P0420 on a Lexus: Causes, Fixes & Repair Cost

🟠 Moderate severity · drivable

A Lexus with P0420 is drivable short-term, but a clogged converter will eventually reduce fuel economy and performance.

What P0420 means on a Lexus

On a Lexus, P0420 means the rear oxygen sensor on bank 1 sees the catalytic converter isn't cleaning the exhaust efficiently. The engine computer compares the front and rear O2 sensors, and when the readings get too similar it sets P0420. Lexus V6 models often have a second, smaller converter downstream, so the code can point to either one.

Is P0420 common on Lexuss?

P0420 appears on higher-mileage RXs, ESs, and GXs just as it does on Toyotas. Lexus catalytic converters and oxygen sensors wear with age, and a slow rear sensor is often misdiagnosed as a dead converter.

Commonly reported on: RX, ES, GX, IS.

Symptoms

Check engine light on
Slightly worse fuel economy
Rotten-egg smell from the exhaust
Often no change in how it drives

Common causes

most common: Worn catalytic converter (front or second converter)
common: Faulty or slow rear oxygen sensor
common: Exhaust leak before the rear sensor
less common: Engine misfire or rich condition that damaged the converter

How to fix it

Fix	DIY?	Typical cost
Inspect the exhaust for leaks and repair them before replacing parts	Yes	$50–200
Test the rear oxygen sensor and replace it if slow	Yes	$80–220
Replace the catalytic converter with an OEM or quality part	Mechanic	$700–2500

Frequently asked questions

Can I drive my Lexus with P0420?

Yes, usually for a short time. The engine will keep running, but a failing converter can reduce fuel economy and eventually clog, so have it diagnosed soon.

Is a bad sensor often the cause of P0420 on Lexus models?

Yes. The rear O2 sensor on many Lexus models gets slow as it ages, and a slow sensor can set P0420 even when the converter is still fine, so it's worth testing first.

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

A rear O2 sensor is about $80–$220, while a converter replacement is typically $700–$2,500 because Lexus parts cost more than standard Toyota parts.