SPN 3698 FMI 13: Meaning and Fix
SPN 3698 FMI 13 signals that the ECM has detected the exhaust high-temperature lamp command signal is out of calibration, meaning the voltage or duty cycle from the lamp driver does not match the expected calibrated range. This fault often appears after a forced DPF regeneration or when a replacement ECM has not been properly calibrated to the vehicle’s specific exhaust aftertreatment system. Technicians frequently encounter this after swapping ECUs without performing the required manufacturer calibration routine.
Common Symptoms
- Lamp Stays On: Exhaust high-temperature indicator remains solid despite normal exhaust temperatures below 250°C.
- No Lamp Response: Lamp fails to illuminate during key-on self-test or commanded regeneration events.
- Derate Active: Engine torque is reduced by 25% to prevent thermal damage to aftertreatment components.
- Fault Code Active: SPN 3698 FMI 13 is stored as active or previously active in the ECM memory.
Probable Causes
- ECM Calibration Error: ECM software version does not match the lamp driver calibration table from the factory.
- Wiring Resistance Shift: Increased resistance in the lamp control circuit due to corrosion or damaged pins.
- Lamp Driver Failure: Internal short or open in the high-temperature lamp driver circuit within the ECM.
- Aftertreatment Retrofit: Non-OEM DPF or exhaust system installed without updating the lamp calibration parameters.
Advanced Technical Analysis
The ECM monitors the lamp command output using a pulse-width-modulated (PWM) signal. The calibration fault is triggered when the actual feedback voltage deviates more than ±5% from the stored calibration curve. This check occurs at key-on and during regeneration events. The ECM compares the commanded state (on/off) to the measured voltage across the lamp load. A mismatch lasting longer than 500 ms sets the fault.
Electrical analysis reveals that the lamp driver uses a low-side MOSFET with a pull-up resistor. Out-of-calibration errors often stem from a floating gate voltage due to a broken ground path or an upstream short to battery. The debouncing timer of 2 seconds prevents false triggers, but persistent deviation forces the ECM to log FMI 13. Technicians should measure the duty cycle at the lamp connector with a scope.
When FMI 13 is active, the ECM enters a safety mode that disables the lamp command and sets a torque derate of 30% to protect the DPF from uncontrolled temperature spikes. The engine management system also limits maximum rail pressure to 1600 bar and reduces boost pressure. These fallbacks remain until the fault is cleared and a successful calibration self-test completes.
Long-term prevention requires that after any ECM replacement, the technician performs a ‘Calibration Download’ using the manufacturer’s diagnostic tool. For example, on a MAN D2676 engine, the lamp calibration is stored in the EEPROM and must be matched to the vehicle’s VIN. Failure to do so results in recurring SPN 3698 FMI 13, as seen in many fleet workshops after ECU swaps without proper parameter setting.
Step-by-Step Troubleshooting Guide
- Verify Calibration: Use OEM software to read and compare the lamp calibration value to the factory specification.
- Inspect Wiring: Check the lamp control circuit for resistance > 2 ohms or intermittent opens at connector pins.
- Test Lamp Driver: Command the lamp on via diagnostic tool and measure voltage at the ECM output pin.
- Clear and Relearn: After repairs, clear the fault and perform a key-on self-test to confirm calibration is accepted.