P0430 fault code: Catalyst System Efficiency Below Threshold
Last updated · OBDCode UK editorial
A bank-2 catalyst-efficiency flag is not, on its own, a reason to abandon the car. Stop if the engine management light flashes, the engine judders, or exhaust fumes enter the cabin.
Can I DIY?
Caution — conditions apply. We do not publish DIY repair or strip-down steps for this code on this page. Read codes and observe only; leave the repair to a qualified garage.
Some checks may be possible for a careful DIY driver with the right tools and a safe place to work. Stop if anything feels beyond your skill or kit.
Never open a hot coolant cap — pressure can scald you. Exhaust and catalytic converter get hot enough to burn and ignite debris. Stay clear of electric fans; they can run after shut-off.
Never idle in a closed garage to read codes — an open door is not enough. CO is odourless and can kill. Run diagnostics in open air. Headache or nausea with exhaust smell: get fresh air and stop.
What does P0430 mean?
P0430 is a generic (SAE standard) powertrain code reported by your car’s engine control unit. It sits in the P04xx block, which covers auxiliary emission controls. The stored description is “Catalyst System Efficiency Below Threshold”. The code itself tells you which circuit or system tripped the fault — not always the exact part that failed.
How to diagnose P0430 yourself
- Confirm P0430 versus P0420: Scan with an OBD-II reader. Record whether P0430 is current or pending, and whether P0420 is stored as well. Do not assume bank 1 and bank 2 share one converter on every engine. Stop if: The engine management light is flashing, or you intend to go under the car on a jack.
Park safely and switch off before locating the socket. Its position varies, so use the vehicle handbook and never force or dismantle trim. - List bank-2 companion codes: From the same scan, write down misfire or oxygen-sensor codes that specify bank 2. Leave the car on its wheels. Do not probe sensors on a hot exhaust. Stop if: You also have a flashing lamp, a strong exhaust smell in the cabin, or you plan to touch the converter to ‘test’ heat.
- Note whether the lamp is steady: Look at the engine management light with the engine running in open air. Steady illumination matches this efficiency flag. A flashing light is a different, more urgent condition. Stop if: The lamp flashes, the engine judders, or fumes enter the cabin.
A flashing engine management light means stop safely and arrange recovery, whether or not you feel judder. A steady light still needs prompt diagnosis, and becomes a stop if it starts to flash, judder or lose power.
Will P0430 fail an MOT?
An illuminated engine management light is a Major defect at MOT until the lamp stays off. If bank 2’s converter is genuinely inefficient, the vehicle may also fail the exhaust emissions test. That is a garage measurement, not a driveway guess.
Before booking your test, check what usually fails on your car: see our MOT pass rates by make and model, built from 33.1 million DVSA tests.
What the MOT data says
These figures are copied from the MOT fail cohort for the certificate wording below. They are not a diagnosis of this car.
Certificate wording: “Catalyst emissions Lambda coefficient outside the default limits or the range specified by the manufacturer”.
Of 166,607 Class 4 vehicles that failed a 2024 MOT on that wording, 7.5% (95% CI 7.4–7.6) failed on the same wording again in 2025. The not-tested gap versus control was 13.4 points.
Certificate wording: “Catalyst emissions Emissions levels exceed default limits”.
Of 82,254 Class 4 vehicles that failed a 2024 MOT on that wording, 9.6% (95% CI 9.4–9.8) failed on the same wording again in 2025. The not-tested gap versus control was 18.7 points.
Certificate wording: “Catalyst emissions Emissions levels exceed the manufacturer's specified limits”.
Of 73,521 Class 4 vehicles that failed a 2024 MOT on that wording, 9.4% (95% CI 9.2–9.6) failed on the same wording again in 2025. The not-tested gap versus control was 18.5 points.
Engine management light MOT outcomes
Related guides
Related fault codes
P0430 — quick answers
Is it safe to drive with P0430?
Drive with care. A bank-2 catalyst-efficiency flag is not, on its own, a reason to abandon the car. Stop if the engine management light flashes, the engine judders, or exhaust fumes enter the cabin.
Can I fix P0430 myself?
Caution — conditions apply. We do not publish DIY repair or strip-down steps for this code on this page. Read codes and observe only; leave the repair to a qualified garage. Basic tools only — for example a household socket set, tyre gauge, or a consumer OBD reader.
Will P0430 fail my MOT?
An illuminated engine management light is a Major defect at MOT until the lamp stays off. If bank 2’s converter is genuinely inefficient, the vehicle may also fail the exhaust emissions test. That is a garage measurement, not a driveway guess.
Fault-code identifiers are standard diagnostic facts. Their explanations and diagnostic guidance on this site are original editorial summaries, not reproduced SAE wording. MOT defect rules reflect the DVSA MOT inspection manual (2026). This page is general guidance for UK drivers, not a substitute for a professional diagnosis.