P20EE: SCR NOx Catalyst Efficiency Below Threshold Bank 1
Severity: medium
What it means: The diesel SCR catalyst isn't reducing NOx as efficiently as expected.
Safe to drive? Usually drivable, but many diesels count down to a speed limit or no-restart if it isn't fixed.
Common symptoms
- Check-engine light
- Possible DEF warning or derate countdown
Likely causes, ranked
- Poor-quality or contaminated diesel exhaust fluid (DEF)DIY
- Faulty NOx sensorShop
- DEF dosing problem or a worn SCR catalystShop
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What it means: The diesel SCR catalyst isn't reducing NOx as efficiently as expected.
Severity: Medium. Safe to drive? Usually drivable, but many diesels count down to a speed limit or no-restart if it isn't fixed.
DIY or shop?
Check the DEF quality first. Then go to a shop.
Before you replace parts
- Read all stored codes and freeze-frame data. Fix misfire, fuel or sensor codes before chasing emissions codes.
- Check the simple things first: connectors, vacuum hoses, fluid levels and the gas cap.
- Clear the code only after a repair, then confirm it stays off after a few drives.
- Check your VIN for recalls and service bulletins.
Safety / mechanic note
Verify with a mechanic if you notice drivability changes, warning lights or overheating. A code points to a circuit or system, not always a specific part.
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