P0420 (catalyst system efficiency below threshold, bank 1) means the computer no longer sees enough difference between the upstream and downstream oxygen sensors — usually because the catalytic converter has degraded. But P0420 is not always a converter, and the fixes range from a $30 sensor to a $2,000+ converter. This guide breaks down the realistic costs and how to avoid replacing a good converter. For the full diagnosis, see how to diagnose catalytic converter efficiency.
Rule out the cheap causes first
Before buying a converter, confirm it's actually the problem. A downstream oxygen sensor that reads lazy, an exhaust leak ahead of the sensor, or an untreated misfire or oil-burning issue can all set P0420 with a perfectly good converter — and replacing the converter without fixing those means the new one fails too. Check the O2 sensors, look for exhaust leaks, and clear any misfire codes first.
Repair cost (2026 US ranges)
US independent shop at about $110–$160/hour plus parts. Planning figures, not a quote.
Fix
Shop (parts + labor)
DIY parts
Diagnosis
$100–$200
—
Oxygen (O2) sensor
$200–$450
$40–$250
Exhaust leak repair
$150–$600
$20–$200
Catalytic converter (aftermarket)
$800–$2,200
$150–$900
Catalytic converter (OEM / CARB states)
$1,500–$3,000
$600–$2,000
Catalytic converter cleaner (additive)
—
$15–$40
Does catalytic converter cleaner work?
A fuel-tank catalytic cleaner is cheap and occasionally clears a borderline P0420 caused by light contamination, but it will not fix a physically failed or melted converter. Think of it as a low-cost long shot on an early P0420, not a real repair. If the code returns quickly, the converter (or an underlying cause) needs proper attention.
FAQ
Can I drive with a P0420 code?
Usually yes, if the light is steady and the car runs normally — P0420 is an emissions/efficiency code, not an immediate hazard. But it won't pass an emissions test, and an underlying misfire or oil problem can worsen, so don't ignore it indefinitely.
Why is my catalytic converter so expensive?
Converters contain precious metals (platinum, palladium, rhodium), and emissions-legal (CARB) converters in states like California cost more. Aftermarket units are cheaper but must be the correct emissions grade for your state.
Will replacing the O2 sensor fix a P0420?
Sometimes — if a lazy downstream sensor set the code with a good converter. That's why testing the sensors before condemning the converter can save you over a thousand dollars.
General diagnostic information for US-market vehicles, not a substitute for a hands-on inspection. Costs are 2026 ballpark ranges for planning only.