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Mercedes-Benz · Sedans
Catalyst System Efficiency Below Threshold Bank 1
Quick Answer
Error code P0420 on your Mercedes-Benz Sedans 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
W177W206W214V297A220 2024A220 4MATIC 2024AMG A35 4MATIC 2024AMG C43 4MATIC 2024AMG C63 S 2024AMG E53 4MATIC+ 2024AMG E63 S 2024AMG EQS 53 2024AMG S63 E 2024C300 2024C300 4MATIC 2024E350 4MATIC 2024E450 4MATIC 2024EQS 450+ 2024EQS 580 4MATIC 2024Maybach S580 2024S500 4MATIC 2024S580 4MATIC 2024W223
Likely Causes
  1. Worn or contaminated catalytic converter
  2. Faulty downstream O2 sensor providing incorrect readings
  3. Exhaust leak before the catalytic converter
  4. Engine misfire causing unburned fuel to damage the catalyst
  5. Use of leaded fuel or oil contamination in the catalyst
How to Fix It
1
Verify no other codes are present that could cause catalyst damage (misfires, rich/lean conditions)
2
Compare upstream and downstream O2 sensor waveforms to confirm catalyst degradation
3
Inspect exhaust system for leaks near the catalytic converter
4
Test downstream O2 sensor for proper response time and voltage range
5
Replace 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 clamps
Related Error Codes
P0171P0174P0300P0410P0455
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