SCR NOx Catalyst Efficiency
This code indicates a problem with the NOx catalyst efficiency 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)
Your truck's computer thinks the NOx catalyst isn't cleaning the exhaust gases well enough, which can lead to higher pollution and possible engine damage.
What the computer is actually seeing
The engine control module (ECM) monitors the NOx levels before and after the SCR (Selective Catalytic Reduction) system. If the difference in readings is too low, it triggers this code.
What a healthy reading looks like
NOx level before SCR: < 1000 ppm; NOx level after SCR: should drop significantly if working; efficiency expected to be above 80%.
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: Must be above minimum to ensure proper operation. If low, refill and retest.
- 22. Inspect wiring and connectors to the NOx sensor and SCR catalyst: Look for corruption, loose connections or shorts. Repair as needed.
- 33. Test the NOx sensor output: Using a scan tool, compare NOx readings before and after the SCR. Ensure there is a significant drop in ppm.
- 44. If readings show little to no change, inspect the SCR catalyst for blockage or damage. A pressure differential test may be needed.
- 55. If the catalyst is faulty, replace it and clear the codes; retest for proper function.
Common causes
- Faulty NOx sensor
- Clogged or damaged SCR catalyst
- Low DEF (Diesel Exhaust Fluid) level or quality
- Wiring issues in SCR circuit
Typical repair cost
$$$
Related codes
Frequently asked questions
Is it safe to drive with this code?
Not recommended. It can lead to higher emissions and potentially damage the engine.
Will the code come back after clearing?
Yes, if the underlying issue isn't fixed, the code will likely return.
Why does this code sometimes pair with SPN 4432?
SPN 4432 indicates a similar problem within the SCR system, usually pointing to issues with different parts or sensors used to monitor NOx efficiency.
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