If P0420 returns after a converter replacement, the original cause or another system condition may remain. Save the new scan and test results before buying another part.
This reference supports the stated topic only. Model, model year, equipment, current rule or individual vehicle conditions may differ; check the original source before acting.
Read the EPA original: Emissions Warranties for 1995 and Newer Cars and Trucks Source date: 2012; research checked: 2026-11-01.
Start with what changed
Record the new code status, freeze frame, mileage since repair, other codes and whether exhaust work included sensors, gaskets or pipe joints. A returning lamp does not identify which new component is bad.
Ask for a system check
Ask the shop to check exhaust integrity, misfire and fuel-control data under the documented operating condition. Compare the pre-repair diagnosis and post-repair verification.
Warranty and compliance
Ask whether the installed converter and original vehicle fall under any applicable parts, repair or emissions warranty before paying twice.
Sources and scope
- EPA — Emissions Warranties for 1995 and Newer Cars and Trucks Document: 2012 · Research checked: 2026-11-01
Research snapshot: 2026-11-01. Source documents may have newer revisions. Verify current instructions and VIN eligibility with the manufacturer.