SPN 241 FMI 0: Meaning and Fix
SPN 241 FMI 0 signals that the tire pressure data from the Tire Condition Message 1 exceeds the normal operational range, typically above 1100 kPa for a heavy-duty truck. This code commonly appears after a forced DPF regeneration when heat causes tire pressure to rise, or when a technician overinflates tires during service. The ECM logs the fault to prevent blowout risks.
Common Symptoms
- Dashboard warning: Tire pressure warning lamp illuminates on the dash, often accompanied by a chime or pop-up message.
- Reduced ride comfort: Excessive tire stiffness causes harsh ride quality, particularly on uneven surfaces or at highway speeds.
- Uneven tire wear: Center tread wear accelerates due to overinflation, reducing tire lifespan and traction.
- Vehicle handling drift: Overinflated tires reduce contact patch, causing unstable steering and reduced cornering grip.
Probable Causes
- Sensor calibration error: Tire pressure sensor (TPS) returns offset values due to incorrect calibration or factory drift.
- Wiring short to power: Signal line shorted to battery voltage causes the ECM to read a falsely high pressure value.
- Overinflation during service: Technician inadvertently overinflates tire above 1200 kPa during routine air top-up.
- ECM software threshold: ECM firmware has an overly sensitive upper limit, triggering fault at 1050 kPa when safe limit is 1100 kPa.
Advanced Technical Analysis
The ECM monitors the tire pressure sensor via a 5V reference, signal, and ground circuit. The microcontroller samples the analog voltage at 100 Hz and compares it to a calibrated pressure-to-voltage lookup table. If the raw ADC value exceeds the upper threshold (typically 4.8V corresponding to 1300 kPa) for 500 ms, the ECM sets FMI 0.
Electrical noise or a short to 5V supply on the signal line can cause the ECM to see a saturated high reading. The ECM applies a debouncing timer of 200 ms to filter transients, but sustained high voltage triggers the fault. Per Bosch factory manuals, a wiring resistance below 5 ohms to power indicates a short.
When SPN 241 FMI 0 is active, the ECM may log the event but typically does not derate engine torque, as tire pressure is not critical for propulsion. However, on integrated TPMS systems, the ECM may inhibit cruise control above 1100 kPa to prevent stability risks. Deutz manuals confirm no torque reduction for this code.
Long-term diagnosis involves verifying the TPS sensor output with a calibrated pressure gauge. Technicians frequently encounter this fault after replacing the ECM without reprogramming the tire pressure thresholds. MAN factory procedures recommend resetting the threshold via diagnostic tool to match the tire manufacturer’s cold inflation specs.
Step-by-Step Troubleshooting Guide
- Verify actual pressure: Use a calibrated tire gauge to check pressure; if below 1100 kPa, suspect sensor or wiring fault.
- Inspect TPS wiring: Measure signal voltage at sensor connector with key on; should be 0.5-4.5V. Above 4.8V indicates short.
- Check ground circuit: Measure resistance between sensor ground and chassis ground; should be less than 0.5 ohms.
- Relearn thresholds: Use J1939 diagnostic tool to reset tire pressure limits per OEM specification; test drive to confirm.