SPN 4335 FMI 4: Meaning and Fix
This fault indicates the ECM has detected a voltage below the normal operating range on the air assist absolute pressure sensor circuit for the SCR dosing system. The sensor, typically a 5V reference piezoresistive type, monitors atomization air pressure. Technicians often see this after a forced DPF regeneration when wiring harnesses near the exhaust become heat-damaged or chafed against the frame rail, causing a short to ground.
Common Symptoms
- Reduced Atomization: Poor reagent atomization leads to white ammonia slip or incomplete NOx conversion.
- Engine Derate: ECM initiates a torque reduction of up to 40% to protect aftertreatment components.
- MIL Illumination: Red or amber malfunction indicator lamp activates immediately on the dash.
- Frozen DEF Dosing: Dosing valve is disabled; no DEF injected, causing eventual catalyst damage.
Probable Causes
- Shorted Sensor Wiring: Signal wire chafed against exhaust bracket or frame rail causing a direct ground.
- Failed Pressure Sensor: Internal bridge circuit shorted; sensor outputs less than 0.2V at key-on.
- ECM Internal Fault: Pull-up resistor or ADC input damaged, often from previous overvoltage events.
- Connector Corrosion: Moisture ingress in the 3-pin Deutsch connector, bridging signal to ground.
Advanced Technical Analysis
The ECM monitors the sensor signal line through a 5V pull-up resistor to a 10-bit ADC. A reading below 0.5V (approx. 6.25 kPa) for 2 consecutive seconds sets the fault. The ECM expects 0.5–4.5V corresponding to 0–400 kPa absolute. A short to ground pulls the line below 0.2V, triggering FMI 4.
Electrical analysis shows the sensor uses a Wheatstone bridge with a typical output of 20–40 mV/V. The ECM amplifier stage multiplies this. A short at the connector or internal bridge failure causes the ADC input to saturate low. A debouncing timer of 500 ms prevents false triggers from transient noise.
Upon fault confirmation, the ECM sets the air assist solenoid to a fixed duty cycle of 0% and disables the dosing valve. Torque is derated by 25–40% depending on OEM calibration. The SCR efficiency monitor is suppressed to prevent false ammonia slip faults during the derate.
Long-term diagnostics should include thermal imaging of the harness near the exhaust manifold. In MAN D08 series engines, this fault frequently recurs after turbocharger replacement if the wiring clip is not reinstalled. Always perform a wiggle test with the sensor powered to identify intermittent shorts.
Step-by-Step Troubleshooting Guide
- Visual Inspection: Check harness from sensor to ECM for chafing, burns, or corrosion near exhaust components.
- Voltage Check: Measure signal pin voltage at sensor connector; should be >0.5V with key-on, engine-off.
- Sensor Resistance Test: Disconnect sensor; measure resistance between signal and ground – infinite resistance is normal.
- ECM Pin Integrity: Backprobe ECM connector pin; if voltage is low with sensor disconnected, suspect ECM fault.