P0420: Catalyst system efficiency below threshold (Bank 1)

What P0420 means

The catalytic converter is not cleaning the exhaust as well as it should. It is often an aging converter, but a bad oxygen sensor or an exhaust leak can cause the same code.

Common symptoms

• Failed emissions test
• Rotten egg smell from the exhaust
• Worse fuel economy
• Loss of power
• Often no noticeable change in how the car drives

Common causes

• Worn-out catalytic converter
• Faulty or worn oxygen sensor
• Exhaust leak near the sensor
• Worn spark plugs

Can I keep driving?

Fix soon. Usually drivable for a short time, but get it checked soon. Ignoring it can waste fuel, raise emissions and lead to more damage.

What to do next

1. Write down the code and any symptoms you notice.
2. Check the simple things first: the fuel cap, loose connectors and fluid levels where they apply.
3. Do not just clear the code. If the light comes back, the cause is still there and needs a proper diagnosis.
4. Read live data and freeze frame data with an OBD-II scanner app to see what the car was doing when the code was set.

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