Which side is bank 2 on my engine?

Bank 2 is whichever cylinder bank does not contain cylinder number 1, as the manufacturer numbers the cylinders. There is no universal rule tying it to the nearside or offside. On many Japanese transverse V6s bank 1 is the rear bank against the bulkhead, on several American V8s bank 1 is the left-hand side as you sit in the car, and other makers do it differently again. Check the manufacturer's data for your specific engine before removing anything, because replacing the wrong catalytic converter or oxygen sensor is a costly error that does not clear the code.

Can an inline four-cylinder engine have a P0430?

Not genuinely. An inline engine has one cylinder bank, so there is no bank 2 to report on. If the code appears on a four-cylinder car, the usual explanations are a basic code reader misinterpreting a manufacturer-specific code, a generic definition list being applied to a code the manufacturer uses for something else, or a non-standard engine or ECU calibration in the car. Have the codes read on equipment that supports the manufacturer's own definitions before acting.

Will P0430 fail an MOT?

Yes. The engine management lamp it triggers is a major defect for petrol cars first used on or after 1 July 2003, and that alone produces a failure certificate. The gas test is a separate matter and a car with one weak bank quite often passes it, because both banks mix before the tailpipe and the analyser sees the average. Passing the gas test does not rescue the MOT while the lamp is on.

I have P0420 and P0430 together — do I need two catalytic converters?

Almost certainly not. Two converters degrading at the same moment is very unlikely; a single cause affecting both banks is the ordinary explanation. Look at fuel pressure, an engine consuming oil or coolant, a purge valve dumping vapour, or a mixture that has been running rich long enough to overheat both units. Fix that first, then reassess the converters, because fitting two new ones to an engine that killed the last pair simply repeats the exercise.

Powertrain code · standard

P0430Catalytic converter on the second cylinder bank is underperforming

Standard definition: Catalyst System Efficiency Below Threshold (Bank 2)

Last updated 27 July 2026

Quick answer

The same catalyst efficiency check as P0420, but reported for bank 2. Bank 2 only exists on V6, V8, V12 and flat engines, so before spending anything the first job is establishing which physical side of the engine your manufacturer calls bank 2 — getting that wrong is the most expensive mistake attached to this code.

How the car works this out

Identically to P0420: the control unit compares the switching of the oxygen sensor ahead of that bank's catalytic converter with the sensor behind it, and sets the code when the rear signal starts to mirror the front one closely enough to show the converter has stopped storing oxygen. What differs is only which half of the engine is being reported on.

What you might notice

Usually none — the code is the symptom
A rotten-egg smell that seems to come from one side of the car
On a V engine, a rattle traceable to one of the two front pipes
Slightly higher fuel consumption

What causes it

Most likely first. A code names the check that failed, not the part that failed it, so more than one of these can produce the same number.

1The bank 2 catalytic converter has degraded, exactly as bank 1 canCommon
How to check: Watch that bank's rear sensor live. A converter with no oxygen storage left lets the rear signal swing in step with the front one.
2The bank 2 rear oxygen sensor is tired and misreporting a healthy converterCommon
How to check: Compare the two banks side by side. On a V engine you have a free reference — if bank 1's sensors behave crisply and bank 2's do not, the sensor is the more likely suspect.
3An exhaust leak on that bank ahead of the rear sensorPossible
How to check: V engines put one manifold hard against a bulkhead or subframe, and that side's manifold gasket and flexi joint fail more often. Check the awkward side first.
4A bank-specific fuelling fault — one leaking injector, one blocked injector, one vacuum leak — that has cooked the converter on that sidePossible
How to check: Compare the fuel trim figures for both banks. A large difference between them points at something specific to one side rather than the converter.
5The wrong converter was replaced during an earlier repair, because bank numbering was misreadLess common

What to do about it

1Establish which bank is which for your engine before anything else. Bank 1 is the bank containing cylinder number 1 as the manufacturer numbers the cylinders. It is not necessarily the nearside, it is not necessarily the front, and on several common V6 layouts it is the rear bank against the bulkhead. Manufacturer data settles it; guesswork does not.
2If you have an inline four, treat a P0430 with suspicion. There is no bank 2 on an inline engine, so the code usually points at a generic code reader misreporting, a previous owner's ECU remap, or a swapped engine.
3Use the good bank as your control. Being able to compare identical sensors on the same car under the same conditions is a diagnostic advantage a four-cylinder owner does not have — use it before buying parts.
4If P0420 and P0430 are both stored, stop thinking about converters. Two converters do not fail in the same week; a shared cause such as rich running, a failing fuel pressure regulator or an engine burning oil does.
5Fix the underlying fuelling fault before fitting a converter, or the new one will follow the old one.

Can you keep driving?

Yes

As with P0420, the car is not unsafe and will usually feel entirely normal. The exception is a rattling converter, whose broken substrate can migrate and restrict that bank's exhaust, which does cost power and can eventually damage the engine on that side. The lamp still fails the MOT.

Will it fail an MOT?

The MOT consequence is the lamp, which is a major defect and ends the test in a failure regardless of which bank is at fault. Where P0430 differs from P0420 in practice is the gas test: a V engine's two banks merge before the tailpipe, so the analyser samples a blend. One weak converter and one healthy one often average out to a pass on carbon monoxide and hydrocarbons, which leaves owners genuinely puzzled that a car with clean-looking gas readings has still failed. It failed on the lamp. It is also worth knowing that a tester checks the catalytic converter is present, and on a V engine that means both of them.

The light this turns on

Engine management lightThe engine management system has found a fault it cannot correct and has stored a code for it. The car will usually still drive, but an illuminated engine light is an automatic MOT failure on petrol cars first used from 1 July 2003 and diesels from 1 July 2008.

Common questions

Which side is bank 2 on my engine?: Bank 2 is whichever cylinder bank does not contain cylinder number 1, as the manufacturer numbers the cylinders. There is no universal rule tying it to the nearside or offside. On many Japanese transverse V6s bank 1 is the rear bank against the bulkhead, on several American V8s bank 1 is the left-hand side as you sit in the car, and other makers do it differently again. Check the manufacturer's data for your specific engine before removing anything, because replacing the wrong catalytic converter or oxygen sensor is a costly error that does not clear the code.
Can an inline four-cylinder engine have a P0430?: Not genuinely. An inline engine has one cylinder bank, so there is no bank 2 to report on. If the code appears on a four-cylinder car, the usual explanations are a basic code reader misinterpreting a manufacturer-specific code, a generic definition list being applied to a code the manufacturer uses for something else, or a non-standard engine or ECU calibration in the car. Have the codes read on equipment that supports the manufacturer's own definitions before acting.
Will P0430 fail an MOT?: Yes. The engine management lamp it triggers is a major defect for petrol cars first used on or after 1 July 2003, and that alone produces a failure certificate. The gas test is a separate matter and a car with one weak bank quite often passes it, because both banks mix before the tailpipe and the analyser sees the average. Passing the gas test does not rescue the MOT while the lamp is on.
I have P0420 and P0430 together — do I need two catalytic converters?: Almost certainly not. Two converters degrading at the same moment is very unlikely; a single cause affecting both banks is the ordinary explanation. Look at fuel pressure, an engine consuming oil or coolant, a purge valve dumping vapour, or a mixture that has been running rich long enough to overheat both units. Fix that first, then reassess the converters, because fitting two new ones to an engine that killed the last pair simply repeats the exercise.

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