P0420 on a Toyota: Causes, Fixes & Repair Cost

🟠 Moderate severity · drivable

You can usually keep driving a Toyota short-term with P0420, but a clogged converter will eventually hurt power and fuel economy.

What P0420 means on a Toyota

On a Toyota, P0420 means the rear oxygen sensor on bank 1 reports that the catalytic converter isn't cleaning the exhaust well enough. Toyota's engine computer compares the front and rear O2 sensor readings, and when they move too similarly, it flags the converter as underperforming. Many Toyota V6 models have a second, smaller converter downstream, so the code can point to either one.

Is P0420 common on Toyotas?

P0420 is one of the most common check-engine codes on higher-mileage Toyotas, especially Camrys, Corollas, and RAV4s. Toyota catalytic converters and oxygen sensors wear with age and mileage, and a slow-reading rear sensor is often mistaken for a dead converter.

Commonly reported on: Camry, Corolla, RAV4, Highlander.

Symptoms

Check engine light on
Slightly worse fuel economy
Rotten-egg smell from the exhaust
No obvious driving problems at first

Common causes

most common: Worn catalytic converter on bank 1
common: Faulty or slow rear oxygen sensor (bank 1, sensor 2)
common: Exhaust leak before the rear O2 sensor
less common: Engine misfire or rich condition that damaged the converter

How to fix it

Fix	DIY?	Typical cost
Check for exhaust leaks and repair them before replacing anything	Yes	$50–200
Test the rear oxygen sensor and replace it if it reads slowly	Yes	$60–200
Replace the catalytic converter with an OEM or quality aftermarket unit	Mechanic	$600–2200

Frequently asked questions

Can I drive my Toyota with a P0420 code?

Yes, usually for a short time. The engine will still run, but a failing converter can hurt fuel economy and eventually clog, so have it checked soon.

Is a bad oxygen sensor a common cause of P0420 on Toyotas?

Yes. The rear O2 sensor on many Toyotas gets slow as it ages, and a slow sensor can trigger P0420 even when the converter is still okay, so it's worth testing before replacing the converter.

How much does it cost to fix P0420 on a Toyota?

Most Toyota repairs run from about $60 for a rear sensor to $600–$2,200 for a converter. Diagnosis usually starts with checking the O2 sensor readings and the exhaust for leaks.