SPN 803 FMI 31: Meaning and Fix
SPN 803 FMI 31 signals that the ECM has detected a continuous condition on Relay Diagonal 2, typically the engine run relay circuit. This code commonly appears after a forced DPF regeneration when high current draw damages relay contacts, or following an ECM replacement where the new unit misinterprets the existing wiring resistance. Technicians frequently encounter this fault when a relay coil fails open or a diode snubber shorts, causing the ECM to see a stuck-on condition.
Common Symptoms
- No Crank: Starter motor remains inactive as ECM inhibits cranking due to relay circuit fault.
- Dash Warning Light: Red STOP lamp illuminates immediately on key-on, indicating a critical relay failure.
- Intermittent Power Loss: Engine randomly stalls when vibration causes the faulty relay to momentarily open circuit.
- No Diagnostic Communication: J1939 data link remains active, but relay 2 failure prevents ECM from powering sensors.
Probable Causes
- Weld Relay Contacts: High current from preheat or fan clutch cycles fuses relay contacts shut permanently.
- Open Coil Winding: Internal coil break due to thermal stress or manufacturing defect, verified by resistance check.
- Shorted Flyback Diode: Suppression diode inside relay or ECM driver fails short, keeping relay energized constantly.
- Corroded Connector: Moisture ingress at the relay base connector creates intermittent high resistance or short.
Advanced Technical Analysis
The ECM microcontroller monitors Relay Diagonal 2 via a feedback voltage divider. When the relay is commanded OFF, the ECM expects battery voltage across the coil. If voltage remains below 4V for 200ms across three consecutive key cycles, FMI 31 sets. This logic prevents false positives from transient spikes during engine cranking.
Electrical analysis reveals a debouncing timer of 150ms to filter contact chatter. A stuck relay shows feedback voltage >2V when commanded OFF. Using a lab scope, technicians often see a flat line at battery voltage instead of the expected decay ramp, confirming the relay fails to de-energize.
The ECM enters a safety fallback by disabling all outputs on Relay Diagonal 2, including the intake heater and exhaust brake, to prevent runaway loads. Torque is derated to 40% and vehicle speed limited to 30 km/h. This protects wiring from thermal overload if the relay remains welded.
Long-term strategy involves upgrading to a sealed relay base with dielectric grease to prevent corrosion. In real workshops, this fault recurs if the ECM driver transistor is damaged from a previous shorted relay. Replacing both relay and ECM driver circuit yields a permanent fix, as seen after DPF regen failures.
Step-by-Step Troubleshooting Guide
- Check Relay Coil: Measure resistance across relay coil terminals 85-86; spec is 70-90 ohms at 20°C.
- Verify Feedback Signal: Backprobe ECM pin for relay 2 feedback; should toggle 0V to battery voltage with key cycle.
- Inspect Connector Pins: Remove and visually check for corrosion or bent pins at relay base and ECM connector.
- Test Diode Integrity: Diode test across relay coil; forward voltage 0.6-0.8V, reverse open. Replace if shorted.