P0420: what it means, common causes, and whether you can drive

What does P0420 mean?

P0420 is the generic code for "Catalyst System Efficiency Below Threshold (Bank 1)", the wording Subaru and Acura use in their service bulletins (Subaru, 2023; Acura, 2019). The engine computer has run its catalyst test and decided the converter on bank 1 isn't cleaning the exhaust as well as the manufacturer's threshold requires.

• Bank 1 is the side of the engine with cylinder 1. A four-cylinder engine has only one bank.
• P0430 is the same fault on bank 2, on V6 and V8 engines. GM's break-in bulletin treats the two codes together (GM PIP3119M).

The code does not measure tailpipe pollution directly. It is a judgment the car makes from its own sensors, and that is why things other than a worn converter can trigger it.

How does the car decide the converter is weak?

A catalytic converter stores and releases oxygen as the engine's mixture swings slightly rich and lean. There is an oxygen sensor before the converter (upstream) and one after it (downstream).

• The upstream sensor sees the mixture switching back and forth as the engine computer trims the fuel.
• Behind a healthy converter, the downstream signal stays much steadier, because the converter's stored oxygen smooths out those swings.
• As the converter loses oxygen storage, the downstream signal starts to copy the upstream one, switching more often (Ford patent EP1118753A2).

There are two main ways of testing this:

• a switch ratio, which counts how often the downstream sensor switches between rich and lean against how often the upstream one does; a ratio above a calibrated threshold means the converter is deteriorating (Ford patent EP1118753A2);
• an oxygen storage capacity test, in which the engine computer deliberately swings the mixture rich and lean and calculates how much oxygen the converter stores; too little means it has deteriorated (Toyota patent US8020371B2).

Which method your car uses, and its exact threshold, is set in its manufacturer's calibration.

Either way, anything that upsets those two sensor signals can make a good converter look bad.

What causes P0420?

A tired converter is the obvious answer, but it is not the only one. These causes come from manufacturer bulletins and patents, California's OBD-II rule and, where marked, our own reading. Subaru's P0420 procedure (Subaru SIB 09-110-23) is a useful model: it has technicians rule out the engine, the sensors and the exhaust before they condemn the converter.

The misfire and oil rows matter most. A P0420 can be the result of a problem further up the engine. If you replace the converter without fixing that problem, the new converter can end up the same way.

Can you keep driving with P0420?

Usually yes, for a while, if the light is steady and the car drives normally. The California Air Resources Board says most manufacturers advise having the car serviced "as soon as conveniently possible". It also warns that driving for a long time without fixing a problem could damage other components, and that replacing a catalyst can be expensive.

It changes when you see any of these:

• The check-engine light is flashing. CARB says a blinking light means a malfunction is happening that could damage the catalytic converter, and many manufacturers recommend service as soon as possible. Reduce speed and load, drive only as far as you need to, follow your owner's manual, and get it looked at the same day if you can. Our guide Check engine light on: can I keep driving? covers the flashing light in detail.
• A strong rotten-egg smell from the exhaust. Wisconsin's consumer protection agency says a continuous burning-sulfur smell usually means a problem in the catalytic converter or other emission control devices, and advises: "Do not delay diagnosis and repair" (Wisconsin DATCP).
• Loss of power, rough running, or a burning smell. Stop somewhere safe and have the car checked before driving further. A converter that is breaking up or overheating is not something to push through.

One more practical point: P0420 turns the check-engine light on, and a lit light fails an OBD-based emissions test even if the car feels fine. Our readiness guide covers the test rules.

What should you do about P0420?

1. Read every code, not just P0420. Look for misfire codes (P0300–P0308), fuel trim codes such as P0171, and oxygen sensor codes. Check pending codes too; our explainer on pending, stored and permanent codes covers why they matter.
2. Fix the engine first. Deal with any misfire, fuel trim fault or oil consumption before you judge the converter.
3. Check for exhaust leaks ahead of and around the converter and the sensors, as Subaru tells its technicians to.
4. Ask a dealer about bulletins and software updates for your model. The Acura case above was fixed by a software update, not a new converter.
5. Don't just clear the code. Clearing it resets the readiness monitors, so the car will read "not ready" for an emissions test until it has been driven enough. See why a car says "not ready".
6. If the converter really is worn, use a legal replacement. In California, a replacement must be a dealer part for your vehicle or an aftermarket converter approved by CARB for your specific vehicle (CARB). Other states and provinces have their own rules, so check yours.

Cost: converter prices vary widely by vehicle and region. We haven't found a current, independently sourced range we trust, so we don't quote one. Get a written estimate that says what diagnosis was done.