The SCR catalyst uses diesel exhaust fluid to convert nitrogen oxides into harmless gases, and it is the heart of the emissions system on modern diesels.
What it does
Selective catalytic reduction (SCR) injects diesel exhaust fluid (DEF) into the exhaust ahead of the catalyst, where a reaction converts NOx into nitrogen and water. NOx sensors monitor how well it works, and the computer doses DEF accordingly.
Signs it is failing
- SCR efficiency or NOx fault codes
- A countdown to a power derate
- Excessive DEF consumption
- Reduced performance in severe cases
Replacement and what to know
A genuinely degraded SCR catalyst is replaced, but because NOx sensor faults and DEF dosing problems mimic catalyst failure, those are ruled out first. Using quality DEF and keeping the sensors healthy protects the catalyst.
Frequently asked questions
What does the SCR catalyst do?
It uses DEF to convert nitrogen oxides in the exhaust into harmless nitrogen and water.
Is it always the catalyst when I get an SCR code?
No. NOx sensor faults and DEF dosing problems commonly cause SCR codes; rule those out first.
How do I protect it?
Use quality DEF, keep the NOx sensors and dosing system healthy, and fix upstream faults promptly.
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