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Detroit Diesel SPN 3251 FMI 16 indicates that the downstream NOx sensor, located after the SCR catalyst, is detecting NOx levels above the acceptable threshold. This code appears on DD13, DD15, and DD16 engines and signals that the SCR system is not reducing NOx emissions to the required level after the exhaust passes through the catalyst.
Symptoms
- Check engine light on
- SCR or emissions warning light illuminated
- Engine derate condition possible
- Reduced SCR system performance
- May appear alongside SCR efficiency and NOx rationality codes
Cause
This code triggers when the downstream NOx sensor detects exhaust NOx levels exceeding acceptable limits after the SCR catalyst. Common causes include:
- Degraded or failing SCR catalyst Insufficient
- DEF dosing reaching the catalyst
- Failed or contaminated downstream NOx sensor
- Exhaust leaks downstream of the SCR catalyst
- Poor DEF quality reducing SCR conversion efficiency
- DEF dosing system fault reducing catalyst effectiveness
- Damaged sensor wiring or connector
Can You Keep Driving?
Limited driving may be possible but high downstream NOx levels indicate the SCR system is not controlling emissions correctly. The ECM will trigger a derate condition if the fault persists. Address this promptly to avoid escalation to a more severe SCR fault.
Recommended Action
Start by checking DEF quality and level as poor DEF is a common contributor to high downstream NOx. If DEF is good inspect the downstream NOx sensor and wiring for damage or contamination. Have the DEF dosing system checked for adequate flow. If all components test correctly the SCR catalyst should be evaluated for degradation as a worn catalyst is a frequent cause of persistently high downstream NOx readings.
It means the downstream NOx sensor is detecting NOx levels above the acceptable threshold after the SCR catalyst, indicating the catalyst is not reducing emissions to the required level.
The most common causes are a degraded SCR catalyst, insufficient DEF dosing, a failed downstream NOx sensor, poor DEF quality, or exhaust leaks near the sensor.
Short distances may be possible but high downstream NOx indicates the emissions system is not functioning correctly and a derate condition is likely to follow if not addressed.
Yes. Persistently high downstream NOx readings will cause the ECM to trigger progressive derate conditions to limit emissions output.
Check DEF quality and dosing system first, then inspect the downstream NOx sensor and wiring. If those are good have the SCR catalyst evaluated. The code should clear once downstream NOx readings return to acceptable levels.
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.