SPN 5848 FMI 6: Meaning and Fix
SPN 5848 FMI 6 indicates the ECM detected current above normal or a grounded circuit on the Aftertreatment 1 SCR intermediate NH3 sensor (Bank 1). This fault often appears after a forced DPF regeneration when the sensor harness is heat-damaged or chafed against the exhaust housing. The ECM monitors sensor supply current; a reading exceeding the calibrated threshold triggers the code and disables SCR control logic.
Common Symptoms
- Reduced Power: Engine enters torque derate of up to 40% to protect SCR components from unmonitored NH3 levels.
- SCR Lamp ON: Amber SCR lamp illuminates with a flashing red stop lamp if derate exceeds 25%.
- No DEF Dosing: DEF dosing stops completely to prevent NH3 slip, causing NOx conversion to cease.
- Frequent Regens: Passive and active regenerations increase as soot loading rises due to disabled aftertreatment controls.
Probable Causes
- Shorted Sensor Wiring: NH3 sensor signal wire shorted to ground or power, often at exhaust bracket chafing points.
- Failed NH3 Sensor: Internal sensor short circuit due to cracked ceramic element or moisture ingress in the connector.
- ECM Driver Fault: Internal ECM output driver failed shorted, causing continuous high current draw on the sensor supply.
- Corroded Connector: Pin corrosion in the 4-pin NH3 sensor connector increases resistance, misleading current sense circuit.
Advanced Technical Analysis
The ECM microcontroller reads the NH3 sensor supply current via a precision shunt resistor and an analog-to-digital converter. A current above the calibrated threshold (typically >500 mA) for 200 ms triggers FMI 6. The ECM debounces the fault over three consecutive key cycles before latching the code, preventing intermittent nuisance trips.
Electrical analysis reveals that a grounded circuit causes the sensor heater line to draw excessive current, often exceeding 2 A. The ECM’s smart power switch detects this via a current sense feedback loop and shuts down the sensor supply within 50 ms. The fault remains active until the short is cleared and the ECM performs a self-test on the next power-up.
Safety fallback: The ECM sets a torque derate of 25% immediately, escalating to 40% after 30 minutes of runtime. DEF dosing is inhibited, and the SCR catalyst efficiency monitor is disabled. The engine runs in a safe mode with reduced NOx conversion, increasing the risk of DPF clogging if not repaired promptly.
Long-term strategy: Technicians frequently encounter this fault after replacing the ECM without verifying the NH3 sensor harness. Always perform a resistance check between sensor pins A and B (supply and ground) before ECM replacement. Real-world example: A 2020 MAN TGS had repeated FMI 6 after a DPF burn—found the sensor harness melted against the exhaust pipe. Replacing the harness and sensor cleared the code permanently.
Step-by-Step Troubleshooting Guide
- Visual Inspection: Check NH3 sensor harness for chafing, melting, or pinched wires near exhaust and frame rails.
- Resistance Test: Measure resistance between sensor signal and ground; should be >10 kΩ. A low reading indicates a short.
- Connector Check: Disconnect sensor and ECM; test for continuity between sensor connector pins and ground. Repair if shorted.
- Sensor Replacement: If wiring is intact and resistance is normal, replace the NH3 sensor. Clear fault and test drive.