P0420 Code: What It Means and How Much It Costs to Fix
By the Auto Repair Home service team· Reviewed by an ASE-certified technician· Updated October 2026· 2 min read
The short answer
P0420 means the downstream oxygen sensor sees that your catalytic converter isn't storing oxygen efficiently. The fix ranges from a $150 O2 sensor or exhaust leak repair to a $900–$2,500 catalytic converter replacement.
Rule out cheaper causes first
Exhaust leaks ahead of the rear sensor, a lazy O2 sensor, misfires and engine running rich can all trigger P0420 without a bad converter.
Why converters fail
Usually an engine problem kills them: misfires, oil burning or coolant leaks overheat or poison the catalyst. Fix the root cause or a new converter will fail too.
Texas inspection impact
An active P0420 fails emissions testing in Travis and Williamson counties.
