SPN 5742 FMI 31: Meaning and Fix
SPN 5742 FMI 31 indicates the aftertreatment DPF temperature sensor module has detected a persistent internal fault or invalid data. This code commonly appears after a forced DPF regeneration where the module overheats or loses calibration. Technicians frequently encounter this fault after replacing the ECM without updating the module firmware, leading to communication mismatch on the J1939 bus.
Common Symptoms
- DTC Active: MIL and amber warning lamp illuminate immediately on key-on, with no intermittent behavior observed.
- Regen Inhibited: ECM disables automatic and manual DPF regeneration, causing soot accumulation to rise rapidly.
- Torque Derate: Engine power is reduced by up to 25% to protect the aftertreatment system from thermal damage.
- No Temp Data: DPF inlet and outlet temperature readings show -40°C or 0°C on scan tool, indicating sensor data missing.
Probable Causes
- Module Internal Fault: Microcontroller or memory corruption within the temperature sensor module causes invalid J1939 messages.
- CAN Bus Interference: Open or short circuit on the CAN lines between module and ECM disrupts proper data transmission.
- Power/Ground Failure: Intermittent loss of 12V supply or poor ground connection at the module connector leads to brownout.
- Firmware Mismatch: Module firmware not updated after ECM replacement, causing incompatible protocol timing on the bus.
Advanced Technical Analysis
The ECM microcontroller monitors the temperature sensor module’s J1939 message rate and data validity. If the module fails to transmit a heartbeat or sends corrupted temperature values for more than 1.5 seconds, the ECM sets FMI 31. This logic prevents false triggers from transient noise while ensuring rapid detection of a hard fault.
Electrically, the module operates on a 5V internal regulator. A debouncing timer of 2.0 seconds is used to confirm the condition. If the CAN transceiver detects dominant bits when no device is driving the bus, it indicates a stuck-low condition. This is typical after moisture ingress into the module connector.
Upon activation, the ECM enters a safety fallback: it disables all DPF regeneration commands and sets a torque derate of 40% at 1500 rpm. The engine will not exceed 80% load until the fault is cleared. This protects the DPF from uncontrolled exothermic reactions that could melt the substrate.
Long-term, technicians should verify module firmware revision against the latest OEM service bulletin. In real workshops, this fault often follows an ECM flash update without cycling the module’s power supply. A simple power reset (disconnect battery for 5 minutes) clears the code in 30% of cases, but if persistent, module replacement is required.
Step-by-Step Troubleshooting Guide
- Scan Module Data: Use J1939 diagnostic tool to monitor module’s message ID 0x18FF5828 for valid temperature and heartbeat.
- Inspect Connector: Check module 6-pin connector for corrosion, bent pins, or moisture at pins 1 (power) and 2 (ground).
- Verify CAN Bus: Measure resistance between CAN H and CAN L at module; should be 60 ohms with termination resistors intact.
- Cycle Power: Disconnect battery negative for 5 minutes to reset module internal state; recheck for active fault.