SPN 803 FMI 13: Meaning and Fix
SPN 803 FMI 13 indicates the ECM has detected that Relay Diagonal 2 is out of calibration, meaning its actuation parameters do not match the learned reference values. This fault commonly appears after an ECM replacement or software update when the relay’s coil resistance or switching time deviates from factory specs. Technicians often encounter this after replacing the relay without performing the required calibration routine, leading to intermittent system behavior.
Common Symptoms
- Engine derate: ECM reduces torque output to protect the relay circuit from potential overcurrent or welding.
- Intermittent fan operation: Engine cooling fan fails to engage or disengages erratically, causing temperature fluctuations.
- Warning lamp active: Amber or red malfunction indicator lamp illuminates on the dashboard persistently.
- Starter lockout: Starter motor is inhibited to prevent engagement during a fault condition.
Probable Causes
- Relay replacement: Installing a non-factory or incorrect relay without performing the calibration procedure.
- ECM software mismatch: Outdated or corrupted ECM firmware causing incorrect relay reference values.
- Wiring corrosion: High resistance in the relay control circuit due to oxidized terminals or loose pins.
- Coil degradation: Internal relay coil resistance drifted outside the calibrated range over time.
Advanced Technical Analysis
The ECM monitors Relay Diagonal 2 by comparing the measured coil current rise time against a stored calibration table. If the current profile deviates by more than 15% over 10 consecutive actuations, the ECM sets FMI 13. This diagnostic is based on the J1939 DM1 message and triggers a permanent fault until a successful calibration cycle is completed.
Electrically, the relay control circuit uses a low-side MOSFET driver with a flyback diode. The ECM measures the back-EMF decay time after de-energization. An out-of-calibration condition occurs when the decay time is outside the 5–15 ms window, often caused by a failing diode or inductive load mismatch.
When FMI 13 is active, the ECM enters a safety fallback mode that disables the relay output and sets a torque derate of 25% to prevent uncontrolled actuator behavior. The derate remains until the fault is cleared and a successful calibration is performed using a factory scan tool.
Long-term prevention involves verifying relay part numbers against OEM specifications (e.g., Bosch 0 332 019 150) and performing the ECM calibration routine after any relay or ECU replacement. In workshops, this fault is often resolved by running the ‘Relay Learn’ procedure via a diagnostic tool like DAVIE or WABCO.
Step-by-Step Troubleshooting Guide
- Verify relay specs: Check relay part number and coil resistance (typically 60–120 ohms) against OEM data.
- Inspect wiring: Measure resistance from ECM pin to relay coil; should be below 0.5 ohms.
- Run calibration: Use a factory scan tool to perform the Relay Diagonal 2 calibration routine.
- Check ECM firmware: Update ECM software to the latest version if calibration fails repeatedly.