SCR NOx Catalyst Efficiency Below Threshold
The SCR system is not reducing NOx emissions as expected.
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)
The Selective Catalytic Reduction (SCR) system isn't converting enough harmful nitrogen oxides (NOx) into harmless substances, which should typically happen to meet emissions regulations.
What the computer is actually seeing
The engine control module (ECM) monitors NOx sensor readings. If the readings from the NOx sensor before the SCR catalyst compared to after the SCR catalyst show insufficient reduction (usually a lower voltage than 0.2-0.8V change), it triggers this code.
What a healthy reading looks like
NOx sensor voltage should change between approx. 0.2V to 0.8V under normal operation, indicating proper catalyst function.
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 tank level; it should be between 10-100% full. If low, refill with the correct DEF fluid.
- 22. Examine exhaust system for leaks; look for cracks or broken hangers that may allow gases to escape.
- 33. Check wiring and connectors to the NOx sensor for damage or corrosion; replace if necessary.
- 44. Test NOx sensor voltage and resistance; it should show a proper range during operation. If it fails, replace the sensor.
- 55. Consider SCR catalyst condition; if it's damaged or clogged, it may need cleaning or replacement. Make sure the exhaust temperature exceeds 200°C (392°F) for it to function properly.
Common causes
- Faulty NOx sensor
- Clogged SCR catalyst
- Insufficient DEF supply
- Exhaust leaks
- Faulty wiring or connectors to sensors
Typical repair cost
$$$
Related codes
Frequently asked questions
Is it safe to drive with this code?
No, it's not safe to drive long-term. It may lead to increased emissions and potential limp mode.
Will this code come back after clearing?
If the underlying issue isn't fixed, yes, this code will likely return.
Why does this code pair with other codes?
It's common for SCR issues to affect NOx readings and be related to DEF system performance, triggering multiple faults.
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