P0420 code: catalyst efficiency below threshold

What each part of the code means

P: Powertrain
0: Generic code, defined by SAE
4: Auxiliary emissions controls
20: The specific fault

How serious is it?

DeepDiag rates a stored P0420 or P0430 as minor, and a pending one as pending. The rating is a starting point, not a safety check.

A flashing check engine light means stop driving, to avoid damaging the catalytic converter. Have the car checked by a professional.

Possible causes

The engine computer compares the oxygen sensor ahead of the catalytic converter with the one behind it. A working converter changes the exhaust as it passes through, so the two readings differ. When the rear reading starts to look like the front one, the computer sets P0420. Bank 1 is whichever side of the engine has cylinder 1.

In no particular order: which one is likeliest depends on your car and its data.

A worn or damaged converter: it can melt or become contaminated. A P0420 on its own isn't proof that the converter has failed.
An oxygen sensor fault: a faulty sensor ahead of or behind the converter can set the code.
An exhaust leak: a leak in the exhaust, or a cracked exhaust manifold.
Engine problems that damage the converter: misfires, a rich mixture, or oil or coolant reaching the exhaust can overheat the converter or coat it.
Engine software: some makers have released software updates for P0420 on particular models.

What to check in the data

Catalyst monitor tests: pass/fail results and measured values for the catalytic converter on each bank, where the car reports them.
Oxygen sensor tests: results for each sensor on the same bank, where the car reports them.
Oxygen sensor readings: live readings from the sensors ahead of and behind the converter, where the car supports them.
Catalyst temperature: live readings, where the car supports them.
Emissions readiness: whether the catalyst monitor has completed its self-test since codes were last cleared.
Freeze frame: the sensor snapshot from the moment the fault was recorded.
Other codes: a failure in one component can set a code in another, so read every stored and pending code.

Before you clear it or buy parts

Clearing erases stored and pending codes and resets the emissions readiness checks, so the car may fail an emissions test until it has been driven enough for them to complete again. If a stored code comes straight back, that points to an active fault.

Rank the possible causes by likelihood, then run the simplest, cheapest test that would confirm the cause before you buy a part. Read what to do before you clear codes.

Fix the cause first: a misfire, a fuel problem or an oil or coolant leak can damage a converter. Find and fix what damaged it before you replace it.
Look for a service bulletin: search NHTSA's recalls page by year, make and model. The results include manufacturer communications, such as the bulletins makers send to dealers.
Check the emissions warranty: for cars from the 1995 model year on, federal law requires the maker to warrant the catalytic converter for 80,000 miles or 8 years, whichever comes first. Check your warranty booklet before you pay.
Buy a converter your state allows: some states, including California, Colorado, Maine and New York, require a factory replacement or an aftermarket converter that the California Air Resources Board (CARB) has approved, depending on the car. Check your state's rules before you buy.

How DeepDiag helps

DeepDiag reads P0420 and the data above, then explains in plain language what the data shows, what the AI suspects and how confident it is. It also suggests checks to run before you buy parts, and says whether it's a job you can do yourself.

DeepDiag gives an AI-assisted reading of the data your car reports. It isn't a professional inspection, a safety check or a guarantee, and the AI can be wrong. Every feature needs an active Premium subscription.

Coming soon

Get told when it's ready See a sample report

Sources

The possible causes, definitions and repair tips on this page come from these sources, checked in November 2026. What DeepDiag reads and how it rates a code come from the app itself.

P0420: catalyst efficiency below threshold.