P0420 — catalyst efficiency below threshold

Bank 1 · Emissions · Generic OBD2 (P0430 is the same fault on Bank 2)

The short version

P0420 means your engine computer compared two oxygen sensors and decided the catalytic converter isn't cleaning the exhaust as well as it should. It is a conclusion the computer drew from a comparison — not a part number, and not a measurement of the converter itself.

An exhaust leak near the rear sensor, a lazy downstream oxygen sensor, or an engine running rich will all produce this exact code with a perfectly good converter underneath. Every one of those is cheaper to fix, and every one of them looks different on live data.

In order, before you price a converter:

• Check for other codes first. A misfire, lean, rich, or oxygen-sensor code sitting alongside P0420 is almost always the real story. Fix that one first — P0420 frequently disappears with it.
• Watch both oxygen sensors at a warm idle. Five minutes, from the driver's seat. This is the single most informative thing you can do, and it's described in full below.
• Listen for an exhaust leak anywhere ahead of or near the rear sensor. Air sneaking in reads as a healthy-looking oxygen level to a sensor that has no idea where it came from.

Reading codes, watching live sensor data, and checking emissions readiness are free in Fixiom — always, no credits and no subscription. The entire test on this page costs you nothing but a Bluetooth LE adapter you likely already have.

What the code is actually measuring

Your exhaust has an oxygen sensor before the catalytic converter and another one after it. The front sensor is there to run the engine — it tells the computer whether the mixture is rich or lean, and the computer trims fuel constantly in response. That's why a healthy front sensor swings: it's watching the computer chase the target back and forth, several times a second.

The rear sensor has a different job. It exists mostly to grade the converter.

A working converter stores oxygen when there's extra and releases it when there's a shortage — that buffering is how it does its chemistry. The practical consequence is that the exhaust coming out of a healthy converter is much steadier than the exhaust going in. So the rear sensor should look calm and lazy compared to the front one.

The computer watches both, and when the rear sensor starts mirroring the front one — switching nearly as fast and nearly as wide — it concludes the converter has stopped buffering. That's P0420.

Bank 1 means the side of the engine with cylinder 1 on it. Four-cylinder and inline engines only have one bank, so they can only set P0420. V6 and V8 engines have two, and can set P0420, P0430, or both — and both at once points away from the converters and toward something affecting the whole engine.

The five-minute test

This is done from the driver's seat with the engine idling. No jack, no crawling under the car, nothing hot to touch.

• Get the engine fully warm first. This test is meaningless cold. The converter has to be up to operating temperature and the fuel system has to be in closed loop — roughly 10–15 minutes of running, or a normal drive.
• Put both oxygen sensors on screen. You want the Bank 1 upstream sensor (sensor 1) and the Bank 1 downstream sensor (sensor 2). In Fixiom they're under the Oxygen Sensors group on the live data screen; tap either gauge to chart it so you can see the shape over time rather than a jumping number.
• Watch for a minute at idle. Then, if you can do it safely and legally where you're parked, briefly raise the engine to a steady fast idle and hold it. Changes in load make the difference more obvious.

What you're looking for

• Healthy converter: front sensor swinging rapidly, rear sensor comparatively flat — often sitting somewhere in the middle-to-upper part of its range and drifting slowly rather than switching. The two traces look like completely different signals. If this is what you see, be very suspicious of the converter theory and go looking for an exhaust leak or a lazy sensor instead.
• Converter not doing its job: the rear sensor's trace starts to look like a slightly softened copy of the front one — same rhythm, same general swing, just a bit smoothed. The more the two traces resemble each other, the more the code is telling the truth.
• Something wrong with the rear sensor itself: a trace that's dead flat at the very top or very bottom of its range, doesn't move at all when engine load changes, or reads implausibly. That's a sensor or wiring conversation, not a converter one.

This is a comparison, not a pass/fail number, and it's genuinely readable by eye once you've seen it. Two traces that look alike is a bad sign. Two traces that look nothing alike is a good sign.

The number behind the code

There's a layer underneath the code that most people never look at. Your engine computer doesn't just decide pass or fail — it runs an actual catalyst monitor test, records a measured value, and compares that value against a limit. All of it is available over the same OBD2 port, in what's called Mode 06.

Fixiom shows these on the emissions screen: the monitor, its measured result, and the limit it was graded against — so instead of "the computer thinks your cat is bad," you get to see how bad, and how close to the line it actually is. A converter sitting just over the threshold is a very different conversation from one that's nowhere near it.

Honest caveat: some Mode 06 tests are manufacturer-defined rather than standardized, so their labels and units vary by carmaker. Fixiom shows those results but marks them as manufacturer-specific rather than pretending to know exactly what a given carmaker's private test measures. Emissions readiness and Mode 06 are part of the free emissions check — no credits.

