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Detroit Diesel SPN 3230 FMI 2 indicates that the NOx sensor is producing a reading that does not match what the ECM expects based on current engine operating conditions. This rationality fault appears on DD13, DD15, and DD16 engines and typically points to a failing NOx sensor or an underlying issue with the SCR system affecting emissions output.
Symptoms
- Check engine light on
- SCR or emissions warning light illuminated
- Engine derate possible
- Other NOx or SCR related codes may appear alongside this fault
- Reduced aftertreatment system performance
Cause
This code triggers when the ECM compares the NOx sensor reading against expected values and finds the data does not correlate correctly. Common causes include:
- Failing or degraded NOx sensor
- NOx sensor contaminated by DEF crystallization
- Exhaust leaks affecting sensor readings
- Wiring or connector damage at the NOx sensor
- Failed SCR catalyst causing abnormal NOx readings
- Water intrusion in the sensor connector
Can You Keep Driving?
Limited driving may be possible but this fault indicates the ECM cannot trust the NOx sensor data, which means it cannot verify the SCR system is controlling emissions correctly. A derate condition is likely to follow if the fault is not diagnosed and resolved.
Recommended Action
Have the NOx sensor and its wiring inspected first as sensor failure is the most common cause. Check for exhaust leaks near the sensor location. If the sensor and wiring check out, have the SCR catalyst evaluated as a degraded catalyst can produce abnormal NOx readings that trigger this rationality fault.
It means the NOx sensor is producing a reading that does not match what the ECM expects, indicating a rationality fault with the sensor or an issue within the SCR system.
The most common causes are a failing or contaminated NOx sensor, exhaust leaks near the sensor, damaged sensor wiring, or a degraded SCR catalyst producing abnormal NOx levels.
Short distances may be possible but the ECM cannot verify SCR system performance with an unreliable NOx sensor reading and a derate condition is likely to follow.
Yes. When the ECM cannot confirm emissions are being controlled correctly it will trigger a derate condition to limit engine output.
Inspect and replace the NOx sensor if faulty, check for exhaust leaks, and verify sensor wiring integrity. If the sensor is good, have the SCR catalyst tested. The code should clear once the ECM receives rational NOx readings within expected parameters.
Related codes
Need Immediate Help With This Fault Code?
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Important Notice
The information provided above is for general informational purposes only and should not replace official OEM service procedures. Always consult the manufacturer’s service manual or a qualified diesel technician before performing repairs. Operating a vehicle with active fault codes may result in further damage.