SPN 5742 FMI 4: Meaning and Fix
SPN 5742 FMI 4 indicates the aftertreatment DPF temperature sensor module reports a voltage below normal or a short to ground on its internal circuit. This module digitizes multiple DPF temperature sensor signals and transmits them via J1939 to the engine ECU. Technicians frequently encounter this fault after a DPF regeneration or following component replacement, when a sensor wire is pinched against chassis ground or moisture enters the module connector. The code typically triggers an amber warning and may inhibit passive regeneration until resolved.
Common Symptoms
- DPF Regeneration Inhibited: ECM disables passive or active regeneration to prevent thermal damage from unmonitored temperatures.
- MIL Illuminated: Amber malfunction indicator lamp on dash stays on; fault code stored in ECM non-volatile memory.
- Erratic Temperature Readings: J1939 data stream shows implausible low values (e.g., -40°C) or frozen readings from affected sensor.
- Torque Derate Active: Engine power reduced by up to 25% as ECM enters safety fallback mode to protect aftertreatment.
Probable Causes
- Sensor Signal Short to Ground: Internal wiring or connector pin damaged, causing direct short between signal line and chassis ground.
- Module Supply Voltage Low: 5V reference or battery supply to the module drops below 4.5V due to corroded terminals or harness damage.
- Corroded Connector Terminals: Moisture ingress at module connector causes oxidation, increasing contact resistance and simulating a low voltage condition.
- Failed Temperature Sensor Element: Internal NTC thermistor shorts to ground due to thermal stress or manufacturing defect, pulling signal low.
Advanced Technical Analysis
The ECM microcontroller monitors the module’s J1939 message for the DPF temperature sensor value. If the reported voltage falls below a calibrated threshold (typically <0.5V for >2 seconds), the FMI 4 is set. The ECM uses a debouncing timer to filter transient noise; only a sustained low condition latches the fault. Real-world data from Deutz TCD engines shows this timer prevents false flags during engine cranking voltage dips.
Electrically, a short to ground can occur on the module’s internal PCB trace, the sensor input pin, or the harness. Using a digital multimeter, technicians measure resistance between signal pin and ground; values below 100 ohms confirm a short. Bosch factory manuals emphasize checking the module’s internal protection diode, which can fail short after a voltage spike from a failed glow plug relay.
When FMI 4 is active, the ECM substitutes a default temperature value (often -40°C) and disables regeneration. This triggers a torque derate to prevent soot buildup from incomplete burns. MAN service bulletins note that the derate is staged: 10% after 1 hour, 25% after 2 hours, forcing the operator to address the fault. The module’s watchdog timer also resets if the low-voltage condition persists.
Long-term prevention includes inspecting the module harness for chafing near the DPF housing, where heat and vibration accelerate insulation wear. In Mercedes-Benz OM471 engines, this fault commonly appears after a forced regeneration if the technician reconnects the module while the engine is running, causing a transient ground loop. Always disconnect the battery before servicing the module to avoid latch-up failures.
Step-by-Step Troubleshooting Guide
- Visual Inspection: Check module connector for bent pins, corrosion, or moisture. Verify harness routing away from exhaust heat shields.
- Measure Supply Voltage: At module connector, measure between supply pin and ground. Should be 4.75–5.25V or 12/24V per OEM spec.
- Check Sensor Signal Wire: Disconnect module, measure resistance from signal pin to chassis ground. If <100 ohms, repair short.
- Swap Module Test: Install known-good module, clear fault, and perform key-on test. If code returns, inspect harness; if gone, replace module.