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Enter a 5-character code such as P0420, P0301, or P0457.

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Codes starting with P are engine and transmission, B are body, C are chassis, and U are network communication. A 0 as the second character means a generic code shared by all makes; a 1 means a manufacturer-specific code, so its meaning depends on your brand.

A steady check engine light usually means get it checked soon. A flashing check engine light means an active misfire: reduce speed and stop driving as soon as it is safe.

That doesn't look like an OBD-II code

A valid code is one letter (P, B, C, or U) followed by four characters, for example P0420.

P0420 Catalytic converter efficiency below threshold (Bank 1)

Can I drive with P0420?

Drive soon

OK to drive for now, but get it fixed within days to a couple of weeks, or the damage and the bill can grow.

Not dangerous right now, but a failing converter can clog and cost power, and the car will fail an emissions test.

What does P0420 mean?

The oxygen sensor after the catalytic converter shows the converter on bank 1 is no longer cleaning the exhaust as well as it should. Bank 1 is the side of the engine with cylinder 1; four-cylinder engines have only one bank.

What causes P0420?

A worn converter is most common, but a rear oxygen sensor or exhaust leak can set the same code. Misfire or fuel codes like P0300 or P0172 often come first.

  1. Worn-out catalytic converter
  2. Faulty downstream (rear) oxygen sensor
  3. Exhaust leak near the oxygen sensors
  4. Misfires or a rich mixture that damaged the converter
  5. Outdated engine computer software

How much does it cost to fix P0420?

$150 – $2,500

Oxygen sensor or exhaust leak repair: $150–$500. New catalytic converter: $1,000–$2,500 or more, depending on the vehicle. Rule out the cheaper causes first, and fix any misfire before replacing the converter, or the new one can be damaged the same way.

What are the symptoms of P0420, and how can I confirm it?

Often there are none besides the light. A badly worn or clogged converter can cause a rotten-egg smell, weak acceleration, or a rattle underneath. To confirm, scan for other codes, check for exhaust leaks near the sensors, and on live data see whether the rear oxygen sensor switches as fast as the front one.

Will P0420 fail an emissions test, and should I clear it?

It fails any inspection that reads the OBD-II computer. Clearing it just before the test does not help, because the readiness monitors reset too. After a repair, clear it and drive normally for several days, including some highway driving. If P0420 returns, the converter or rear sensor still needs attention.

Confirm the code yourself

A low-cost scanner lets you read the code, clear it, and see if it comes back before paying for a diagnosis.

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