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All codes/P0420

P0420

Catalyst System Efficiency Below Threshold (Bank 1)

Moderate severity

Your car's computer thinks the catalytic converter on the engine's first bank isn't cleaning exhaust gases as well as it should, which means it's not working efficiently.

The powertrain control module (PCM) has detected that the catalytic converter system for Bank 1 is not operating at the required efficiency.

Common causes

  • Failing Catalytic Converter

    This is the most common cause, especially on older vehicles or those with high mileage, indicating the catalyst material is degraded.

    high

    Typical part cost: $500-2500+

  • Oxygen Sensor (O2 Sensor) Malfunction

    A faulty upstream or downstream O2 sensor can send incorrect readings, leading the PCM to falsely believe the converter is inefficient.

    medium

    Typical part cost: $100-300

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First things to check

  1. 1Scan for other related codes (misfires, O2 sensor codes).
  2. 2Visually inspect exhaust system for leaks, damage, or loose connections.

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Tools you may need

Already have a compatible scanner? Use it. If not, a Bluetooth OBD2 adapter can help you confirm engine codes and check supported live sensor data before replacing parts.

Illustration of a Bluetooth OBD2 adapter, not a product photo

Basic checks

Veepeak OBDCheck BLE

A Bluetooth LE adapter for reading standard engine codes and supported live data.

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Illustration of a Bluetooth OBD2 adapter, not a product photo

Regular diagnostics

vgate iCar Pro BLE 4.0

A Bluetooth LE adapter with an automatic sleep feature for routine code and sensor checks.

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Illustration of a Bluetooth OBD2 adapter, not a product photo

Another option

OBDLink CX

A Bluetooth LE adapter to consider when comparing supported vehicles and diagnostic apps.

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Illustrations shown, not product photos. Check vehicle, phone and app compatibility before buying. These adapters do not guarantee access to ABS, airbags or manufacturer-specific systems. Clearing a code does not repair the fault and can reset emissions readiness monitors.

As an Amazon Associate, Wrenchify earns from qualifying purchases. This supports the app at no extra cost to you.

Related codes

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Frequently asked questions

Can I just clear the P0420 code?

Clearing the code will turn off the Check Engine Light temporarily, but if the underlying problem isn't fixed, the code will return. It's a diagnostic indicator, not the problem itself.

Is a P0420 code always a bad catalytic converter?

No, while it's a common cause, it's not always the case. Faulty oxygen sensors, exhaust leaks, or even engine performance issues can trigger this code. Proper diagnosis is crucial before replacing the converter.

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