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Diagnostic code DIAG 03404-02 on MAN heavy-duty vehicles is interpreted by the MAN-Cats / J1939 diagnostic convention as:
• SPN 3404 – SCR (DEF/AdBlue) return-line / purge circuit
• FMI 02 – “Data erratic, intermittent or incorrect”
In plain language the ECU detects an implausible pressure/flow behaviour during the post-key-off purge that draws AdBlue back from the dosing line into the tank.
Key points
• Concerns the Selective Catalytic Reduction (SCR) system, not the cooling circuit.
• Typical reasons: partial crystallisation (urea), kinked or pinched return hose, defective supply-module reversing valve, pressure-sensor or wiring noise.
• Code is often logged as “passive” because the purge is executed only after ignition OFF; it becomes “active” if the fault repeats.
Code structure
SPN (Suspect Parameter Number) 3404 uniquely points to the SCR return-line / purge subsystem in MAN’s J1939 implementation.
FMI (Failure Mode Identifier) 02 = signal is present but implausible (erratic, intermittent, out of range).
Functional background
• At key-off the supply module reverses and sucks DEF out of the injector line to prevent freezing (-11 °C) and crystallisation.
• Line pressure is monitored by a transducer inside the supply module; the ECU expects a smooth pressure drop to ≈0 bar within a calibrated time.
• Any restriction or bad sensor feedback produces pressure traces that violate the expected profile, triggering SPN 3404/FMI 02.
Most frequent technical root causes
A. Deposits / crystallisation inside return line, quick-couplings or filter.
B. Kinked / pinched hose after repair work or chassis movement.
C. Supply-module internal fault – sticking reversing valve or worn pump vanes.
D. Pressure sensor drift or water intrusion into its connector.
E. Intermittent wiring/contact – fretting, corrosion, unlocked pin.
F. Air ingress on suction side causing oscillating pressure.
Symptom pattern
• Fault usually “passive”; no MIL while driving.
• May be accompanied by SPN 3364, 4334, 5246 (general SCR faults) if purge repeatedly fails.
• If ignored, crystallisation progresses → dosing blockage → active derate (max 20 km/h) mandated by Euro VI OBD.
Most recent MAN-Cats software (v15.x, 2024) keeps the same definition for SPN 3404 FMI 02. Field reports in 2023–24 document an increase in winter-related crystallisation events after fleets switched to cheaper DEF suppliers. Newer supply-modules (HW-03 and later) include a fine-mesh inlet filter to mitigate the issue.
Pressure profile example (good vs. faulty purge):
Pressure (bar)
4 | ________ ___
3 | / \ / \
2 |----------/ \----------------/ \-----
1 | / \ / \
0 |________/ \____________/ \__ time
good purge erratic/fluctuating (FMI 02)The ECU samples this curve; overshoot, ripple > ±0.3 bar or time-to-zero > x s sets the fault.
Manipulating or disabling SCR components to mask this code violates Euro VI / EPA-10 emissions legislation and can lead to heavy fines and loss of licence. Repairs must maintain DEF purity (ISO 22241) and use approved parts.
Validate the fault
• Read freeze-frame; note purge duration, start & end pressures.
• Check for additional SCR codes to gain context.
Visual inspection
• Tank level & DEF quality (clear, colourless).
• Hoses for kinks/chafe; white crust = leak & crystallisation.
Actuation test (with MAN-Cats or Jaltest)
• Run “Return-flow / purge test” while watching live pressure.
• Good system: drop from 4–5 bar to <0.2 bar within spec time (≈6–10 s).
Isolation steps
a) Blow return hose with low-pressure dry air; flush with warm de-ionised water if resistance felt.
b) If hose is clear, suspect supply-module. Measure current draw; >2 A ripple points to worn pump.
c) If mechanical path OK but pressure trace still noisy, scope the sensor line (expected 0–5 V, <20 mV noise).
Rectification
• Replace or refurbish blocked hose, inline filter, or supply-module as verified.
• Fit new O-rings; lubricate with DEF-compatible grease.
• Replace defective sensor; repair wiring using sealed, crimp-solder splice sleeves.
Validation
• Clear codes; execute two consecutive purge cycles.
• Road test, key-off, wait >5 min, re-scan.
Preventive measures
• Use ISO 22241-1 certified DEF.
• Insulate exposed lines in cold climates.
• Schedule annual system flushing where ambient <-15 °C.
Potential challenges & mitigation
• Fleet schedules rarely allow cool-down time – plan tests at shift end.
• Crystallised quick-couplings may crack – keep spares on hand.
Contradictory on-line sources sometimes list 03404-02 as “low coolant level”. Cross-checking with official MAN SPN/FMI tables (Edition 2023) shows coolant level is SPN 0111; therefore 03404-02 does not correspond to coolant level in MAN Euro V/VI trucks. Treat any coolant-related interpretation with caution unless your specific model deviates from the standard SPN mapping.
• Investigate supply-module HW-03 reliability vs. earlier HW-01/02 in cold climates.
• Explore ultrasonic flow sensing as a non-contact alternative to pressure-profile purge validation.
• Monitor DEF conductivity correlation with crystallisation frequency.
Reference material
• MAN Truck & Bus AG – Fault Code Compendium, Rev. 2023.
• SAE J1939-73 (2022) – Application Layer – Diagnostics.
• ISO 22241-1 (2021) – Diesel engines — NOx reduction agent AUS 32.
DIAG 03404-02 flags an erratic feedback from the SCR/AdBlue return-purge circuit (SPN 3404, FMI 02). The majority of cases stem from partial urea crystallisation or sensor/wiring issues. Perform a pressure-monitored purge test, clear physical blockages, verify sensor integrity and wiring, and re-validate. Ignore coolant-level interpretations unless confirmed by model-specific documentation. Timely repair prevents full SCR blockage and legally mandated power derate.