⚠ P0420 LOW SEVERITY

P0420: what it means

Officially? A wall of jargon. To most people? Absolutely nothing. That's exactly why we built the app.

The official definition vs. an actual explanation

Plug into any code reader and you'll get the same line every other app shows you. Technically correct. Practically useless.

What other apps tell you

"P0420 — The catalytic converter on bank 1 isn't reducing emissions as efficiently as it should."

What OBD Mechanic tells you

"This one means your catalytic converter on bank 1 isn't cleaning up the exhaust as well as the car expects. Nine times out of ten it's a genuinely tired cat, but before you spend big, rule out the cheaper culprit: a lazy or duff rear (post-cat) oxygen sensor giving a false read, or an exhaust leak upstream fooling it. If there are any fuel or misfire codes alongside it, sort those first, because they'll cook a cat and bring this code straight back."

What actually causes it, cheap to dear

Worth checking in roughly this order. Don't jump straight to the expensive fix:

Exhaust leak near the cat or around the sensors throwing the reading off
Lazy or failed rear (downstream) oxygen sensor giving a false efficiency read
Running rich or misfiring (fix any fuel/misfire codes first — they kill cats)
Genuinely worn-out catalytic converter (the expensive one, but common on high-mileage cars)

usually cheap depends pricey

Suggested checks

Scan for any other codes first — misfires or fuel trim faults are often the real cause
Have a listen and look for exhaust leaks between the engine and the cat
Compare the front and rear O2 sensor readings live — a good cat keeps the rear voltage steady while the front swings
If all that's clean, the cat itself is the likely answer

Which dials to watch for P0420

The code itself is a snapshot. The fault sets after the car's watched the numbers over a whole drive cycle. Here's what's worth keeping an eye on:

O2 B1S2 post-cat voltage

The whole point of this code. A healthy cat holds this rear sensor fairly steady; if it swings around copying the front sensor, the cat's not doing its job.

Not simulated live Our free demo drive doesn't model this reading yet. Watch it live on your own car in the app.

Short fuel trim

Tells you if the engine's running rich, which is what wears cats out in the first place. Worth watching so you fix the cause, not just the symptom.

Not simulated live Our free demo drive doesn't model this reading yet. Watch it live on your own car in the app.

Illustrative panels. These readings aren’t in our free demo drive, but the app shows them live from your own car.

It spots things you'd never see in the numbers

Codes rarely tell the whole story. The clever part is reading them together, and against your exact car. Here's the kind of thing it catches:

AI Mechanic

What our mechanic says

P0420

This one means your catalytic converter on bank 1 isn't cleaning up the exhaust as well as the car expects. Nine times out of ten it's a genuinely tired cat, but before you spend big, rule out the cheaper culprit: a lazy or duff rear (post-cat) oxygen sensor giving a false read, or an exhaust leak upstream fooling it. If there are any fuel or misfire codes alongside it, sort those first, because they'll cook a cat and bring this code straight back.

P0420: quick questions

Can I still drive it?

Yes, it's safe to drive — the car will run fine and it won't leave you stranded. It's an emissions fault, not a mechanical danger. Only worry if it comes with rough running or a loss of power, which points to a different problem feeding into it.

How much will it cost to fix?

If it's just a rear O2 sensor, you're looking at maybe £40-120 in parts plus fitting. A genuine cat replacement is the pricey one — anywhere from £150 for a universal aftermarket unit to £600+ for an OEM part on some cars, plus labour. That's exactly why it pays to rule out the cheap causes first.

Will it pass the MOT?

Not with the light on — an illuminated engine management light has been an automatic MOT fail in the UK since 2018, so it'll fail on that alone. And if the underlying cat is knackered, the car may fail the actual tailpipe emissions test on top, so it pays to sort the real cause first.