relearn: Perform a zero-pressure calibration using the manufacturer’s diagnostic tool to reset the pressure offset value.