OBD-II Code P0420 — What It Means & How to Fix It

What Does This Error Mean?

P0420 means your catalytic converter isn't cleaning exhaust gases as well as it should. Your car has oxygen sensors before and after the catalytic converter. When both sensors show similar readings, the computer knows the converter isn't doing its job. Your car will still run, but it's producing more pollution. This is one of the most common check engine light codes.

Affected Models

All vehicles 1996+
Common in Toyota Corolla and Camry
Common in Honda Accord and Civic
Common in Subaru Outback and Forester
Common in Ford Focus

Common Causes

Worn-out catalytic converter that has lost its ability to process exhaust
Faulty downstream oxygen sensor giving false readings to the computer
Exhaust leak before or near the catalytic converter
Engine misfire sending unburned fuel into the converter and damaging it
Using leaded fuel or certain fuel additives that poison the converter

How to Fix It

First, check for other codes alongside P0420. If you see misfire codes like P0300 or lean/rich codes like P0171, fix those first. An engine problem can trigger P0420 as a secondary issue.

Always fix the root cause before replacing expensive parts like the catalytic converter.
Inspect your exhaust system for leaks. Look for rust holes, broken hangers, or loose connections between the exhaust manifold and the catalytic converter. An exhaust leak can fool the oxygen sensors.

You can sometimes hear exhaust leaks as a ticking or hissing sound when the engine is running.
Try cleaning the catalytic converter with a fuel additive designed for this purpose. Add it to a full tank of gas and drive at highway speeds for 30-50 miles. This can sometimes restore a marginally failing converter.

Products like Cataclean or Solder-It are available at most auto parts stores for $15-$25.
Test or replace the downstream oxygen sensor (the one after the catalytic converter). A lazy or failing sensor can trigger P0420 even when the converter is fine. Sensors cost $30-$100 and are often a DIY job.

Oxygen sensors typically last 60,000-100,000 miles. If yours is old, it's worth replacing as a first step.
If the code returns after trying the above steps, the catalytic converter itself likely needs replacement. Get quotes from a few shops — prices vary widely. Make sure they use a converter that meets your state's emissions standards.

In California and states that follow CARB standards, you must use a CARB-compliant converter, which costs more.

When to Call a Professional

Catalytic converter replacement is best left to a professional. It involves working under the car and often requires welding or specialized tools. A mechanic can also confirm whether the converter is truly bad or if it's just a sensor issue. Expect to pay $100-$200 for diagnosis. Converter replacement ranges from $500 to $2,500 depending on your vehicle.

Frequently Asked Questions

Will P0420 cause my car to fail an emissions test?

Yes, in most states a P0420 code will cause an automatic emissions test failure. The check engine light must be off to pass. You'll need to fix the issue and clear the code before testing. After clearing, you may need to drive 50-100 miles for the system to complete its self-check.

Can I just delete the code and ignore it?

You can clear the code, but it will come back if the problem isn't fixed. Driving with a bad catalytic converter won't damage your engine. But it increases pollution and will fail emissions testing. Also, if the converter breaks apart internally, debris can clog your exhaust and cause serious engine problems.

Why is catalytic converter replacement so expensive?

Catalytic converters contain precious metals like platinum, palladium, and rhodium. These metals are what actually clean the exhaust gases. The raw materials alone can cost hundreds of dollars. Labor adds another $100-$300 depending on how accessible the converter is on your vehicle.