Code P0420: Read This Before Buying a Converter

Overview

P0420 means catalyst efficiency is below threshold. But the code is derived from comparing two oxygen sensors, so the real fault can be a lazy sensor or a small exhaust leak.

Symptoms

• Steady check engine light with normal performance
• Rotten-egg sulphur smell from the exhaust
• Failed emissions test
• Power loss if the converter is badly clogged
• Code returns 60–120 miles after clearing with no drivability change
• Rear oxygen sensor trace swinging almost like the front one on a scan tool

Likely causes

• Genuine catalyst degradation after 90k+ miles or chronic misfire
• Slow-responding downstream oxygen sensor
• Exhaust leak before or between the sensors
• Oil or coolant burning poisoning the catalyst
• Small exhaust leak ahead of the front sensor drawing in air and skewing readings
• Rear O2 sensor gone lazy without failing outright, so it sets no code of its own
• Missing ECU calibration update on certain models

Diagnosis & repair steps

1. Inspect and listen for exhaust leaks on a cold start
2. Compare both sensor waveforms on a scan tool
3. Fix any misfire or oil burning first
4. Replace the converter last, using a certified part
5. Inspect the exhaust visually and by sound on a cold start before buying parts
6. Compare sensor traces: a healthy rear sensor stays fairly flat around 0.6–0.8 V
7. Fix any misfire or oil burning first, or the new converter fails quickly too
8. Ask for a converter temperature-differential test before approving replacement

Expected cost

Exhaust leak repair $60–$200, rear oxygen sensor $120–$350, EPA-compliant aftermarket converter $400–$1,100, OEM converter $900–$2,500 installed.

Figures are a guidance range calculated with a published method — read the methodology.

How to prevent it

Keep plugs on schedule, fix misfires immediately, and never ignore oil burning. Converters rarely die of age — they die from unburned fuel reaching them.

FAQ