P0420 — Catalyst Efficiency Below Threshold (MG Context)
Standard meaning: the rear oxygen sensor is seeing too little difference from the front one — the ECU concludes the catalytic converter isn’t cleaning as it should. It is one of the most misdiagnosed codes in any brand: the catalyst is the most expensive suspect, and it is frequently innocent. On MG’s ageing petrol fleet (older MG3, first-generation ZS and HS) this code deserves method, not a parts cannon.
The usual suspects — cheapest first
- Exhaust leaks upstream of the rear sensor — a small leak skews the reading and fakes catalyst failure.
- Oxygen sensors themselves — an ageing rear sensor produces exactly this picture.
- Engine running rich or burning oil — relevant on early older-generation MG3s, where oil leaks and head-gasket issues are a documented pattern (see the MG3 problems page); a contaminated catalyst is a consequence, and replacing it without fixing the cause buys the same code back.
- The catalyst, last — genuinely failed cats happen, usually past 150,000 km or after sustained misfires.
MG-specific context worth knowing
ZS petrol (Nov 2020 – Feb 2024 build): these cars are subject to the NOx emissions recall (UK R/2025/481, EU-wide KBA 15828R) — a free software recalibration of the emissions system. If your car shows emissions-related codes and hasn’t had the recall work, check your VIN first: get the official recalibration before paying for any diagnosis, and tell the workshop it was done, because it changes the ECU’s emissions behaviour they’re diagnosing against.
Safe owner checks
- Listen cold: an exhaust blow or tick near the manifold when first started points at a leak.
- Check oil consumption between services — topping up regularly on an older MG3 is evidence, not trivia.
- Fuel quality: a tank of the correct RON (older MG3: 95 RON per the UK handbook) and a highway run before re-testing — catalysts self-clean at temperature.
- History: was the code preceded by misfires (P030x)? Fix those first, always.
When it’s a workshop job
Comparative front/rear sensor readings, smoke-testing for leaks, and catalyst temperature checks need tools and judgement. Insist on seeing the rear-sensor data before agreeing to a catalyst — a competent workshop will show you willingly. Emissions equipment removal or “emulator” workarounds are illegal in our covered markets and fail inspections; we don’t document them.