P0430 is the same test as P0420 but on bank 2, the other cylinder bank of a V6, V8 or flat engine. The converter on that side is not cleaning the exhaust as well as the computer expects, or the rear sensor watching it is misreading. Diagnose soon and drive gently. Indicative UK repair cost is £60 to £900.
Bank 2 only exists on an engine that has two cylinder banks. If your car has a straight-four there is one bank, one pair of sensors, and no bank 2 for a code to be about, so the very first thing worth doing is confirming the code was read correctly. On a V6 or V8 you have two converters and two rear sensors, which gives you two separate chances to see this fault, one extra question to answer, and one genuine advantage that a four-cylinder owner does not get.
Can I drive with P0430?
Diagnose soon, drive gently. The car will almost certainly feel completely normal and you can keep using it. What is worth avoiding is letting it sit for months, because the cheap explanations for this code stay cheap while the expensive one gets steadily worse. On a V engine there is a second cost to that delay: if the real cause is an engine-wide fuelling problem, the converter on the other bank is ageing at exactly the same rate and you will be reading about it again before long.
What P0430 means for your MOT
The position matches a bank 1 catalyst fault, with one addition that applies only to a two-bank engine. Both converters are inspected, so one good side does not carry a bad one.
- The lamp. A major defect under section 8.2.1.2 of the DVSA MOT inspection manual for petrol cars first used on or after 1 July 2003, or 8.2.2.2 for diesels first used on or after 1 July 2008.
- The tailpipe figures. The probe goes in one tailpipe and measures the gas after both banks have joined up. A converter that has failed on one side only can still drag the combined hydrocarbon or lambda reading outside the permitted limits, and that is a separate major defect. People assume the good side will dilute the bad one. Sometimes it does. Often it does not, and that assumption is why this comes as a surprise.
- The converters as parts. Section 8.2.1.1 names catalytic converters among the components a tester inspects. On a V engine that means both of them, and a decat on either side is a major defect in its own right.
What usually causes it
- The bank 2 converter has lost efficiency. The genuine version of this fault. Age, mileage and heat cycles have taken the coating past the point where it can hold and release oxygen the way the test expects.
- The rear oxygen sensor on that bank. With a practical twist unique to a V engine: both rear sensors are the same age and have covered the same miles, so if one has gone lazy the other is unlikely to be far behind it.
- An exhaust leak on that side. Manifold studs and flexi joints on the rear bank of a transverse V6 sit in the most awkward place on the car, get inspected the least often, and corrode exactly as fast as the ones you can see.
- An underlying misfire or a rich mixture. If P0420 is stored alongside P0430, start here. Two converters do not normally fail in the same fortnight. An engine-wide ignition or fuelling problem damages both at once, and it is both the cheaper explanation and the one that stops you doing the job twice.
What it costs to fix in the UK
£60 to £900, which looks identical to the bank 1 figures and comes with two warnings attached. The rear bank of a transverse V6 often sits against the bulkhead, and labour there can be twice what the front bank costs. And if a quote covers both converters, ask what the evidence is for replacing the second one, because the answer should be test data rather than symmetry.
Before agreeing to anything at that end of the range, it is worth comparing the investigation fee itself against what a car diagnostic costs in the UK.
These are indicative UK ranges for guidance only. Actual causes and prices vary by vehicle, and a stored code points to a system, not always the exact part.
What to check first
- Establish which side is bank 2 on your engine, from manufacturer data. Manufacturers do not number banks consistently, and fitting an expensive converter to the wrong side of a V6 is a mistake that genuinely gets made. Do not rely on a rule of thumb from a forum.
- Use the good bank as your reference. This is the one real advantage of a two-bank engine. Put both rear sensor traces side by side with the same engine feeding both. If bank 2's rear sensor is behaving in a way bank 1's is not, that comparison tells you far more than any single reading could.
- Check whether bank 1 has stored anything. One bank flagged points at that bank. Both banks flagged points upstream at something they share.
- Inspect the bank 2 manifold and flexi joint for leaks. Cold start, listen, and look for soot tracking around the joints. It is awkward to get to and that is exactly why it gets skipped.
Related codes
- P0420, catalyst efficiency below threshold (Bank 1). The same fault on the other side, and the page to read if both are stored.
- P0171, system too lean (Bank 1). A fuelling condition that can be hard on a converter over time, and one of the upstream causes worth eliminating first.
- P0300, random or multiple cylinder misfire. The classic shared cause when both banks have flagged a catalyst problem at once.
Get it narrowed down before you spend
A stored code tells you which system is unhappy, not which part has failed. If you want it narrowed down before you spend anything, send the code and your symptoms and a mechanic will read them personally. Quick Diagnosis is £20, and if we can't help, you don't pay.
Written by the First Diagnostic mechanic. Over a decade in the trade: a VW-only workshop running diagnostics and electrical repair, then BMW-only through to full engine rebuilds. Now an independent all-makes workshop, plus building and wiring drift cars for competition, including trackside work as head mechanic. More about who reads your codes.