SPN 3362 FMI 2: Meaning and Fix
SPN 3362 FMI 2 indicates the ECM detects erratic, intermittent, or incorrect data from the diesel exhaust fluid dosing unit input lines for Bank 1. This fault commonly appears after a forced DPF regeneration when thermal cycling stresses connectors, causing intermittent contact on the DEF pump or air pressure sensor lines. Technicians frequently encounter this fault after replacing the ECM if harness pins are not fully seated.
Common Symptoms
- Intermittent MIL: Malfunction indicator lamp flashes on and off unpredictably during highway cruising or light load.
- Derate Active: Engine power gradually reduces to 25% after 60 minutes of continuous fault presence.
- No DEF Dosing: Dosing unit fails to inject DEF, leading to NOx conversion inefficiency and increased tailpipe emissions.
- Fault Code Log: Multiple logged occurrences of SPN 3362 FMI 2 with timestamps showing random activation intervals.
Probable Causes
- Harness Chafing: DEF dosing unit harness rubs against chassis frame, causing intermittent short or open circuits.
- Connector Corrosion: DEF fluid wicking into the 6-pin connector creates green corrosion and erratic pin resistance.
- Air Pressure Fluctuation: Compressed air supply to dosing unit drops below 5.5 bar intermittently due to wet tank issues.
- ECM Pin Fretting: Micro-motion at ECM J2 connector pins causes intermittent contact on DEF pump signal lines.
Advanced Technical Analysis
The ECM microcontroller continuously monitors the DEF dosing unit input lines using a pull-up voltage (typically 5 VDC) and a series resistor. When the line voltage deviates outside the valid window (0.5 V to 4.5 V) for more than 50 ms, the firmware sets FMI 2. This debouncing timer prevents false triggers from transient noise but allows genuine erratic signals to be captured. The fault is stored as active only when the voltage crosses the thresholds at least three times within a 10-second window.
Electrically, a broken shield ground on the twisted pair can induce common-mode noise that the differential receiver interprets as erratic data. The ECM uses a 10 kHz sampling rate on these lines; any frequency above 12 kHz from alternator ripple or injector pulse interference will be flagged as intermittent. Real-world examples include aftermarket LED lighting installed near the DEF harness, which injects high-frequency noise into the signal lines.
When FMI 2 is active, the ECM enters a safety fallback: it disables DEF dosing and commands a torque derate of 1.5% per minute until a maximum of 40% reduction is reached. Simultaneously, the aftertreatment controller records the event in the non-volatile memory. The derate persists until the fault becomes inactive for a continuous 120-second period, after which dosing resumes at a reduced rate for 10 minutes before full power is restored.
Long-term diagnostic strategy involves comparing the DEF dosing unit input line voltage against the known good value of 2.5 V ±0.3 V at key-on engine-off. In workshops, a common finding after repeated SPN 3362 FMI 2 occurrences is a loose terminal in the DEF pump connector (Deutsch DT06-6S). Technicians should perform a 10-cycle connector mate/unmate test and measure pin retention force with a calibrated pull tester to prevent recurrent faults.
Step-by-Step Troubleshooting Guide
- Voltage Check: Measure voltage at DEF dosing unit connector pin A (signal) and pin B (return) with key on; should be 2.5 V ±0.3 V.
- Harness Wiggle: Wiggle the DEF harness while monitoring live data; look for voltage spikes outside 0.5–4.5 V range indicating intermittent contact.
- Air Pressure Test: Connect a pressure gauge to the dosing unit air inlet; verify steady 6.0–8.5 bar during engine idle and load.
- Connector Overhaul: Remove and inspect the 6-pin connector for corrosion; replace terminals if retention force is below 5 N.