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EmissionsSeverity Low: usually safe to drive

P0421Warm Up Catalyst Efficiency Below Threshold (Bank 1)

What it means

The warm-up (close-coupled) catalyst on bank 1 is not working well enough.

Symptoms

  • Check engine light
  • Failed emissions test

Common causes

  • Worn warm-up catalyst
  • Exhaust leak
  • Faulty oxygen sensor

What to check first

  1. Compare the oxygen sensor signals before and after the catalyst
  2. Check for exhaust leaks
  3. Fix misfires and fuelling faults first
Can I drive with P0421? Usually yes. P0421 rarely affects how the car drives, but it keeps the check engine light on and can fail an emissions test.

Generic OBD-II meaning. Manufacturer-specific meanings and checks can differ; always check the service information for the car.

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P0421: questions and answers

What does P0421 mean?

P0421: Warm Up Catalyst Efficiency Below Threshold (Bank 1). The warm-up (close-coupled) catalyst on bank 1 is not working well enough.

What are the symptoms of P0421?

Check engine light. Failed emissions test.

What causes P0421?

Worn warm-up catalyst. Exhaust leak. Faulty oxygen sensor.

How do you diagnose P0421?

1. Compare the oxygen sensor signals before and after the catalyst 2. Check for exhaust leaks 3. Fix misfires and fuelling faults first

Can I drive with P0421?

Usually yes. P0421 rarely affects how the car drives, but it keeps the check engine light on and can fail an emissions test.