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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 severity

powertrain

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)

  1. Worn catalytic converter
  2. Faulty downstream O2 sensor
  3. Engine running rich/lean
  4. 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.