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Detroit Diesel SPN 3216 FMI 2 indicates that the upstream NOx sensor, located before the SCR catalyst, is producing readings that do not match what the ECM expects based on current engine conditions. This rationality fault appears on DD13, DD15, and DD16 engines and can affect the ECM’s ability to properly control DEF dosing and monitor SCR system performance.
Symptoms
- Check engine light on
- SCR or emissions warning light illuminated
- Engine derate possible
- DEF dosing may be affected
- Other NOx or SCR related codes may appear alongside
Cause
This code triggers when the upstream NOx sensor reading does not correlate with expected values based on engine load, fuel rate, and other parameters. Common causes include:
- Failing or degraded upstream NOx sensor
- Exhaust leaks upstream of the sensor affecting readings
- DEF crystallization contaminating the sensor
- Damaged sensor wiring or connector
- Water intrusion at the sensor connector
- Engine running abnormally rich or lean affecting NOx output
Can You Keep Driving?
Limited driving may be possible but the ECM relies on the upstream NOx sensor to control DEF dosing accurately. Without reliable upstream data the entire SCR system performance is compromised and a derate condition is likely to follow.
Recommended Action
Inspect the upstream NOx sensor and its wiring for damage, corrosion, or contamination. Check for exhaust leaks at manifold joints, flex pipes, or fittings upstream of the sensor location. If the sensor wiring is intact and no exhaust leaks are found, replace the upstream NOx sensor as sensor degradation is the most common cause of this fault.
It means the upstream NOx sensor is producing readings that do not match what the ECM expects, indicating a rationality fault that affects SCR system monitoring and DEF dosing control.
The most common causes are a failing upstream NOx sensor, exhaust leaks near the sensor, contaminated or damaged sensor wiring, or water intrusion at the connector.
Short distances may be possible but without accurate upstream NOx data the ECM cannot properly control DEF dosing and a derate condition is likely.
Yes. When the ECM cannot trust the upstream NOx sensor data it will typically trigger a derate condition to limit emissions output.
Inspect and replace the upstream NOx sensor if faulty, repair any exhaust leaks, and verify wiring integrity. The code should clear once the ECM receives consistent and rational upstream NOx readings.
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.