← Back to Detroit Diesel Fault Codes
Detroit Diesel SPN 4334 FMI 16 indicates that the SCR (Selective Catalytic Reduction) system is not converting NOx emissions efficiently enough to meet required thresholds. This code appears on DD13, DD15, and DD16 engines and is a moderate severity warning that can escalate to a derate if left unaddressed.
Symptoms
- Check engine light on
- SCR or DEF warning light illuminated
- Possible reduction in engine performance Increased
- NOx emissions Engine derate may follow if efficiency continues to drop
Cause
This code triggers when the ECM detects that the SCR catalyst is not reducing NOx at the required efficiency level. Common causes include:
- Low quality or contaminated
- DEF fluid Faulty NOx sensor (upstream or downstream)
- Worn or damaged SCR catalyst
- DEF dosing system malfunction
- Exhaust leaks affecting sensor readings
Can You Keep Driving?
Short term driving is possible but this code typically escalates. If the SCR system efficiency continues to fall, the engine will enter a progressive derate. Address this promptly to avoid a more severe code or breakdown.
Recommended Action
Start by checking DEF fluid quality — contaminated or diluted DEF is a very common cause. If DEF quality is good, have the NOx sensors, DEF dosing system, and SCR catalyst inspected by a qualified diesel technician.
It means the SCR system is operating but not converting NOx emissions at the required efficiency level, triggering a moderate severity fault.
The most common causes are poor quality or contaminated DEF, a failing NOx sensor, a worn SCR catalyst, or a DEF dosing system fault.
Limited driving is possible but the code is likely to escalate to a more severe fault and derate if the underlying issue is not addressed.
Yes, if SCR efficiency continues to drop the ECM will trigger progressive derate conditions to reduce emissions output.
Inspect and replace DEF if quality is suspect, then have the NOx sensors and SCR system diagnosed. The code will clear once the system confirms acceptable conversion efficiency.
Related codes
Need Immediate Help With This Fault Code?
If your engine is derating, losing power, or unable to complete regeneration, don’t risk further damage or costly downtime.
Use 4RoadService to locate mobile truck repair, dealer diagnostics, or emergency roadside assistance near your location.
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.