SPN 5743 FMI 5: Meaning and Fix
SPN 5743 FMI 5 indicates the module detects current below normal or an open circuit on its temperature sensor input. This fault commonly appears after a forced DPF regeneration when thermal stress cracks a solder joint on the module’s circuit board, interrupting the sensor signal to the ECM via J1939.
Common Symptoms
- MIL Illumination: Red or amber malfunction indicator lamp activates on the dashboard within one drive cycle.
- Derated Power: Engine torque is reduced by up to 40% to protect the SCR system from thermal damage.
- No Regeneration: Passive and active DPF regeneration is inhibited due to missing SCR temperature data.
- Fault Code Active: SPN 5743 FMI 5 stored in ECM memory, often accompanied by related sensor codes.
Probable Causes
- Open Sensor Wire: Broken conductor between the SCR temperature sensor and the module connector, common near exhaust brackets.
- Module Connector Corrosion: Moisture ingress at the 8-pin Delphi connector causes pin oxidation and intermittent open circuit.
- Failed Sensor Element: Internal thermistor open circuit due to thermal cycling or vibration fatigue in the exhaust stream.
- Module Power Loss: Missing 12V supply or ground at the module due to blown fuse or corroded chassis ground stud.
Advanced Technical Analysis
The ECM continuously monitors the SCR Temperature Sensor Module’s J1939 message for a valid temperature value. When the module detects an open circuit (current < 0.5 mA) on the sensor input, it transmits a diagnostic message with FMI 5. The ECM then sets the fault and initiates a 30-second debounce timer before logging the code to prevent false triggers from transient noise.
Electrical analysis reveals that a true open circuit yields infinite resistance across the sensor pins. The module’s internal pull-up resistor (typically 10 kΩ to 5V) will pull the signal line to 5V, which the module interprets as an open. Technicians should measure resistance between sensor pins: an open circuit reads > 10 MΩ, while a good sensor reads 1-10 kΩ depending on exhaust temperature.
Upon activation, the ECM enters a safety fallback mode. It substitutes a default SCR inlet temperature of 200°C to allow limited operation but disables active regeneration to prevent uncontrolled thermal events. Torque derate is applied linearly, reaching 40% reduction after 10 minutes of continuous fault presence. The vehicle may still move at reduced speed for limp-home purposes.
Long-term prevention requires inspecting the module mounting bracket for exhaust heat damage. Technicians frequently encounter this fault after replacing the ECM because the new unit requires a J1939 configuration reset. Always verify module firmware version against OEM service bulletins; Bosch Gen-2 modules are prone to internal solder cracks and should be replaced with the Gen-3 revision when possible.
Step-by-Step Troubleshooting Guide
- Visual Inspection: Check module connector for corrosion, bent pins, and wire chafing near exhaust heat shields.
- Resistance Measurement: Disconnect sensor, measure resistance across pins 1 and 2. Open circuit confirms failed sensor.
- Module Power Check: At module connector, measure voltage between pin 3 (12V) and pin 4 (ground). Must be 11-14V.
- J1939 Signal Test: Monitor CAN Hi/Lo at module pins 7 and 8 with oscilloscope; look for 2.5V differential.