SPN 201 FMI 31: Meaning and Fix
This fault indicates the Engine Exhaust Gas Temperature (EGT) sensor signal is present but invalid, often due to a short to ground or internal failure. Technicians frequently encounter this after a forced DPF regeneration, where thermal shock damages the sensor element, causing the ECM to flag a condition exists error.
Common Symptoms
- Engine derate: ECM reduces torque by up to 40% to protect aftertreatment from unmonitored exhaust heat.
- DPF regen lockout: Active regeneration is inhibited, leading to soot buildup and eventual DPF clogging.
- Check engine lamp: Amber warning lamp illuminates steadily on the dash, indicating a non-critical fault.
- Erratic EGT reading: Scan tool shows sensor output stuck at a fixed value, typically near 0°C or 1000°C.
Probable Causes
- Sensor short circuit: Internal short to ground or supply voltage within the EGT sensor probe element.
- Wiring harness damage: Chafed or melted wires near the exhaust manifold causing intermittent signal loss.
- Connector corrosion: Moisture ingress at the sensor connector creates high resistance, triggering condition exists flag.
- ECM input failure: Internal ECM pull-up resistor failure or damaged input channel, rare but possible after voltage spike.
Advanced Technical Analysis
The ECM monitors the EGT sensor via a pull-up resistor to 5V. Under normal operation, the sensor’s negative temperature coefficient thermistor creates a voltage divider. When FMI 31 activates, the ECM detects a signal that is outside the valid range but not an open or short circuit—often a stuck value from a partially damaged sensor. This triggers the condition exists code after a debounce timer of 5 seconds.
From an electrical standpoint, a sensor with a cracked ceramic element can produce a fixed resistance near 100 ohms, causing the ECM to read a steady 4.5V. The debouncing algorithm requires the fault to be present for 5 consecutive seconds before logging. Voltage spikes from nearby glow plugs or injectors can also induce transient errors that mimic a condition exists state.
The ECM’s safety logic responds by setting the aftertreatment temperature to a default model value, disabling active regeneration, and requesting a torque derate of 25-40%. On MAN D26 engines, this also forces the exhaust throttle valve to a safe position. The fault remains active until a valid sensor reading is restored for at least 10 seconds.
Long-term prevention includes inspecting the sensor mounting torque (30 Nm per Deutz specs) and using anti-seize compound. In workshops, this code often appears after replacing the ECM without reprogramming the sensor calibration. Always verify sensor resistance at 20°C (should be 2.5 kΩ) and compare with the Bosch factory table. Replace if deviation exceeds 5%.
Step-by-Step Troubleshooting Guide
- Visual inspection: Check EGT sensor wiring for chafing, melting, or corrosion near the exhaust manifold and connectors.
- Resistance test: Measure sensor resistance at ambient temperature; compare to manufacturer spec (e.g., 2.5 kΩ at 20°C).
- Voltage check: With key on, engine off, measure signal pin voltage at ECM; should be 4.5-5V if open, 0V if shorted.
- Sensor substitution: Install a known-good EGT sensor and clear fault; if code returns, inspect ECM input circuit for damage.