SPN 5848 FMI 18: Meaning and Fix
This fault indicates the aftertreatment SCR intermediate NH3 sensor reports data valid but below the normal operating range (moderately severe). In practice, this code often appears after a forced DPF regeneration when urea dosing is temporarily suspended, causing a transient drop in NH3 concentration. The ECM monitors the sensor signal against model-based expected values; persistent below-range readings trigger the fault.
Common Symptoms
- Reduced NOx Conversion: SCR system efficiency drops, leading to elevated tailpipe NOx emissions and potential regulatory non-compliance.
- Torque Derate Active: ECM initiates a gradual torque reduction to limit exhaust output, typically 25% derate after 1 hour.
- DEF Warning Lamp: Dashboard illuminates the DEF warning lamp, often accompanied by a check engine light for operator notification.
- Increased Regeneration Frequency: DPF regeneration cycles occur more often due to altered exhaust thermal management from low NH3.
Probable Causes
- NH3 Sensor Drift: Sensor element aging or contamination causes output below actual ppm, common after 8000 engine hours.
- Insufficient DEF Dosing: Clogged dosing injector or low DEF pressure reduces NH3 formation, confirmed by DEF pump duty cycle analysis.
- Exhaust Leak Upstream: Air ingress before the NH3 sensor dilutes sample gas, lowering measured ppm below true value.
- ECM Calibration Error: Incorrect model parameters for NH3 expected values, often after ECM software update without proper adaptation.
Advanced Technical Analysis
The ECM evaluates the NH3 sensor signal every 100 ms against a virtual model that predicts expected ppm based on engine load, DEF dosing rate, and exhaust temperature. When the measured value falls below the model’s lower confidence bound for 10 consecutive seconds, the FMI 18 is set. This logic prevents false triggers from transient events like DEF line purging.
Electrically, the NH3 sensor uses a 5V reference and a CAN-based output. A slow negative drift in the sensor’s internal reference voltage, often due to thermal cycling of the zirconia element, can cause a below-range reading without a hard circuit fault. The ECM debounces the signal for 3 seconds to distinguish drift from noise.
Upon activation, the ECM sets a torque derate of 0.5% per minute until a 25% limit is reached, protecting the aftertreatment from over-temperature while maintaining drivability. The SCR catalyst temperature is increased by 30°C via post-injection to attempt to restore NH3 storage, but if the sensor remains low, the derate persists.
Long-term diagnosis requires comparing NH3 sensor raw voltage to a known-good sensor in the same exhaust stream. Technicians frequently encounter this fault after replacing the ECM without performing the NH3 sensor adaptation reset procedure. Using a J1939 scan tool to monitor PID 5848 while manually dosing DEF at 50% rate confirms sensor accuracy.
Step-by-Step Troubleshooting Guide
- Read Freeze Frame: Record engine speed, load, and exhaust temperature at fault onset to identify operating conditions.
- Verify DEF Quality: Test DEF concentration with a refractometer; below 32.5% urea reduces NH3 generation and triggers low readings.
- Inspect Dosing System: Check DEF injector for clogging using a flow test; typical flow should be 2-4 ml/s at 8 bar.
- Perform Sensor Cross-Check: Compare NH3 sensor output with upstream NOx sensor; a large disparity indicates sensor drift needing replacement.