SPN 3598 FMI 5: Meaning and Fix
This fault indicates the ECU detected current below normal or an open circuit on its second power output supply (#2). In practice, this code commonly appears after a forced DPF regeneration when a secondary fuel pump relay fails to energize, or after replacing the ECM without properly reseating the 120-pin connector, causing a pin to push back and break the circuit.
Common Symptoms
- No power to aux loads: Secondary loads like aftertreatment fuel pumps or cooling fans fail to operate.
- Interrupted system operation: DPF regeneration cycles abort prematurely due to missing power supply.
- ECM warning lamp active: Red or amber stop lamp illuminates on the dash immediately after key-on.
- Voltage reading zero: Multimeter shows 0.0 V at the ECU power output #2 pin with ignition on.
Probable Causes
- Open supply wire: Broken conductor or corroded terminal between ECU pin and the load device.
- Blown internal fuse: ECU internal solid-state fuse or external inline fuse for output #2 is open.
- Failed load device: Relay coil or solenoid connected to output #2 has an internal open circuit.
- ECM connector damage: Bent, pushed-back, or oxidized pin in the ECU harness connector for output #2.
Advanced Technical Analysis
The ECM microcontroller monitors the output #2 voltage via a feedback resistor divider. When the output driver is commanded ON, the feedback must read within 90% of battery voltage. If the feedback stays near 0 V for more than 500 ms, the diagnostic sets FMI 5, as per Bosch ED17 controller logic.
A debouncing timer of 3 consecutive fault samples (each 100 ms apart) is required before the code becomes active. This prevents false triggering from transient loads. The open-circuit threshold is typically 50 mA; below this, the driver detects infinite resistance and flags the fault.
Upon activation, the ECM disables the output driver for that channel and may enter a torque derate strategy (typically 25% reduction) to protect the aftertreatment system. For Mercedes-Benz OM471 engines, this derate remains until a successful key-cycle and fault-clearing procedure is completed.
Long-term prevention requires dielectric grease on all power output pins and annual torque checks on the ECU connector. In Deutz TCD 7.8 applications, technicians frequently resolve this fault by replacing the 48-pin header on the engine harness, which suffers from fretting corrosion after 5000 hours of operation.
Step-by-Step Troubleshooting Guide
- Verify fuse and relay: Check external fuse F12 and relay K03 for continuity; replace if open.
- Measure output at ECU pin: Backprobe ECU pin A23 with ignition ON; expect battery voltage when commanded.
- Inspect harness continuity: Disconnect load, measure resistance from ECU pin to load; must be < 1 ohm.
- Check connector integrity: Remove ECU connector, inspect pin A23 for corrosion or push-back; repair as needed.