SCR Catalyst Efficiency Below Threshold
The SCR is not reducing NOx 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 doing its job properly, which means it's not converting harmful nitrogen oxides (NOx) into less harmful substances as it should.
What the computer is actually seeing
The engine computer measures NOx concentrations coming in and going out of the SCR system. It calculates the efficiency of the SCR by comparing these two readings and expects a certain reduction percentage. If the reduction falls below a specified threshold (often around 85% efficiency), it triggers this code.
What a healthy reading looks like
NOx concentration readings before SCR should be around 500-600 ppm and after SCR, they should be less than 100 ppm for a properly functioning system.
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. Start with a visual inspection of the SCR system including all wiring and connectors. Look for signs of damage or corrosion.
- 22. Check the level and quality of the diesel exhaust fluid (DEF). It should be clear and free of contamination. Replace if needed.
- 33. Using a scan tool, monitor NOx sensor readings before and after the SCR catalyst while the engine is running. Check for significant differences in the values.
- 44. If the NOx reduction is below 85% and fluid quality is good, perform a back pressure test to ensure there are no blockages in the exhaust system.
- 55. If everything checks out but the code persists, inspect and possibly swap the NOx sensors to see if one is faulty.
Common causes
- Faulty SCR catalyst
- Contaminated diesel exhaust fluid (DEF)
- Leaking exhaust system
- Faulty NOx sensors
- Wiring issues in the SCR system
Typical repair cost
$$$
Related codes
Frequently asked questions
Is it safe to drive with this code?
It’s generally drivable, but emissions may be higher, and you may fail an emissions test.
Will this code come back after I clear it?
Yes, if the underlying issue isn't fixed, the code will likely return.
Why does this code pair with other SCR-related codes?
Codes related to SCR often occur together because they concern the same system, and issues in the SCR can impact multiple sensors and functions.
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