SPN 803 FMI 5: Meaning and Fix
SPN 803 FMI 5 indicates the ECM detected current below normal or an open circuit on Relay Diagonal 2. This relay typically controls auxiliary loads like fuel heaters or cooling fans. Technicians frequently encounter this fault after a wiring harness chafes against the engine block during a clutch replacement, severing the control wire. The ECM logs the fault immediately when the commanded load current fails to reach the minimum threshold.
Common Symptoms
- No Fan Operation: Engine cooling fan fails to engage when commanded by the ECM, causing gradual coolant temperature rise.
- Inoperative Fuel Heater: Fuel heater remains off in cold conditions, leading to waxing and hard starting below -10°C.
- Active Warning Lamp: Amber warning lamp illuminates on the dashboard; no immediate drivability loss but fault active.
- Intermittent Load Drop: Load connected to Relay Diagonal 2 cuts out randomly due to intermittent open circuit in the harness.
Probable Causes
- Open Relay Coil: Internal coil resistance exceeds 1000 ohms or is infinite, preventing current flow through the driver.
- Chafed Control Wire: Wire from ECM pin to relay coil rubbed through against metal bracket, causing an open circuit.
- Corroded Connector: Moisture ingress at relay or ECM connector creates high resistance, dropping current below threshold.
- Failed ECM Driver: Internal MOSFET in ECM open-circuit due to overcurrent or thermal stress, no output current possible.
Advanced Technical Analysis
The ECM microcontroller monitors the current flow through the relay driver circuit using a precision shunt resistor. When the driver is activated, the ECM expects a minimum current of 150 mA to flow through the relay coil. If the sensed current remains below 50 mA for more than 200 ms, the ECM sets SPN 803 FMI 5. This debouncing period prevents false triggers from transient voltage dips during engine cranking.
The relay coil presents an inductive load. Upon deactivation, the ECM’s internal flyback diode clamps the voltage spike to -0.7 V. An open circuit eliminates this spike, which the ECM can detect as a secondary confirmation. The diagnostic routine runs continuously once per 100 ms loop. Real-world data from Bosch ED17 ECUs shows this fault often occurs after a water pump replacement where the harness is disturbed.
When this fault is active, the ECM disables the relay driver output and stores the fault in non-volatile memory. No torque derate is applied for this specific code, but if the relay controls the engine brake or fan, the ECM may activate a backup strategy like forcing the fan to run continuously via a secondary circuit. This is common on MAN D26 engines where the fan relay is duplicated for redundancy.
Long-term prevention requires inspecting the harness routing near the relay panel, especially on Deutz TCD 2015 engines where the relay block sits near the exhaust manifold. A common workshop scenario is a technician finding a melted relay socket due to high resistance. Replacing both the relay and the mating connector housing with gold-plated terminals eliminates recurrence. Always verify coil resistance between 70-120 ohms before ECM replacement.
Step-by-Step Troubleshooting Guide
- Verify Relay Coil: Measure resistance across relay terminals 85 and 86. Should be 70-120 ohms; if open, replace relay.
- Check Control Wire: Disconnect ECM connector. Measure continuity from ECM pin to relay terminal 86. Resistance must be below 1 ohm.
- Inspect Connectors: Look for green corrosion or bent pins at ECM and relay sockets. Clean with contact cleaner and dielectric grease.
- Test ECM Driver: With relay removed, activate output via diagnostic tool. Measure voltage at ECM pin; should pulse to battery voltage.