p0420
Moderate
Cadillac · Sedans
Catalyst System Efficiency Below Threshold (Bank 1)
Quick Answer
Error code P0420 on your Cadillac Sedans 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 oxygen sensor signals before and after catalyst
2
Inspect exhaust system for leaks
3
Check for engine running rich condition
4
Test catalytic converter with exhaust analyzer
5
Replace catalytic converter if efficiency is below 80%
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 sensors
Exhaust clamp
Gasket
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