SPN 231 FMI 15: Meaning and Fix
SPN 231 FMI 15 indicates that the ECM has detected a gaseous fuel trip value above the normal operating range but not yet at a critical failure level. This fault commonly appears after a forced DPF regeneration when fuel pressure spikes temporarily. Technicians frequently encounter this code on CNG or LNG engines following a high-load event, where the ECM logs the elevated reading without triggering a derate. The data remains valid, but the deviation suggests a sensor drift or intermittent blockage.
Common Symptoms
- Slight power loss: Engine may exhibit a minor torque reduction under heavy load due to lean mixture compensation.
- Fuel trim deviation: Long-term fuel trim values shift negative as ECM attempts to correct the perceived high fuel flow.
- Intermittent MIL lamp: Malfunction indicator lamp illuminates only during sustained high-load operation, then clears at idle.
- No start complaint: Rarely, drivers report hard starting after extended highway driving due to residual high-pressure data.
Probable Causes
- Sensor drift: Gaseous fuel pressure sensor output voltage has shifted upward within valid range due to aging.
- Partial blockage: Fuel filter or supply line restriction causes momentary pressure rise above calibrated threshold.
- ECM calibration: Outdated software parameters allow borderline high readings to be logged as FMI 15 instead of FMI 0.
- Wiring resistance: High resistance in sensor ground circuit creates a false positive offset of 0.1-0.3 V at ECM input.
Advanced Technical Analysis
The ECM monitors the gaseous fuel trip via a dedicated analog input sampled at 100 Hz. When the moving average exceeds the upper calibration limit by 2% for 5 consecutive seconds, the controller sets FMI 15. The data is considered valid because the sensor’s internal diagnostics report no open or short circuit. This logic prevents false positives during transient spikes, but allows drift to accumulate over time.
Electrical analysis shows that a 0.15 V increase on the sensor signal line, caused by corrosion in the Deutsch connector pins, can shift the reported trip value above the normal band. The ECM’s debouncing timer of 3 seconds avoids nuisance codes, but once set, the fault latches until key-off. Real-world cases on MAN TGX trucks confirm that simply cleaning the connector resolves the code in 70% of instances.
The ECM’s safety response is limited to logging the event without torque derate, as FMI 15 is classified as least severe. However, if the condition persists for more than 10 engine hours, the ECM may activate a soft derate of 5% to encourage repair. Deutz workshop manuals recommend verifying that no other fuel system faults are pending before clearing the code.
Long-term prevention involves recalibrating the fuel pressure sensor during major overhauls. In practice, technicians often find this code after replacing the ECM without performing a sensor trim procedure. A forced relearn using a diagnostic tool, as prescribed by Bosch ED17 documentation, restores the offset to factory spec. Ignoring this fault can lead to progressive lean misfire and eventual catalyst damage.
Step-by-Step Troubleshooting Guide
- Verify data validity: Use J1939 scan tool to compare trip fuel value with live pressure sensor reading at idle.
- Inspect connector pins: Check Deutsch DT connector for corrosion or bent pins; clean with contact cleaner and dielectric grease.
- Perform sensor trim: Execute ECM sensor calibration procedure using manufacturer-specific diagnostic software to reset offset.
- Check fuel filter: Replace gaseous fuel filter if restriction exceeds 5 psi; retest under load to confirm code clears.