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Key points
• System involved: Selective Catalytic Reduction (SCR) / AdBlue dosing module
• Type of fault: Electrical or hydraulic signal implausibility (FMI 02) – often stored as “passive/inactive”
• Typical causes: Crystallised AdBlue in return hose, kinked line, leaking connections, failing pump/3 ⁄ 2-way valve, or erratic DEF-line-pressure sensor/wiring.
Code structure
• First digit “5” – MAN system group “Engine / exhaust after-treatment” (ECU: EDC/PLD or EDC17CV).
• SPN 3404 – “SCR return (purge) line pressure / flow”.
• FMI 02 – “Data erratic, intermittent or incorrect” per SAE J1939-73.
Function of the purge circuit
After ignition OFF the supply module reverses and draws AdBlue out of the dosing line (4–5 bar ➜ ≈0 bar in ≤ 10 s) to avoid crystallisation. Pressure is measured continuously. Deviation from the learned pressure-drop curve raises 5 3404-02.
Electrical / hydraulic path
Dosing Control Unit → 3 ⁄ 2-way valve ↔ pump ↔ line ↔ dosing valve → pressure sensor feedback → ECU (5 V ref, signal, ground). Any mismatch between command and observed pressure/flow triggers the fault.
Observable symptoms
• DTC shown as passive; MIL may stay off until a certain counter is exceeded.
• Recurrent message on key-on self test.
• Possible derate if accompanied by SPN 5246 FMI 0/16.
• Extended purge time or “buzzing” noise after switch-off.
Probability ranking of root causes
Diagnostic workflow (field-proven)
a. Read freeze-frame – coolant temp, pressure at start/end of purge, purge duration.
b. Visual check: DEF level ≥ 20 %, crystal traces on quick-couplings, hose kinks.
c. Warm-water (≤ 60 °C) dissolve test for visible crystals; replace 6 × 4 mm hose if stiff.
d. Electrical:
– 5 V ref, signal, ground continuity (< 0.1 Ω) to EDC pin A38, A55, B11.
– Sensor resistance (disconnected) 220–260 Ω @ 20 °C.
e. Actuation test with MAN-cats III: command purge, watch pressure 5 bar ➜ 0 bar in 6–10 s.
f. Current draw of pump 1.2–1.6 A (steady); spikes > 2.2 A indicate mechanical drag.
Corrective actions
• Flush and refill with ISO 22241-1 compliant AdBlue; vacuum-evacuate crystals.
• Replace return hose & O-rings; torque quick couplings 10 Nm.
• Supply module repair kit (MAN 81.15408-6070) or full module 81.15408-6041 if valve sticking.
• Replace pressure sensor (BOSCH 0 281 006 XXX) after verifying PID drift.
• Update ECU software (EDC17CV42 SW-level ≥ ZGS 006) to extend purge window.
• GEN2 dose modules (introduced late 2022) use internal temperature-compensated MEMS pressure sensors and a brush-less pump, largely eliminating 5 3404-02.
• MAN now recommends winter grade DEF (– 37 °C crystallisation point) for Nordic fleets.
• Remote diagnostics via RIO or MAN-Telematics pushes SPN/FMI decoding directly to workshop PCs, cutting troubleshooting time.
• Analogy: Think of the purge like sucking liquid out of a straw; if the straw is pinched or blocked you feel resistance (pressure drop not matching time). The ECU observes the same mismatch.
• A passive FMI 02 is similar to a stored “misfire history” – a hint something was wrong, even if it is OK now.
• Tampering or bypassing the purge circuit to silence the DTC violates Euro V/VI emissions law (Reg. (EC) 595/2009) and may lead to fines and loss of type approval.
• Always dispose of contaminated AdBlue according to local environmental regulations; it is classified as non-hazardous but high-nitrogen.
• Keep DEF above 10 % to minimise air ingestion.
• Store AdBlue at 0–25 °C, sealed; older fluid (> 12 months) greatly accelerates crystallisation.
• During winter service add line-heater inspection to maintenance sheet; an iced line fools the purge pressure profile.
Possible challenges & mitigation
• Intermittent faults: Use oscilloscope logging during purge; vibration-induced wiring breaks become visible.
• Remote sites: Carry premixed warm-water bottles and hose couplings to do on-road de-crystallisation if workshop is far away.
• Some early MAN BMS software versions map SPN 3404 to “retarder pressure control valve”. Cross-check the control unit that stored the code:
– If code is under ECU 5 (EDC/Engine) ➜ SCR purge (most cases).
– If under ECU 5 (BMS) in special coach chassis, the same numeric code can relate to the retarder valve. Always read the source ECU ID to avoid mis-diagnosis.
• Study MAN TSBs on “SCR GEN2 supply module diagnostics” (2023-08).
• Investigate machine-learning models that predict crystallisation events based on ambient RH/temperature logs.
• Examine alternative DEF formulations with lower evaporation rate to reduce crystal formation.
Recommended resources
– MAN-cats III user manual, chap. 8.3 “AdBlue pressure analysis”
– ISO 22241-1/-2 for DEF quality & handling
– SAE paper 2022-01-0598 on next-gen SCR pumps
Error 5 3404-02 flags an implausible pressure/flow signal in the SCR return-line purge. In 90 % of cases the root cause is physical blockage (crystals, kink) or a drifting pressure sensor. A structured approach—visual check, electrical tests, live purge monitoring—pinpoints the fault quickly. Keep DEF clean, hoses unkinked, and software up to date to prevent recurrence and maintain legal emission levels.
