Symptoms › P0420 code (catalyst efficiency below threshold)
🧪 P0420 code (catalyst efficiency below threshold)
Before paying for a $2,000+ converter, rule out the cheaper causes. Jump to the card to show your mechanic ↓
Not a diagnosis. This explains common causes so you can ask better questions. A technician who inspects your car is the only one who can diagnose it.
What you're noticing
- A steady check engine light, and a code reader or shop says P0420: "Catalyst System Efficiency Below Threshold (Bank 1)".
- Often the car drives normally. You may fail an emissions inspection.
- Sometimes a rotten-egg smell or reduced power.
Likely causes (most to least likely)
- An engine problem that is hurting the converter. RepairPal notes converters usually fail because something else was not fixed, such as a misfire, or an internal oil or coolant leak. Fix that first or a new converter can fail too.
- An exhaust leak. A leak near the oxygen sensors can throw off the readings the computer uses to judge the converter.
- A faulty oxygen sensor. The computer compares the sensors before and after the converter. A bad sensor can trigger P0420 without a bad converter.
- A worn-out catalytic converter. A real possibility, especially on high-mileage cars, but it should be the conclusion of testing, not the first guess.
Safe to drive?
Usually OK to drive short-term if the car runs normally. Get it diagnosed soon.
- If the check engine light starts flashing, treat it as urgent and see our flashing-light guide.
- Stop and call for help if you smell burning, see smoke, or the car loses power badly. A clogged converter can overheat.
- Leaving it can mean a failed emissions test and, if the root cause is a misfire or leak, more damage.
Fair price ranges
| Job | Parts | Labor | Total |
|---|---|---|---|
| Catalytic converter replacement | $2,044–$2,133 | $236–$347 | $2,280–$2,480 |
| Check engine light diagnosis and testing | $0 | $122–$179 | $122–$179 |
Source: RepairPal Fair Price Estimator (national averages, estimates last updated Sep 3, 2026; excludes taxes, fees and your location). Checked October 24, 2026. Converter prices vary a lot by vehicle and by whether a direct-fit or OEM part is required. Some states (like California) require specific certified converters.
Questions to ask
- Did you test the oxygen sensors and check for exhaust leaks and misfires before blaming the converter?
- Is my car within the federal emissions warranty (8 years / 80,000 miles for the catalytic converter)? Can you check my VIN for extended coverage?
- Is this an OEM or aftermarket converter, and is it legal in my state?
- What caused the converter to fail, and is that being fixed too?
Upsells to question
- Replacing the converter without testing sensors or checking for leaks first.
- Replacing every oxygen sensor "while we're in there" without test results for each.
- Paying out of pocket when the part may be under emissions warranty. Ask the dealer to check your VIN.
👋 Hand this to your mechanic
Re: P0420 code (catalyst efficiency below threshold). Only do this:
- I have code P0420.
- Please check for misfires, exhaust leaks and oxygen sensor problems before replacing the converter.
- Please check my VIN for emissions-warranty or extended converter coverage.
- Written estimate first. Call me before any work.
From greasemonkeylingo.com
Helpful tools
OBD2 code reader: recommendations coming soon.
Sources
- RepairPal: P0420 Catalyst System Efficiency Below Threshold
- RepairPal: Catalytic converter replacement cost
- 40 CFR 85.2103: Federal emission warranty (8 years/80,000 miles for catalytic converters)
- US EPA: Frequent questions about vehicle emissions
- FTC: Auto Repair Basics
- NHTSA: Check your VIN for open safety recalls
- Your vehicle's owner's manual (warning lights, maintenance schedule)
Last reviewed October 24, 2026. How we research · Affiliate disclosure