Rule these out first

Roughly in order of how often they turn out to be the answer, and how cheap they are to address:

• An exhaust leak ahead of or near the rear sensor. The classic false P0420. A leak lets fresh air into the pipe; the rear sensor sees oxygen and reports it, and the computer reads that as a converter that isn't consuming oxygen. Often audible as a tick or a puff that changes with engine speed, and frequently worse when the engine is cold and the metal hasn't expanded yet. A cracked flex pipe, a leaking gasket, or a rusted seam can all do it.
• A tired downstream oxygen sensor. These age. A slow, lazy, or drifting rear sensor can report a converter as failing when it's fine — and unlike the front sensor, a weak rear sensor often sets no code of its own, because there's nothing obviously wrong with it. It's just slow.
• An engine running rich or lean, or misfiring. This matters twice. First, it can cause the code on its own. Second — and more expensively — a rich mixture or a misfire is what kills converters in the first place. Raw fuel reaching a converter overheats it and destroys the internals. If you replace a converter without finding what killed it, the new one goes the same way.
• A recently fitted budget aftermarket converter. Cheap replacement converters frequently don't have enough of the precious-metal coating to satisfy a factory monitor, and can set P0420 within months of being installed while being physically intact.
• Coolant temperature sensor trouble. If the computer believes the engine is colder than it is, it may never fully enter closed loop or may run rich to compensate — which shows up downstream as a converter complaint.

When it really is the converter

Sometimes it is. The picture that points that way: no other codes, fuel trims within a normal range, no audible leak, a rear sensor that clearly tracks the front one, a Mode 06 catalyst result well past its limit — and often high mileage or a known history of the engine running badly at some point.

A converter that's physically breaking up or plugging is a different failure again, and it usually announces itself with symptoms rather than just a code: noticeable power loss especially at higher speeds, a rotten-egg smell, or a rattle from underneath. Those are worth taking seriously and are past the point where live data is the useful tool.

The shop-floor version: a converter almost never dies of old age by itself. Something upstream usually killed it. Finding that something is the difference between one repair and two.

Is it safe to drive?

A steady check engine light with P0420 alone is generally not an emergency. It's an emissions fault. Most vehicles drive completely normally, and you have time to diagnose it properly rather than reacting to it.

The other real consequence is inspection. In states that read OBD2 codes, a stored P0420 with the light on is a failed emissions test — so it's a deadline, even when it isn't a breakdown.

Clearing the code (and why not to, right before an inspection)

Clearing a code doesn't repair anything. It erases the code, and it erases the freeze frame — the snapshot of engine conditions recorded at the moment the fault set, which is often the most useful single piece of evidence you have. If the underlying fault is still there, the code returns as soon as the catalyst monitor runs again.

There's also a specific trap worth knowing. Clearing codes resets the emissions readiness monitors, and unset monitors are themselves a failing result in most states. Clearing the light the morning of an inspection reliably converts "failed for a code" into "failed for not ready" — and the catalyst monitor is one of the slowest to re-set, sometimes taking several drive cycles including sustained highway speed.

Fixiom shows readiness status on the free emissions screen, so you can see which monitors have actually completed before you spend the money on a test. There's a fuller guide to clearing codes honestly if you want the whole picture.

What a phone adapter can't see

Being straight about the ceiling, because a tool that won't tell you its limits isn't being confident — it's being useless.

What a Bluetooth LE adapter and a phone genuinely give you here: the code, the freeze frame conditions when it set, both oxygen sensors streaming live with history, fuel trims, the Mode 06 catalyst monitor result and its limit, and emissions readiness status. For separating a converter from a sensor from a leak, that's most of what a technician would look at first.

What it cannot do:

• Measure exhaust backpressure. Diagnosing a physically plugged converter needs a gauge plumbed into the exhaust.
• Take a temperature reading across the converter. The inlet-versus-outlet temperature check needs an infrared thermometer and hands under a hot vehicle.
• See a crack, a rusted seam, or a broken-up substrate. That's eyes and ears on the car.
• Replace a tailpipe gas analysis. Only a real emissions bench measures what's actually coming out.

A code narrows the field. Tests confirm. This page gets you from "the computer said catalyst" to a short list of specific things worth checking — which is exactly where the free live data ends and hands on the car begin.

Common questions

Related guides

• P0171 — System too lean — where fuel trims tell you the answer in about two minutes.
• P0300 — Random misfire — and the per-cylinder counters that show which one it really is.
• How to clear a check engine light — You can. Here's what it actually costs you, and when not to.