SPN 3720 FMI 18: Meaning and Fix
SPN 3720 FMI 18 signals the ash load percentage of the diesel particulate filter being below the normal operating range. This fault is often detected after routine maintenance where technicians forget to reset the ash load counter, leading to inaccurate readings. In practice, it indicates that the filter may not be accumulating ash as expected, which might point to issues in the regeneration process or sensor inaccuracies. Ignoring this could result in more severe aftertreatment system failures.
Common Symptoms
- Faulty Regeneration: Incomplete or ineffective regeneration cycles, often leading to increased soot accumulation in the filter.
- Sensor Misreading: The ash load sensor provides inaccurate data, leading to faulty diagnostics and unnecessary service actions.
- Unexpected Alerts: Frequent and unexpected alerts in the ECM related to aftertreatment system performance and effectiveness.
- Reduced Power: The vehicle may experience a reduction in power output due to incorrect ash load readings impacting engine management.
Probable Causes
- Sensor Fault: A malfunctioning ash load sensor can deliver incorrect data, causing erroneous ash load percentage readings.
- ECM Software Glitch: Software anomalies in the ECM can lead to improper interpretation of ash load data.
- Incomplete Service Reset: Failure to reset the ash load counter post-service can lead to misreported ash load levels.
- Regeneration Cycle Issues: Problems in the regeneration cycle can cause improper ash accumulation readings.
Advanced Technical Analysis
The ECM microcontroller is crucial in interpreting sensor data. Any glitch in its logic or signal monitoring could lead to incorrect ash load readings. This scenario is common after ECM updates where the microcontroller might not fully synchronize with sensor feedback, leading to false error codes. Technicians must ensure firmware updates are correctly installed and sensors properly calibrated to avoid misinterpretation.
Electrical breakdown in the ash load sensor could result from faulty wiring or poor connections. A debouncing timer helps in filtering noise from the sensor’s signal. If this timer fails, it could allow false readings to pass as genuine. Technicians should inspect wiring integrity and replace any faulty connectors to ensure consistent and accurate sensor data.
ECM safety mechanisms include torque derate strategies to protect the engine when reading anomalies occur. If incorrect ash load data is detected, the ECM might initiate a torque derate to avoid potential damage. This precautionary measure ensures vehicle safety but can lead to performance issues if the underlying cause is not addressed.
Long-term diagnostics should focus on ensuring accurate sensor data and consistent regeneration cycles. Regular workshops reveal that post-service resets of the ash load counter are often overlooked, leading to recurring fault codes. Implementing checklists for post-maintenance procedures can mitigate such oversights and maintain system integrity.
Step-by-Step Troubleshooting Guide
- Verify Sensor Data: Cross-check ash load sensor readings with known good values to confirm data validity.
- Check Wiring and Connections: Inspect wiring and connectors for damage or corrosion that might affect signal quality.
- Reset Ash Load Counter: Ensure the ash load counter is properly reset following any service to avoid false readings.
- Review Regeneration Logs: Analyze regeneration cycle logs to identify irregularities or missed cycles impacting ash accumulation.