SPN 5848 FMI 9: Meaning and Fix
This fault indicates the ECM has not received a valid CAN message from the NH3 sensor within the expected update period, typically 100-500 ms. Technicians often encounter this after a sensor replacement if the new unit was not properly calibrated or if wiring was disturbed during DPF servicing. The ECM logs FMI 9 when data age exceeds the debounce timer threshold.
Common Symptoms
- No SCR conversion: SCR system cannot calculate NH3 slip, leading to reduced NOx reduction efficiency and potential emissions exceedance.
- Check engine lamp: Amber warning lamp illuminates immediately after fault detection, often accompanied by a derate notification on the dash.
- Torque derate active: Engine power is reduced by up to 25% to protect aftertreatment components from unmonitored ammonia levels.
- No sensor data: Diagnostic tool shows NH3 sensor value stuck at 0 ppm or last valid reading, with no live data updates.
Probable Causes
- Open CAN circuit: Broken or corroded CAN high or low wire between sensor and ECM connector, often at chassis harness chafe points.
- Sensor power loss: Missing 12V or ground at sensor due to blown fuse, corroded pin, or relay failure in the power distribution box.
- ECM software fault: Incompatible or corrupted ECM calibration that does not correctly poll the NH3 sensor message ID (PGN 64950).
- Sensor internal failure: NH3 sensor electronics damaged by thermal stress or moisture ingress, preventing CAN message generation.
Advanced Technical Analysis
The ECM continuously monitors the reception rate of the NH3 sensor CAN message (PGN 64950, SPN 5848). A debouncing timer starts when no message is received within 300 ms. If the timer reaches 1.5 seconds, FMI 9 is set. The ECM then freezes the last valid NH3 value and transitions to open-loop SCR control, relying on a map-based ammonia dosing model.
Electrical analysis must include measuring the termination resistor at the sensor connector. The CAN bus requires 120 ohms between CAN_H and CAN_L. Open or shorted termination causes signal reflection and message corruption. A multimeter should show 60 ohms across the bus when power is off, indicating both terminating resistors are intact.
Upon activation of FMI 9, the ECM initiates a gradual torque derate, typically reducing engine power by 5% per minute until a maximum of 25% is reached. This strategy prevents excessive ammonia slip that could damage downstream components. The derate remains active until the fault is cleared and a successful drive cycle with valid sensor data is completed.
In workshop practice, this code often appears after a forced DPF regeneration that overheated the NH3 sensor wiring harness near the exhaust. Technicians should inspect the harness for melted insulation near the SCR inlet. A known fix on MAN D26 engines involves replacing the sensor pigtail with a heat-resistant loom and updating ECM software to version 2.6.1 or later.
Step-by-Step Troubleshooting Guide
- Verify sensor power: Check for 12V at sensor pin 1 and ground continuity at pin 2 with key-on, engine-off.
- Inspect CAN bus: Measure resistance between CAN_H and CAN_L at sensor connector; should read 60 ohms.
- Check message presence: Use oscilloscope to confirm 250 kbps CAN signal with correct PGN 64950 identifier.
- Replace sensor if needed: If power and CAN bus are intact, replace NH3 sensor and perform teach-in procedure via diagnostic tool.