SPN 5298 FMI 15: Meaning and Fix
This fault indicates the aftertreatment 1 DOC conversion efficiency is above the normal operating range, often triggered by excessive exothermic heat from uncontrolled hydrocarbon dosing. Technicians frequently see this after a failed forced DPF regeneration or a leaking fuel injector in the doser unit, causing catalyst temperatures to spike above 650°C and reducing NO2 formation.
Common Symptoms
- High exhaust temp: Exhaust gas temperature sensors report values above 650°C during regeneration or light load operation.
- Increased fuel consumption: Fuel economy drops by 5-10% due to post-injection fuel passing unburned into the DOC.
- White smoke emission: Visible white or blue-gray smoke from tailpipe indicates unburned hydrocarbons entering the aftertreatment.
- DPF regeneration frequent: DPF regeneration cycles occur more often as excess soot loading results from incomplete DOC oxidation.
Probable Causes
- Hydrocarbon doser leak: Leaking HC doser injector introduces raw fuel into DOC, causing uncontrolled exothermic reactions.
- Post-injection timing error: ECM post-injection quantity or timing calibration drift leads to excess fuel reaching the DOC.
- DOC thermal degradation: Aged or poisoned DOC substrate loses precious metal surface area, altering conversion efficiency readings.
- Exhaust gas temperature sensor: Faulty EGT sensor upstream of DOC reports artificially low temps, causing over-fueling during regeneration.
Advanced Technical Analysis
The ECM calculates DOC conversion efficiency using upstream and downstream NOx sensors combined with EGT data. When the efficiency exceeds the calibrated threshold (typically >95% at 350°C), FMI 15 sets. The algorithm uses a sliding window of 300 seconds to filter transient spikes, but persistent high efficiency indicates a real thermal event.
Electrical analysis reveals that the NOx sensor heater circuits must be within 0.5 ohms of spec. A corroded connector at the DOC outlet NOx sensor can cause intermittent high readings. The ECM debounces the signal for 10 seconds before logging the fault to avoid false triggers from vibration or moisture ingress.
The ECM activates a torque derate of 25% after 10 hours of active fault time to protect the DOC from thermal runaway. Simultaneously, it commands a regeneration inhibit and forces the engine into low-load operation. If the catalyst temperature exceeds 700°C, the ECM will shut down the engine to prevent meltdown.
Long-term prevention includes verifying HC doser flow rate with a graduated cylinder test during every major service. In MAN trucks, this fault often appears after replacing the ECM without recalibrating the doser trim values. A 2023 Bosch technical bulletin recommends replacing the DOC if efficiency remains above 110% after doser repair.
Step-by-Step Troubleshooting Guide
- Scan data analysis: Monitor DOC inlet/outlet NOx and EGT via diagnostic tool. Compare actual vs expected efficiency at 350°C.
- HC doser leak test: Perform leak-down test on HC doser valve. Replace if flow exceeds 0.5 L/h with engine off.
- EGT sensor comparison: Compare upstream and downstream EGT sensors during cold start. Difference >15°C indicates sensor drift.
- Post-injection calibration: Verify post-injection fuel quantity using ECM data logger. Adjust timing per manufacturer specs if deviation >5%.