Emissions code
P0421: Catalytic converter slow to clean up after warm-up (bank 1)
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.
The computer compares the oxygen sensors before and after the catalytic converter during the warm-up period, and the converter on cylinder 1's bank is not cleaning up as much as it should. An aging converter is common, but a weak rear sensor or an exhaust leak can fool the test.
- Safe to drive?
- Usually yes, for now
- Most common cause
- Aged catalytic converter
You can keep driving. The car will fail emissions testing, and a failing converter can get worse over time.
What you may notice
- Often none
- Faint rotten-egg smell
- Lower fuel economy
Common causes, most likely first
- Aged catalytic converter
- Failing downstream O2 sensor
- Exhaust leak near the sensors
- Long-term misfire or oil burning
Check this first
Check that there are no misfire codes and that no exhaust leak exists ahead of the rear sensor. Ask the shop to read the live front and rear sensor data to see whether the converter is really failing.
Other repairs that sometimes fix P0421
| 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).
P0421 questions
Is this the same as P0420?
It is a close relative. P0420 is tested after warm-up at steady speed, and P0421 is tested during warm-up. Both point to the same bank, and the usual fix is the same.
Are there warranties?
Some cars have an emissions warranty on the converter, often 8 years or 80,000 miles, but check your own paperwork. If your car is within that range, ask the shop to look into it.
Can I replace just the sensor?
If the converter tests fine, yes. A shop should compare the readings of both sensors, and only replace the converter if the numbers show it is truly worn.
How much does it cost to fix P0421?
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.
Related codes
- P0420: Catalytic converter not working well (bank 1)
- P0430: Catalytic converter not working well (bank 2)
- P0136: Downstream oxygen sensor circuit problem (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.