Aftertreatment SCR Efficiency Below Threshold
The SCR system isn't working efficiently enough to clean exhaust gases.
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 is responsible for converting harmful nitrogen oxides (NOx) in the exhaust into harmless nitrogen and water using DEF (diesel exhaust fluid). When this code triggers, it usually means the system isn't reducing NOx as it should, which can happen if there's not enough DEF, if the injector is clogged, or if there's a problem with the SCR catalyst itself.
What the computer is actually seeing
The engine control unit (ECU) monitors the NOx levels before and after the SCR. If the post-SCR NOx level is too high when compared to the pre-SCR levels, the ECU determines that the SCR is not working effectively and stores this code.
What a healthy reading looks like
NOx levels should drop significantly after passing through the SCR. A typical reading could show NOx levels before SCR at around 2000 ppm and after SCR ideally below 100 ppm, depending on the engine and operating conditions.
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.
- 1Check the DEF level and quality; refill or replace if low or contaminated. Decision Point: Is DEF level okay? Yes: go to step 2. No: correct and retest.
- 2Inspect the SCR injector for clogs or carbon buildup. Ensure it's spraying properly. Decision Point: Is the injector functioning? Yes: go to step 3. No: clean/replace injector and retest.
- 3Use a scan tool to observe the NOx sensor readings. Check pre- and post-SCR levels. Decision Point: Is the drop in NOx levels satisfactory? Yes: go to step 4. No: inspect and possibly replace the SCR catalyst.
- 4Inspect and repair any damaged wiring or connectors in the SCR system. Decision Point: Are all connections secure and in good condition? Yes: finalize with step 5. No: fix wiring/connectors and retest.
- 5If all components check out, consider replacing the SCR catalyst if performance is still not up to par while retesting after each major fix.
Common causes
- Low DEF level or quality
- Clogged SCR injector
- Faulty SCR catalyst
- Wiring issues to SCR components
- Faulty NOx sensors
- Exhaust leaks before or after SCR system
Typical repair cost
$$$
Related codes
Frequently asked questions
Is it safe to drive with this code?
It's best to fix this soon; if ignored, you risk increased emissions and potential engine performance issues.
Will this problem come back after repair?
If the underlying issue is fixed properly, it shouldn't return. Regular maintenance of the DEF system helps prevent recurrence.
Why does this code pair with other codes?
The SCR system is complex and interconnected; issues in one part can trigger multiple fault codes, especially relating to emissions.
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 →