SCR NOx Catalyst Efficiency Below Threshold
The SCR system cannot properly reduce NOx emissions.
Heads up: this code wasn't in our hand-written library yet, so DiagCoach AI built this page on the fly. Double-check critical specs against service info before condemning parts.
What it means (plain English)
Your vehicle's SCR system isn't working right, causing higher than acceptable NOx emissions. This usually means the catalyst isn't doing its job properly, which is critical for keeping the engine clean.
What the computer is actually seeing
The engine control module tracks the NOx levels before and after the SCR. If the reduction in NOx is less than expected under certain conditions, it triggers this code.
What a healthy reading looks like
NOx levels should drop by at least 75% after passing through the SCR system; for example, inlet NOx at 1000 ppm should be reduced to 250 ppm or less after processing.
Guided diagnostic — the DiagCoach way
Don't just throw parts at it. Walk through these in order — each step tells you whether to keep going or stop and fix what you found.
- 11. Inspect the DEF level and quality; ensure it meets OEM specifications (typically at least ASTM D7467). Check for contamination.
- 22. Examine the wiring and connectors on the SCR catalyst and NOx sensors for damage or corrosion. Repair as necessary.
- 33. Use a scan tool to check live data for NOx sensor readings. Before the SCR, values should be high; after the SCR, values should drop significantly. If they don't, continue to the next step.
- 44. Perform a backpressure test on the exhaust system to ensure it's not clogged. Expect readings below 5 PSI at idle and up to 10 PSI at full throttle.
- 55. If all previous tests are clear, replace the SCR catalyst and clear DTC. Monitor for reoccurrence.
Common causes
- Faulty SCR catalyst
- Blocked or restricted exhaust system
- Faulty NOx sensors
- Insufficient DEF quality or level
- Wiring issues to the SCR or sensors
Typical repair cost
$$$
Related codes
Frequently asked questions
Can I drive with this code active?
It's not safe to drive long distances as it may increase emissions and could lead to more damage to the SCR system.
Will this code come back after clearing?
If the underlying issue isn't fixed, yes, this code is likely to return.
Why does this code sometimes pair with other codes?
Multiple codes can indicate a common issue, often related to the same system, like failures in the SCR's components.
Working a real vehicle right now?
Let DiagCoach walk you through it live with your specific symptoms, vehicle, and what you've already checked.
Start guided diagnostic →