SCR NOx Catalyst Efficiency Below Threshold
NOx conversion efficiency is below expected levels in the SCR system.
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 SCR system isn't cleaning up the exhaust gases like it should, which may cause excessive emissions. It collects ammonia and needs at least 100-200 PSI of DEF pressure for proper operation.
What the computer is actually seeing
The engine control module (ECM) monitors the NOx sensor readings before and after the SCR catalyst. If the difference indicates that more than 20% of NOx is not being converted, it triggers this code.
What a healthy reading looks like
NOx levels after the SCR should be less than 50 ppm; if the difference between inlet and outlet exceeds 10% efficiency, it triggers an issue.
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 for low fluid levels; refill if necessary. Did that solve the issue? Yes: Clear codes and retest. No: Go to step 2.
- 22. Check for DEF pump operation; ensure it's delivering at least 10-15 PSI. Is it functioning correctly? Yes: Go to step 3. No: Replace the DEF pump.
- 33. Examine wiring and connectors to NOx sensors for damage or corrosion. Are connections clean and secure? Yes: Go to step 4. No: Repair as needed.
- 44. Test NOx sensors with a scan tool; compare inlet and outlet readings. Do they show a significant difference above expected values? Yes: Replace the defective NOx sensor. No: Go to step 5.
- 55. Inspect the SCR catalyst for blockage or contamination. Is it functioning properly? Yes: Consider SCR replacement. No: Clean or replace as needed.
Common causes
- Clogged or contaminated SCR catalyst
- Faulty NOx sensors
- Low DEF supply or quality
- Mechanical issues in the exhaust system
Typical repair cost
$$$
Related codes
Frequently asked questions
Can I drive the vehicle with this code?
It's drivable, but expect potentially increased emissions. Fix soon to avoid penalty and performance issues.
Will this code come back after clearing it?
If the underlying issue isn't resolved, yes, it will trigger again.
Why does this code sometimes pair with SPN 0426?
Both codes indicate issues with NOx sensor performance and SCR efficiency.
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 →