SPN 5024 FMI 31: Meaning and Fix
SPN 5024 FMI 31 indicates the Engine Exhaust 1 NOx Sensor Heater Ratio has detected a condition where the heater control circuit is unable to maintain the required temperature ramp at the sensor tip. This fault commonly appears after a forced DPF regeneration that overheated the sensor element or when moisture ingress damages the heater winding. The ECM monitors heater resistance and current draw; a ratio outside 0.85–1.15 triggers this code, often accompanied by a gradual torque derate to protect the aftertreatment system.
Common Symptoms
- MIL Illumination: Red or amber warning lamp on dash indicates an active emissions-related fault requiring immediate attention.
- Torque Derate: Engine power is gradually reduced by up to 25% to limit exhaust temperature and protect aftertreatment components.
- Regen Inhibited: DPF regeneration is blocked by ECM until the NOx sensor heater ratio returns to normal range.
- Rough Idle: Incorrect NOx readings cause ECM to alter fuel injection timing, leading to unstable idle and misfire.
Probable Causes
- Heater Open Circuit: Internal break in sensor heater element caused by thermal cycling or vibration fatigue over time.
- Wiring Harness Damage: Chafed or corroded wires at the sensor connector near the exhaust manifold due to heat exposure.
- ECM Driver Failure: Internal ECM MOSFET shorted or open, failing to supply correct PWM voltage to the heater circuit.
- Sensor Contamination: Oil ash or soot deposits on the sensor tip alter thermal conductivity, confusing the heater control loop.
Advanced Technical Analysis
The ECM continuously calculates the NOx sensor heater ratio by dividing the measured heater resistance by a reference value stored in calibration. A ratio below 0.85 indicates a short circuit or excessive current draw; above 1.15 suggests an open circuit or high resistance. The controller applies a debouncing timer of 10 seconds before confirming the fault to ignore transient noise from exhaust pulses.
On the electrical side, the heater circuit operates at a nominal 12V PWM signal with a target resistance of 2.5 ohms at 20°C. Using a precision shunt resistor, the ECM compares actual current to expected current. A deviation of ±15% for more than 5 consecutive samples locks the fault. Technicians frequently encounter this after replacing the ECM without recalibrating the sensor offset.
When FMI 31 is active, the ECM enters a safety fallback: it sets the NOx sensor reading to a default value of 500 ppm, disables closed-loop NOx control, and activates a torque derate of 1% per minute up to 25%. This strategy prevents inaccurate data from causing uncontrolled regeneration or over-fueling, as documented in MAN D2866 workshop manuals.
Long-term prevention requires verifying sensor mounting torque (45 Nm ±5 Nm) and checking for exhaust leaks upstream. In real-world workshops, this code often arises after a technician installs a non-genuine sensor with different heater characteristics. Always perform a sensor learn-in procedure via diagnostic tool after replacement to reset the heater ratio baseline.
Step-by-Step Troubleshooting Guide
- Visual Inspection: Check sensor connector for corrosion, bent pins, and harness for chafing near the exhaust manifold heat shield.
- Heater Resistance Test: Measure between pins A and B with a multimeter; expected value is 2.5 ohms ±0.3 at 20°C.
- ECM Output Check: Back-probe ECM connector and measure PWM voltage at idle; should be 11–13V with 50% duty cycle.
- Sensor Relearn: Use diagnostic tool to perform a sensor replacement procedure and clear fault codes after repair.