P0420 means catalyst system efficiency below threshold (bank 1). Your catalytic converter isn't cleaning exhaust gases as well as it should. The downstream O2 sensor sees too little dif
You can keep driving — get it checked when convenient.
Your catalytic converter isn't cleaning exhaust gases as well as it should. The downstream O2 sensor sees too little difference from the upstream one. This is one of the most common codes on cars over 100k miles.
Converters degrade with age and mileage — the most common root cause.
A lazy sensor can falsely report low efficiency — always rule this out first.
Extra oxygen skews the sensor readings.
Contaminates and kills the catalyst over time.
Overheats and melts the catalyst substrate.
Typically $200–$2500
O2 sensor replacement typically $200–$400; catalytic converter replacement typically $1,000–$2,500. Always diagnose before replacing the converter — it's the most expensive guess. Estimates vary by vehicle. Costs are estimates and vary by vehicle, location, and shop.
Safe to drive — this code won't strand you. But don't ignore it for months: a failing converter can clog, hurting performance and fuel economy, and you'll fail emissions testing.
Typically $200–$2500. O2 sensor replacement typically $200–$400; catalytic converter replacement typically $1,000–$2,500. Always diagnose before replacing the converter — it's the most expensive guess. Estimates vary by vehicle.
You can keep driving — get it checked when convenient.
The most common cause is worn-out catalytic converter. Converters degrade with age and mileage — the most common root cause.