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p0420
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BMW · Sedans
Catalyst System Efficiency Below Threshold Bank 1
Quick Answer
Error code P0420 on your BMW 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
330i530i540i740i760i330e 2024330i xDrive 2024530i 2024530i xDrive 2024540i xDrive 2024545e xDrive 2024740i 2024740i xDrive 2024760i xDrive 2024M3 CS 2024M3 Competition 2024M3 Competition xDrive 2024M340i xDrive 2024i4 M50 2024i4 eDrive35 2024i4 eDrive40 2024M3M340ii4i5
Likely Causes
  1. Worn or contaminated catalytic converter
  2. Faulty downstream oxygen sensor
  3. Exhaust leak before or at the catalytic converter
  4. Engine misfires causing catalyst damage over time
  5. Use of leaded fuel or excessive oil consumption
How to Fix It
1
Verify no other engine codes are present that could damage the catalyst
2
Compare upstream and downstream O2 sensor waveforms with a scan tool
3
Inspect exhaust system for leaks at manifold and catalytic converter connections
4
Check catalytic converter temperature differential with an infrared thermometer
5
Replace catalytic converter if efficiency is confirmed below 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 oxygen sensor
Exhaust gaskets
Exhaust clamps
Related Error Codes
P0171P0174P0300P0335P0340
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