Check Engine Light P0420 — Catalytic Converter Warning

Common causes

• aging catalytic converter (most common — usually >100k miles)
• failing oxygen sensor downstream of the cat
• exhaust leak between the cat and the rear O2 sensor
• engine misfire dumping unburned fuel through the cat (compound problem)
• incorrect octane / contaminated fuel

How a mechanic diagnoses it

1. scan live O2 sensor data — upstream should switch rapidly, downstream should be flat near 0.7V
2. smoke test the exhaust between the cat and the rear O2 sensor
3. check for stored or pending misfire codes (P030x)
4. swap the downstream O2 sensor and retest
5. physical cat inspection — backpressure test or visible damage

Typical fix cost

$150–$1,800 — most common fix: downstream O2 sensor replacement OR full catalyst replacement. Actual cost varies by vehicle year/make and local labor rates; Fixo's AI estimate adjusts for both.

Codes commonly seen alongside P0420

• P0300 · Ignition

  P0300 Random Misfire — What It Means & When It's Serious

  Why it's related: Misfires dump unburned fuel through the cat and can cause P0420 within days — fix the misfire first.

• P0171 · Fuel

  P0171 System Too Lean — Common Causes & Fixes

  Why it's related: A lean condition lets unburned air through the cat, which over time triggers the same downstream-O2 reading P0420 watches for.