P0420

P0420: Catalyst System Efficiency Below Threshold (Bank 1)

• Medium severity
• Usually needs a shop
• Typical cost: $150–$2500

What P0420 means

P0420 is “Catalyst System Efficiency Below Threshold (Bank 1).” The engine computer compares readings from the oxygen sensors before and after the catalytic converter on bank 1 (the engine side containing cylinder 1). When the difference between those two readings gets too small, the converter isn’t scrubbing exhaust gases as effectively as it’s supposed to, and the computer logs P0420.

Common causes

A genuinely worn-out or contaminated catalytic converter (often from overheating) is the most common root cause, but it’s rarely the first thing to check, because several cheaper problems can trigger the same code by damaging or fooling the converter:

• Engine misfires (unburned fuel reaching and damaging the converter)
• A failing or aging oxygen sensor giving a false reading
• Exhaust leaks upstream of the converter
• A wrong air/fuel ratio (too rich or too lean, from an unrelated fuel or intake problem)

Replacing the converter without fixing whatever caused it to fail is a common, expensive mistake — if a misfire or exhaust leak damaged the first converter, an unaddressed root cause will damage the replacement too.

DIY-fixable, or shop repair?

Generally a shop repair. Confirming whether the converter itself is bad versus a sensor or upstream fault needs a scan tool that can graph live oxygen-sensor data, which most home mechanics don’t have. Oxygen sensor replacement is sometimes DIY-accessible; converter replacement usually isn’t (specialized tools, and in many US states, emissions-legal replacement parts requirements).

Typical cost range

An oxygen sensor replacement, if that turns out to be the actual cause, typically runs $150–$400 installed. A full catalytic converter replacement is far more expensive — commonly $500–$2,500 depending on the vehicle, because the converter contains precious metals (platinum, palladium, rhodium) that drive both its manufacturing cost and its resale value to thieves. Diagnose the root cause first rather than defaulting straight to converter replacement — it’s the more expensive fix and won’t hold if something else caused the failure.