P0420 🟡 Moyen ✅ Can drive (carefully)

Catalyst System Efficiency Below Threshold — Bank 1

The ECU has detected that the catalytic converter on bank 1 is no longer reducing exhaust pollutants efficiently. One of the most common fault codes worldwide, particularly on high-mileage vehicles.

🚗 Common on: Vauxhall Astra, Ford Focus, VW Golf, Toyota Yaris, Renault Clio, Peugeot 308

💷

Average repair cost

£10 — £700

🔍 Symptoms

  • → Check engine light on (solid)
  • → Slight power loss
  • → Slightly increased fuel consumption
  • → Rotten egg smell from exhaust

⚙️ Common Causes

  1. 1Worn or poisoned catalytic converter (oil/lead contamination)
  2. 2Faulty downstream O2 sensor
  3. 3Exhaust leak before the cat
  4. 4Engine burning oil
  5. 5Untreated misfires that destroyed the cat

🔧 Recommended Fixes

  1. 1Try a catalytic converter cleaner additive (~£10) — works in ~30% of cases if cat is mildly worn
  2. 2Test O2 sensors with a multimeter
  3. 3Inspect exhaust for leaks
  4. 4Replace downstream O2 sensor (~£30-120)
  5. 5Replace catalytic converter (~£120-700)

🛒 Recommended Parts (Amazon UK)

Catalytic converter cleaner additive~£10
View on Amazon UK →
Universal O2 lambda sensor~£25-70
View on Amazon UK →
Multimeter for sensor testing~£18
View on Amazon UK →
OBD2 scanner to clear the code~£22
View on Amazon UK →

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Frequently Asked Questions

Can I clear this code without fixing it?

Yes, but it will return if the underlying issue isn't fixed. Clearing a code only tells you if the problem is permanent or intermittent.

Will this code fail an MOT?

Probably yes if the check engine light is on during the MOT. The MOT checks OBD readiness monitors since 2018.