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EmissionsSeverity Low: usually safe to drive

P0420Catalyst System Efficiency Below Threshold (Bank 1)

What it means

The downstream oxygen sensor shows that the catalytic converter on bank 1 no longer stores enough oxygen. It isn't cleaning the exhaust as it should.

Symptoms

  • Check engine light
  • Failed emissions test
  • Usually no change in how the car drives (rattle if the substrate broke up)

Common causes

  • Worn or damaged catalytic converter
  • Exhaust leak near the sensors
  • Faulty or slow downstream oxygen sensor
  • Past misfires or oil burning that damaged the substrate

What to check first

  1. Compare upstream and downstream O2 signals: a good catalyst gives a steady downstream signal
  2. Check for exhaust leaks
  3. Fix any misfire or fuelling fault before replacing the catalyst
Can I drive with P0420? Usually yes. P0420 rarely affects how the car drives, but it keeps the check engine light on and can fail an emissions test.
Practise it: Inefficient catalytic converter in the engine simulator shows this fault in live data, tests and on the scope.Open in the simulator

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

How to test the parts behind P0420

Oxygen (lambda) sensorWiring, typical values, multimeter and scope tests

Known engine problems that set P0420

1.6 GDI Gamma: Oil consumption and piston or bearing damageHyundai, Kia · typically 80,000–200,000 km

Cases with P0420 (0)

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P0420: questions and answers

What does P0420 mean?

P0420: Catalyst System Efficiency Below Threshold (Bank 1). The downstream oxygen sensor shows that the catalytic converter on bank 1 no longer stores enough oxygen. It isn't cleaning the exhaust as it should.

What are the symptoms of P0420?

Check engine light. Failed emissions test. Usually no change in how the car drives (rattle if the substrate broke up).

What causes P0420?

Worn or damaged catalytic converter. Exhaust leak near the sensors. Faulty or slow downstream oxygen sensor. Past misfires or oil burning that damaged the substrate.

How do you diagnose P0420?

1. Compare upstream and downstream O2 signals: a good catalyst gives a steady downstream signal 2. Check for exhaust leaks 3. Fix any misfire or fuelling fault before replacing the catalyst

Can I drive with P0420?

Usually yes. P0420 rarely affects how the car drives, but it keeps the check engine light on and can fail an emissions test.