SPN 4360 FMI 5: Meaning and Fix
SPN 4360 FMI 5 indicates the SCR intake temperature sensor circuit has current below normal or an open circuit. The ECM detects no valid signal from the sensor, often after a forced DPF regeneration where thermal stress damages the sensor element. Technicians frequently encounter this fault after replacing the ECM or during post-regen inspections, leading to incorrect DEF dosing and potential system derate.
Common Symptoms
- DEF Dosing Disabled: ECM inhibits DEF injection to prevent under- or over-dosing due to missing temperature input.
- Engine Power Derate: Active torque reduction by up to 25% to protect SCR substrate from thermal shock.
- Diagnostic Lamp Illuminated: Red stop lamp and amber warning lamp activate per SAE J1939 DTC severity.
- Fault Code Stored: SPN 4360 FMI 5 logged with freeze frame data showing ambient temperature readings.
Probable Causes
- Open Sensor Wiring: Broken wire at connector or harness chafing near exhaust manifold from vibration.
- Failed Sensor Element: Internal thermistor open circuit due to thermal fatigue from repeated regenerations.
- Corroded Connector Pins: Moisture ingress at the 2-pin connector causing high resistance or open circuit.
- ECM Internal Fault: Faulty pull-up resistor or ADC channel on ECM board, rare but documented in Bosch EDC17.
Advanced Technical Analysis
The ECM microcontroller monitors the sensor signal through a 5V pull-up resistor. A normal NTC thermistor outputs 0.5–4.5V. When current falls below the normal threshold (open circuit), the ADC reads near 5V, triggering FMI 5 after a 2-second debounce timer. This logic prevents false triggers from transient noise.
Electrical analysis shows the sensor circuit has a nominal resistance of 2.5 kΩ at 250°C. An open circuit creates infinite resistance, causing the ECM to read the pull-up voltage. The debounce timer ensures the fault is persistent before logging. MAN factory manuals specify checking for pin corrosion at X1 connector.
Upon activation, the ECM enters a safety fallback using a default temperature value of 0°C or a model-based estimate. This forces a torque derate to 75% of rated power and disables DEF dosing. Mercedes-Benz guidelines recommend resetting the fault only after verifying sensor continuity with a multimeter.
Long-term prevention includes inspecting the sensor harness routing near the exhaust manifold for heat damage. In practice, this fault often recurs after DPF cleaning if the sensor is not replaced with a genuine Deutz part. Use a breakout box to measure resistance directly at the ECM connector per Bosch EDC17 procedures.
Step-by-Step Troubleshooting Guide
- Visual Inspection: Check sensor connector and harness for chafing, burns, or loose pins near exhaust components.
- Resistance Measurement: Disconnect sensor, measure resistance between pins: 2.5 kΩ at 25°C, open circuit indicates failure.
- Voltage Check at ECM: Backprobe ECM connector, key-on: expect 5V on signal wire; 0V indicates open circuit.
- Bypass Test: Jumper sensor connector pins; if fault changes to FMI 0 (voltage high), replace sensor.