Emissions code
P0420: Catalytic converter not working well (bank 1)
The computer compares the oxygen sensor before the catalytic converter with the one after it. On this code the two readings look too alike, which suggests the converter is not cleaning the exhaust as well as it should on the side with cylinder 1.
- Safe to drive?
- Usually yes, for now
- Most common cause
- Worn-out catalytic converter, the most common reason on higher-mileage cars
Usually safe to drive for now, but get it checked soon; if the check-engine light flashes or the car runs rough, stop because a misfire can overheat the converter.
How this range is built
| Part of the job | Low | High |
|---|---|---|
| Parts | $250 | $1,400 |
| Labor: 1.2 to 2.5 hours × $125 to $175 an hour | $150 | $438 |
| Fees: shop supplies, disposal, diagnostics | $0 | $40 |
| Total, rounded | $400 | $1,900 |
The hourly rate is the BLS Occupational Employment and Wage Statistics, May 2025 average pay for this trade in Boston, $31.54 an hour, times 4 to 5.5 to cover what a shop bills (overhead, tools, insurance, profit). Parts are a band from budget aftermarket to the manufacturer part. Full method.
Not a repair shop, estimates only. Typical range, estimate. Not a quote. Your car, parts and shop change the number.
This is the range for the most common fix, catalytic converter replacement. Which repair your car needs depends on a proper diagnosis.
What you may notice
- Check-engine light on
- Slightly worse gas mileage
- Rotten-egg smell from the exhaust
Common causes, most likely first
- Worn-out catalytic converter, the most common reason on higher-mileage cars
- Failing downstream oxygen sensor giving a false reading
- Exhaust leak near the sensors
- Engine misfires or burning oil that damaged the converter earlier
Check this first
Make sure there is no exhaust leak ahead of the rear sensor, and ask the shop to look at the sensor data before replacing the converter. Ask if a software update exists for your model.
Other repairs that sometimes fix P0420
| Repair | Typical range, Boston |
|---|---|
| Oxygen sensor replacement (one sensor) | $110 to $370 |
| Diagnostic scan (check engine light) | $60 to $290 |
Before you pay
- Ask the shop what test showed this part is the cause, not just that the code appeared.
- Ask for parts and labor listed separately, and for the hours.
- Ask for a written OK before any extra work. Massachusetts rules require written authorization before repairs.
Got a quote already? Check it against the range. Want to see if your car has a recall tied to this problem? Check recalls by VIN.
Need to read a code yourself? OBD-II scanners on eBay (affiliate link, disclosure).
P0420 questions
Can I just replace the rear oxygen sensor?
Sometimes the sensor is the real problem, but a good test of the live readings should show which part is bad before you buy either one.
Will this fail an emissions test?
Yes, in most states a stored catalyst code or a lit check-engine light means a fail until it is fixed.
Why did my converter wear out?
Age and mileage are common, but oil burning, coolant leaks into the engine, or long-running misfires can ruin one early. Fix those first or the new one may fail too.
How much does it cost to fix P0420?
The most common fix, catalytic converter replacement, has a typical range of $400 to $1,900 in Boston. It is an estimate built from parts prices and local labor wages, not a quote. Your car and shop change it.
P0420 repair cost by city
Related codes
- P0430: Catalytic converter not working well (bank 2)
- P0421: Catalytic converter slow to clean up after warm-up (bank 1)
- P0138: Rear oxygen sensor reads high (bank 1, after converter)
- P0171: Engine getting too much air or too little fuel (bank 1)
More emissions codes
Code meaning written in our own words. Some codes mean different things on different makes, so check your scan tool or manual. Not a repair shop, estimates only.