P0430 — catalyst efficiency below threshold (Bank 2)

Bank 2 · Emissions · Generic OBD2 (P0420 is the same fault on Bank 1)

The short version

P0430 means the computer compared the oxygen sensor before the Bank 2 catalytic converter against the one after it, and concluded that converter isn't cleaning the exhaust the way it should. It's the exact same fault as P0420, measured on the engine's other cylinder bank — a conclusion drawn from a sensor comparison, not a part number.

Everything that produces a false P0420 produces a false P0430 too: an exhaust leak near that bank's rear sensor, a lazy downstream sensor, or an engine running rich. All cheaper than a converter, and all visible in live data first.

Before anyone prices a converter:

• Check whether P0420 is stored alongside it. This single fact splits the diagnosis in half — detail below.
• Check for other codes. A misfire, lean, or rich code next to P0430 is usually the real story.
• Watch the Bank 2 sensor pair at a warm idle. Five minutes from the driver's seat, described below.

Reading codes, watching live sensor data, and checking emissions readiness are free in Fixiom — always, no credits and no subscription.

Which side is Bank 2?

Bank 1 is the side of the engine that contains cylinder 1; Bank 2 is the other side. Which physical side that is — driver or passenger, front or rear of a transverse engine bay — varies by manufacturer and layout, so it's worth confirming for your specific engine before anyone lays hands on anything.

Only engines with two cylinder banks can set this code at all: V6s, V8s, V10s, and flat/boxer engines. An inline four or six has one bank and one catalyst monitor, which is why those engines only ever set P0420.

In live data the naming follows the same convention: Bank 2 Sensor 1 is the upstream sensor on that bank, Bank 2 Sensor 2 the downstream one. Those two are the pair this code is about.

The five-minute test

The full walkthrough — what the sensors are for, what healthy and failed traces look like, the wideband-sensor wrinkle — lives on the P0420 page and applies here verbatim. The only change is which sensors go on screen.

• Get the engine fully warm — the test means nothing cold.
• Put Bank 2 Sensor 1 and Bank 2 Sensor 2 on screen and chart them. In Fixiom they're in the Oxygen Sensors group; tap a gauge to see the shape over time.
• Read the comparison. Front sensor swinging busily while the rear one stays comparatively flat and lazy: the converter is doing its job, so go looking for a leak or a tired sensor instead. Rear sensor mirroring the front one — same rhythm, slightly softened: the code is probably telling the truth.
• Then do the same for Bank 1. Having a healthy bank on the same engine is a luxury P0420-only cases don't get: compare the two rear sensors under the same conditions, and the difference between a working converter and the suspect one is right there on screen.

When P0420 and P0430 set together

This is the fact worth establishing before anything else, because two converters failing independently on the same day is genuinely unlikely. When both bank codes arrive together, the smart money is on something the whole engine shares — usually the thing that's damaging both converters at once:

• A rich condition — unburned fuel is what actually kills catalysts. Check fuel trims on both banks: strongly negative trims mean the computer is pulling fuel out of a mixture something else made rich.
• A misfire history. Misfires dump raw fuel into the exhaust. If misfire codes have come and gone, that's the lead.
• Oil or coolant consumption. Both poison catalyst coatings slowly, and both banks breathe the same crankcase.
• A shared sensing problem — a contaminated MAF mis-fuelling every cylinder, or low fuel pressure leaning the whole engine.

Replacing two converters without finding the shared cause is how people buy four converters.

When it's Bank 2 alone

A single-bank code points at single-bank causes, in rough order of cost:

• An exhaust leak ahead of or near the Bank 2 rear sensor. Fresh air sneaking in reads as oxygen the converter failed to consume. Often audible as a tick that changes with engine speed.
• A tired Bank 2 downstream sensor. Rear sensors age quietly — a slow one frequently sets no code of its own.
• That bank running lean or rich on its own. Compare long-term fuel trim between banks. A meaningful split between Bank 1 and Bank 2 trim points at that bank's injectors, intake sealing, or upstream sensor — and the converter complaint is the symptom.
• The converter itself — most convincing when the mileage is high, the rear sensor clearly mirrors the front, the Bank 1 comparison looks obviously different, and there are no other codes.

Is it safe to drive?

A steady light with P0430 alone is generally not an emergency — it's an emissions fault, and most vehicles drive normally while you diagnose it properly. It will fail an OBD2 emissions inspection, so there's a deadline even without a breakdown.

What a phone adapter can't see

What it genuinely gives you: both banks' oxygen sensors live with history, per-bank fuel trims, the freeze frame from when the code set, Mode 06 catalyst monitor results where the vehicle reports them, and readiness status. The bank-versus-bank comparison is most of what a technician would start with.

What it cannot do:

• Measure exhaust backpressure — the definitive test for a physically plugged converter.
• Take temperatures across the converter — an infrared thermometer and hands under a hot vehicle.
• See a crack, a rattling substrate, or a rusted seam.
• Confirm which physical side Bank 2 is on your engine — that's a service-information question, and worth answering before parts move.

Common questions

Related guides

• P0420 — Catalyst efficiency below threshold — the code people get quoted a converter for.
• P0300 — Random misfire — and the per-cylinder counters that show which one it really is.
• P0171 — System too lean — where fuel trims tell you the answer in about two minutes.
• Check engine light flashing — A flashing light is not the same warning as a steady one. What to do right now.