SPN 5743 FMI 8: Meaning and Fix
SPN 5743 FMI 8 indicates the Aftertreatment 1 SCR Temperature Sensor Module is transmitting signals with abnormal frequency, pulse width, or period over the J1939 datalink. The ECU cannot correctly interpret multiplexed temperature values from SCR bank 1 sensors. This fault commonly appears after a DEF system overhaul or sensor module replacement where CAN termination resistors were disturbed, causing corrupted PWM signal timing that the ECM flags as out-of-specification frequency deviation.
Common Symptoms
- SCR Efficiency Degradation: Engine ECU loses accurate SCR inlet and outlet temperature references, causing suboptimal urea dosing and reduced NOx conversion efficiency.
- Active Regeneration Inhibition: Aftertreatment controller suspends active DPF regeneration cycles due to unreliable SCR thermal data received via the J1939 multiplexed network.
- Inducement Derate Activation: Vehicle enters an emissions-based torque derate or speed limitation as the ECM enforces SCR system protection per OBD-HD compliance requirements.
- MIL and Warning Lamps: Amber malfunction indicator lamp and aftertreatment warning icons illuminate on the dashboard, alerting the operator to a critical sensor communication failure.
Probable Causes
- Sensor Module CAN Fault: Internal oscillator drift or microcontroller failure within the SCR temperature sensor module generates out-of-specification PWM output frequency on the J1939 bus.
- Wiring Harness Damage: Chafed, corroded, or broken shield wires between the sensor module and ECU introduce noise, distorting signal pulse width and triggering FMI 8.
- Missing CAN Termination: Absent or incorrect 120-ohm CAN bus termination resistor causes signal reflections that corrupt the multiplexed temperature frame timing and frequency.
- Supply Voltage Instability: Fluctuating reference voltage to the sensor module, often caused by poor ground straps or failing ECU power relay, corrupts PWM signal generation.
Advanced Technical Analysis
The ECM continuously monitors incoming J1939 PGN frames from the SCR Temperature Sensor Module, measuring the period and pulse width of the multiplexed signal against a calibrated tolerance window defined in the application software. When measured frequency deviates beyond this window for a duration exceeding the manufacturer-defined debounce threshold, FMI 8 is latched. Bosch EDC17 and similar platforms use a dedicated CAN receive interrupt handler that flags abnormal timing at the microcontroller level before escalating to fault memory.
Electrically, FMI 8 is often triggered by impedance discontinuities along the CAN High and CAN Low lines connecting the sensor module to the backbone. A damaged shield or improper splice introduces differential noise that corrupts the bit-timing synchronization. Oscilloscope measurements should reveal distorted dominant-to-recessive transitions or asymmetric pulse widths deviating from the nominal 250 kbps J1939 specification. Debounce timers in MAN and Mercedes-Benz Euro VI calibrations typically require 2-5 seconds of continuous abnormal signal before the fault is confirmed active.
Upon confirming SPN 5743 FMI 8, the ECM activates a substitution value strategy for SCR temperature inputs, defaulting to a conservative fixed temperature model. This fallback prevents unsafe urea injection but simultaneously restricts dosing accuracy, causing NOx output to exceed threshold limits. Per SAE J1939-73 OBD-HD protocols, the system then initiates a timed inducement sequence, progressively limiting engine torque output to enforce repair. Deutz TCD and similar Euro Stage V engines implement a two-stage derate: first 25% torque reduction, then idle-only operation after extended fault duration.
Long-term diagnostic strategy requires logging J1939 traffic using a CANalyzer or equivalent tool to capture live PGN 65110 message intervals from the sensor module. Technicians in Mercedes-Benz Actros workshops frequently encounter this fault after replacing the aftertreatment harness without verifying termination resistance, measuring 60 ohms at the bus confirms both terminators are intact. Preventively, inspect the sensor module connector for fretting corrosion every 150,000 km and verify module firmware revision matches ECU calibration to avoid timing incompatibilities introduced by software mismatches.
Step-by-Step Troubleshooting Guide
- CAN Bus Resistance Check: Measure resistance between CAN High and CAN Low at the ECU connector; confirm 60 ohms indicating both 120-ohm termination resistors are properly installed.
- Oscilloscope Signal Capture: Connect oscilloscope to the SCR module CAN lines and verify clean 250 kbps waveform with symmetric pulse widths; abnormal signals confirm harness or module fault.
- Sensor Module Power Verification: Measure reference supply voltage at the SCR temperature sensor module connector; confirm stable 5V or 12V supply within manufacturer-specified tolerance under load conditions.
- Module Replacement Validation: After installing a replacement SCR sensor module, perform ECU parameter reset and confirm correct firmware version match before clearing faults and conducting a road test.