p0420
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Mercedes-Benz · SUVs
Catalyst System Efficiency Below Threshold Bank 1
Quick Answer
Error code P0420 on your Mercedes-Benz SUVs means: Catalyst System Efficiency Below Threshold Bank 1. This is usually fixable — see the step-by-step guide below.
Affected Models
Applies to all models
X243X247X254V167X167AMG GLB35 4MATIC 2024AMG GLC43 4MATIC 2024AMG GLC63 S 2024AMG GLE53 4MATIC+ 2024AMG GLE63 S 2024AMG GLS63 2024EQB 250+ 2024EQB 300 4MATIC 2024EQB 350 4MATIC 2024GLB250 2024GLB250 4MATIC 2024GLC300 2024GLC300 4MATIC 2024GLE350 4MATIC 2024GLE450 4MATIC 2024GLS450 4MATIC 2024GLS580 4MATIC 2024Maybach GLS600 2024
Likely Causes
How to Fix It
1
Ensure no misfire or fuel system codes are present that could damage the catalyst
2
Compare upstream and downstream O2 sensor waveforms to assess catalyst storage capacity
3
Check for exhaust leaks around the manifold and catalytic converter connections
4
Test the downstream O2 sensor response time and voltage switching
5
Replace the catalytic converter if confirmed to be below efficiency threshold
Pro Tip
This issue typically requires professional diagnosis. Attempting this fix without proper training could cause further problems.
Parts You May Need
Catalytic converter
Downstream O2 sensor
Exhaust gaskets
Exhaust mounting hardware
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