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‐ On most MAN TGX Euro-6 engines the EDC fault “05 4591-02” (often displayed as EDC 04591-02) is stored when the Electronic Diesel Control detects charge-air / boost pressure above the upper permissible limit (FMI 02 = “data erratic/intermittent/incorrect”).
‐ The code is triggered when the measured boost pressure deviates positively from the target curve for a configurable time window (typically ≥ 150 mbar for > 5 s).
‐ Prime suspects: defective Pierburg turbo-control valve 51.09413-0022, sticky VGT vanes or waste-gate, leaking/blocked intercooler system, or erroneous boost-pressure sensor signal/wiring.
Code structure
• EDC-05 – subgroup of engine-management diagnostics recorded by the Bosch/Continental MD1 or EDC17 controller.
• SPN 4591 – MAN proprietary mapping that, in the Euro-6 TGX calibration, points to the charge-air pressure regulation loop (other brands may map 4591 to an SCR heater – hence the different interpretations you find on the web).
• FMI 02 – “Erratic, intermittent or implausible signal”. Here it flags “Pressure too high/above control range”.
System description
• Turbocharger: Variable-geometry (VGT) unit actuated pneumatically via a Pierburg pulse-width-modulated (PWM) valve.
• Charge-air path: Compressor → intercooler → throttle/intake manifold, with a MAP (Manifold Absolute Pressure) sensor (5 V ratiometric, 10–90 kPa abs range) feeding the ECU.
• Control loop: ECU calculates desired boost from driver torque request + altitude + EGR state → commands VGT position via the PWM valve → monitors real pressure.
Why the fault appears
• VGT vanes stuck in “closed” (high-boost) position by soot/oil → over-boost during acceleration.
• PWM control valve electrically OK but pneumatically leaking or clogged → ECU demands ~60 % duty, vane does not respond fast → overshoot.
• MAP sensor drift (5 V reference fault, water/oil in sensor) → signal reads higher than real.
• Mechanical blockage/down-stream restriction (collapsed intercooler hose, ice, excessive crankcase fumes) → actual pressure spikes.
• Software: outdated calibration with too aggressive smoke-limiter → turbo overshoots; MAN released several ECU SW updates (e.g., TGX_E6_D26.106.x) to soften transient boost.
Expected symptoms
• MIL / “engine” lamp; sometimes yellow “EDC” only.
• Power cut (torque derate 10–40 %) if the fault becomes “static”.
• Audible turbo whine, hiss from hoses, or pop-off through charge-air joint.
• Elevated exhaust gas temperature in data log.
‐ Field reports (MAN dealer TSB 06-22, Elektroda.de thread 3470299, 2024) show > 70 % of 4591-02 cases cured by replacing the Pierburg valve and retorquing VGT linkage.
‐ MAN has migrated latest TGX Euro-6B engines to an electric VGT actuator (no PWM valve) that eliminates this common failure path.
‐ Remote diagnostics via MAN-Now/ RIO platform can stream real-time boost trace to help confirm overshoot events.
Boost control analogy: think of the VGT vanes as the nozzle of a garden hose; closing the nozzle (small area) raises water jet pressure. If the nozzle sticks or the command valve cannot vent, pressure rises above what the “pressure gauge” (MAP sensor) expects, and the ECU flags 4591-02.
Measured values to record (typical spec at 1 500 rpm, 50 % load, sea level):
• Commanded boost: 170 kPa abs
• Measured boost: 165–175 kPa abs (OK) → > 190 kPa triggers pre-warning; > 200 kPa for > 5 s sets 4591-02.
‐ Do NOT defeat the code by deleting it in the ECU; tampering with boost control or fitting mechanical bleed-valves violates EU Stage-VI emissions law and can over-stress engine hardware.
‐ Repairs must restore manufacturer specs; documentation is required in many EU states for roadworthiness (TÜV/Dekra).
‐ On earlier EDC17CV44 platforms used outside EU, SPN 4591 may indeed reference the DEF return-line heater. Always confirm with the VIN-specific MAN spare-parts portal or MAN-cats because the SPN-to-text mapping is firmware-dependent.
‐ Persistent over-boost can crack pistons/head-gasket and overspeed the turbo; do not ignore repeated 4591-02 events.
‐ Comparative reliability of pneumatic vs. electric VGT actuation on heavy-duty diesels.
‐ Development of self-learning boost controllers using turbo-speed sensors (already on MAN D38).
‐ Impact of low-temperature intercooler icing on transient boost faults.
Fault code EDC 05 4591-02 on a MAN TGX Euro-6 almost always points to intermittent excessive boost pressure. Focus your diagnostics on the VGT/waste-gate actuation loop, the Pierburg control valve, MAP sensor accuracy, and air-path integrity. Confirm with MAN-cats freeze-frame, repair the mechanical/electrical root cause, update ECU software, and erase the code. If in doubt, a MAN-certified workshop can bench-test the turbo and flash the latest calibration.
