P0420 — Catalyst System Efficiency Below Threshold (Bank 1)

P0420 means your catalytic converter on Bank 1 is no longer cleaning exhaust gases efficiently. The most common cause is a worn catalytic converter, but failing oxygen sensors and exhaust leaks trigger it too.

Cost range: $200–$2,500. Severity: medium. Safe to drive? Yes, you can usually drive several hundred miles with P0420, but you'll fail emissions testing and a damaged cat can cause power loss or further sensor damage.

Most common causes of P0420

  1. Failing catalytic converter — Most common on vehicles with 100k+ miles, especially after upstream misfires. ($800–$2,500)
  2. Faulty downstream O2 sensor — The rear sensor reports cat efficiency. A lazy sensor mimics a bad cat. ($150–$400)
  3. Exhaust leak before the cat — Leaks throw off the O2 reading and trigger P0420 falsely. ($100–$500)
  4. Engine misfire damaging cat — Long-term P03xx misfires destroy cats from the inside. (Varies)

Symptoms drivers report with P0420