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P0420: Catalyst System Efficiency Below Threshold (Bank 1)

What this check-engine code means in plain English, what usually causes it, and what a fix typically costs.

Severity: Monitor — not an emergency, but address soon

What P0420 means

The catalytic converter on Bank 1 is not reducing emissions as efficiently as expected. The powertrain control module compares oxygen sensor readings before and after the converter; when they're too similar, it means the catalyst isn't doing its job properly.

What causes P0420?

Likely
Worn or failing catalytic converterNormal wear over time or contamination from oil/coolant consumption reduces the catalyst's ability to convert harmful gases.
Likely
Faulty downstream oxygen sensorThe rear O2 sensor may be giving inaccurate readings, making the computer think the converter has failed when it hasn't.
Possible
Exhaust leak before the catalytic converterA leak upstream can allow extra oxygen in, skewing sensor readings and triggering a false catalyst efficiency code.
Less common
Engine misfires or running rich/leanUnburned fuel or improper air-fuel mixture can damage the converter or cause it to appear inefficient.

Is P0420 safe to drive?

Drive it?
Usually yes, short-term
Watch for
Reduced fuel economy, sulfur smell, or rattling from exhaust
DIY-friendly?
Sensor replacement is moderate DIY; converter replacement requires tools and safety precautions

You can typically continue driving with P0420, as it primarily affects emissions rather than drivability. However, you may fail an emissions test, experience reduced fuel economy, and in some states face fines for operating a vehicle with a malfunctioning emissions system.

P0420 repair cost

~$150 – $2200+

Industry-average estimates. Cost depends heavily on whether you need just an oxygen sensor or a full catalytic converter replacement, and whether OEM or aftermarket parts are used.

Which one is it for your car?

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Related codes

P0430