P0420 — Catalyst System Efficiency Below Threshold

What Does P0420 Mean?

The P0420 code is triggered when the ECU detects that the downstream (post-converter) oxygen sensor is reading similar to the upstream sensor — indicating the converter isn't processing exhaust gases. A healthy converter should show a much steadier downstream O2 signal compared to the constantly fluctuating upstream signal.

Can I Drive with P0420?

Yes, P0420 won't cause the car to break down. But you will fail an emissions test, and ignoring it long-term can cause the converter to collapse internally and restrict exhaust flow, which can cause overheating. Fix it within the next few months.

Common Causes (Ranked by Likelihood)

• Worn or failing catalytic converter — most common cause by far
• Failed downstream O2 sensor — cheap fix, worth testing first
• Exhaust leak between engine and converter — creates false readings
• Engine running rich (fouled converter) — oil burning, coolant leak, bad injector
• Incorrect or counterfeit replacement converter not meeting OEM specs

Diagnosis Before Replacement

Don't immediately replace the converter. First have the downstream O2 sensor tested — a bad sensor costs $100–$250 vs. $800–$2,500 for a converter. Also check for exhaust leaks. If the sensor is good and there are no leaks, the converter is almost certainly the culprit.

Repair Cost

• O2 sensor (if sensor is the cause): $100–$250
• Catalytic converter (aftermarket): $800–$1,400 parts + labor
• Catalytic converter (OEM): $1,200–$2,500
• California-compliant converters cost more and are required in CA and several other states