p0420
Moderate
GMC · Trucks
Catalyst System Efficiency Below Threshold Bank 1
Quick Answer
Error code P0420 on your GMC Trucks means: Catalyst System Efficiency Below Threshold Bank 1. This is usually fixable — see the step-by-step guide below.
Affected Models
Likely Causes
How to Fix It
1
Verify actual catalyst efficiency with scanner
2
Inspect exhaust system for leaks
3
Check oxygen sensor functionality
4
Look for engine misfire codes
5
Replace catalytic converter if confirmed failed
Pro Tip
This fix requires some technical knowledge. If you're unsure about any step, consider contacting support or a qualified technician.
Parts You May Need
Catalytic converter
Oxygen sensor
Exhaust gasket
Heat shield
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