SCR SYSTEM DETERIORATION
Indicates a performance issue 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 (Selective Catalytic Reduction) system is having trouble reducing harmful emissions from the exhaust. This means the system isn't working as efficiently as it should, leading to higher emissions.
What the computer is actually seeing
The ECM sees the aftertreatment efficiency drop, typically measured through NOx sensors or pressure differentials. It expects a certain level of NOx reduction to be maintained (usually over 80% efficiency).
What a healthy reading looks like
NOx sensor readings should indicate a reduction of NOx below 200 ppm after SCR injection, while pressure differentials across the SCR should be less than 5 inches of water column (inH2O).
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 DEF level in the tank. It should be between 10-90% full. Is it low?
- 22. Inspect the wiring and connectors for the NOx sensor and SCR injector. Look for loose connections or signs of damage. Are they intact?
- 33. Verify the quality of the DEF being used. Is it clean and compliant with specifications?
- 44. Check for exhaust leaks before the SCR system. Are there any signs of leaks affecting sensor readings?
- 55. Test the NOx sensor functionality with a multimeter. Do readings match expected values?
- 66. If all above checks out, inspect the SCR catalyst for clogs. Is there excessive back pressure?
Common causes
- Faulty NOx sensor
- Clogged SCR catalyst
- Low DEF (Diesel Exhaust Fluid) levels
- Inadequate DEF quality (contaminated or poor-grade fluid)
- Wiring issues to sensors or injector
Typical repair cost
$$$
Related codes
Frequently asked questions
Is it safe to drive with this code?
No, you may exceed emissions limits, which can lead to further legal or mechanical issues.
Will the code come back after clearing?
Yes, if the underlying issue isn't fixed, the code will reappear.
Why does this code sometimes pair with others like SPN 5791 or SPN 5792?
These codes indicate specific issues with the SCR system and often occur together when there's a common failure in components or lines.
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