Exhaust Aftertreatment SCR Efficiency
The system has detected poor efficiency in the Selective Catalytic Reduction process.
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, which cleans up the diesel exhaust by breaking down NOx gases, isn't working well enough, which can lead to greater pollution and possible engine performance issues.
What the computer is actually seeing
The engine control module (ECM) monitors the NOx sensor readings before and after the SCR unit to calculate efficiency. It expects a certain reduction percentage in NOx; if the reduction is below approximately 75%, it triggers this code.
What a healthy reading looks like
NOx levels before SCR: up to 200 ppm, after SCR: under 50 ppm expected for proper efficiency.
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. Check the DEF level; top it off if low.
- 22. Inspect DEF quality; it should be clear, not frozen or contaminated.
- 33. Inspect wiring and connectors to both NOx sensors for damage or corrosion.
- 44. Evaluate the exhaust system for leaks, especially around the SCR and NOx sensors.
- 55. Use a scan tool to check live data from the NOx sensors; compare pre- and post-SCR readings with expected values.
- 66. If actual NOx levels are out of range, replace the faulty NOx sensor; else, inspect the SCR unit.
- 77. If SCR is clogged or damaged based on visual inspection or live data, replace it.
Common causes
- Low DEF (Diesel Exhaust Fluid) quality or quantity
- Faulty NOx sensors
- Clogged or damaged SCR catalyst
- Exhaust leaks before or after the SCR system
Typical repair cost
$$$
Related codes
Frequently asked questions
Is it safe to drive with this code?
No, this can affect emissions and performance. Fix it soon.
Will this code come back after clearing?
If the underlying issue isn't resolved, the code will likely return.
Why does this relate to other codes?
It often works in conjunction with NOx sensor codes, indicating efficiency loss or sensor issues.
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