SCR Catalyst Efficiency Below Threshold
The SCR catalyst is not reducing NOx emissions effectively.
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 uses a catalyst to convert harmful NOx gases into nitrogen, and if it's not working, those gases aren't being reduced like they should.
What the computer is actually seeing
The engine control unit (ECU) monitors the NOx levels before and after the SCR catalyst. If the post-catalyst NOx levels are above a certain threshold (typically 2-3 times the pre-catalyst levels), the code triggers.
What a healthy reading looks like
Pre-catalyst NOx levels should be less than 150 ppm; post-catalyst levels shouldn't exceed that by more than 30% under 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 tank for contamination or low fluid level; refill or replace if necessary.
- 2Inspect exhaust system for leaks or restrictions (use smoke test if necessary).
- 3Test the NOx sensors — compare pre and post SCR values; replace faulty sensors if needed.
- 4If sensors are good, perform an SCR efficiency test; if catalyst fails the test, replace the SCR unit.
Common causes
- Failed SCR catalyst
- Faulty NOx sensors
- Blocked or contaminated diesel exhaust fluid (DEF)
- Leaking exhaust system components
Typical repair cost
$$$
Related codes
Frequently asked questions
Is it safe to drive with this code?
Not recommended; you risk worsening emissions and damaging the catalyst.
Will this code come back after clearing it?
If the underlying issue isn't fixed, the code will return after a short drive cycle.
Why does it pair with other codes?
It often indicates a systemic problem with the SCR system, including sensor failures.
Working a real vehicle right now?
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