SPN 1569 FMI 0: NOx Sensor - Circuit Malfunction

Overview\n\nSPN 1569 FMI 0 indicates a circuit malfunction in the NOx sensor system. The ECM has detected an electrical issue with the sensor or its wiring.\n\n## Why This Happens\n\nNOx sensors monitor exhaust gas composition. Circuit faults typically point to wiring problems rather than the sensor itself.\n\n## Step-by-Step Diagnosis\n\n1. **Visual inspection** - Check harness for rubbing, burns, or breaks\n2. **Connector check** - Look for corrosion, bent pins, or loose connections\n3. **Sensor resistance test** - Use multimeter to check sensor circuit\n4. **Replace if needed** - If wiring is good, replace sensor\n\n## Common Mistakes\n\n- Replacing sensor without checking wiring first\n- Ignoring exhaust leaks that damage sensors\n- Not clearing codes properly after repair