OBD-II Tool
Fault code decoder
Paste a DTC from your scanner or garage report. We'll tell you what it means, what's likely causing it, and what it usually costs to fix in the UK.
P0420 — Catalyst Efficiency Below Threshold (Bank 1)
medium severitypowertrain
What it means
The catalytic converter is no longer cleaning exhaust gases properly.
Symptoms you'll notice
- EML on
- MOT emissions fail risk
- Slight power loss
- Rotten-egg smell
Likely causes (most common first)
- Worn catalytic converter
- Faulty downstream O2 sensor
- Engine running rich/lean
- Exhaust leak before cat
Fix yourself?
Garage job
Typical UK cost
£200–£1200
Indicative only. Pull all stored codes — a single DTC rarely tells the full story. Confirm with live data before replacing parts.
Common UK codes — quick links
Need a deeper read? Run a free vehicle history check — Sam reads your MOT advisories and tells you which faults match.