The P0420 code means catalyst efficiency below threshold: the catalytic converter is not cleaning exhaust as well as the computer expects. It does not automatically mean you need a $900 to $2,400 converter. A worn oxygen sensor, an exhaust leak, or an engine problem upstream can each trigger the same code, and ruling those out first is the honest order of operations.
On this page: What P0420 means Can you drive? The 4 honest fixes Honda, Chevy, Nissan What to avoid FAQ
What the P0420 code actually means
Your exhaust has an oxygen sensor before the catalytic converter and one after it. When the converter is healthy, the downstream sensor sees smooth, steady readings because the converter is storing and releasing oxygen as it burns off pollutants. When the downstream readings start mirroring the upstream ones, the computer concludes the converter is not doing much anymore and logs P0420, catalyst efficiency below threshold. The subtlety is that “the converter looks lazy” has several causes, and only one of them is a dead converter. That is why P0420 is one of the most over-repaired codes in the book, and why the fair-price rule here is diagnose first, buy second.
Can you keep driving with P0420?
Usually yes, for a steady light. The car typically runs and drives normally, and the immediate stakes are emissions and fuel economy, not a breakdown. Two honest cautions. A P0420 code that arrives with misfire codes or a flashing check engine light is a different situation: active misfires are what kill converters, and that combination deserves attention in days, not months. And a lit check engine light fails emissions testing in every state that tests, so your inspection date is a real deadline.
The 4 honest fixes, cheapest first
| Fix | When it is the answer | Fair 2026 price |
|---|---|---|
| 1. Fix an exhaust leak | Leak before or near the downstream sensor skews readings | $75-$300 |
| 2. Replace the downstream O2 sensor | Sensor is lazy or dead, converter is fine | $150-$400 |
| 3. Fix the upstream engine problem | Misfires, rich or lean running poisoned the readings | $150-$600 |
| 4. Replace the catalytic converter | The converter itself is truly worn out | $900-$2,400 |
Ranges are national, independent-shop figures, July 2026. A good shop earns its $75 to $200 diagnostic hour here by testing sensor response and checking for leaks and misfires before quoting the big number. Ask one plain question: “What did you rule out before the converter?” A shop with a real answer tested; a shop without one guessed, and a $2,000 guess is not one you should fund. Our guide to finding a good mechanic covers how that conversation should sound.
P0420 on a Honda, Chevy, or Nissan
The P0420 code means the same thing on every brand; the patterns differ a little. High-mileage Hondas are famous for triggering P0420 with converters that are merely tired, where an OEM converter fixes it and some cheap aftermarket units re-trigger the code within months. GM trucks and Nissans with higher mileage often trace P0420 to worn sensors or small exhaust leaks first.
The brand lesson is the same lesson: test before replacing, and if a converter is truly needed, be careful how cheap you go, because the $200 universal converter that cannot satisfy your car’s computer is the most expensive part you can buy twice. California and other CARB states also require CARB-compliant converters, which cost more and are not optional there.
What to avoid
Catalytic converter cleaner in a bottle occasionally helps a converter that is contaminated, and does nothing for one that is worn out; treat a $20 bottle as a lottery ticket, not a repair. Clearing the P0420 code the night before an emissions test does not work, because the computer reports “not ready” until it finishes its own drive cycles. And be wary of any quote that jumps straight to a converter without a diagnostic story attached. Converter theft and replacement fraud both ride on the part’s price tag; the FTC’s auto repair basics is blunt about getting the diagnosis in writing before authorizing expensive work.
P0420 FAQ
Is P0420 the same as P0430?
Same problem, other side. P0420 is bank 1, the side with cylinder 1; P0430 is bank 2 on V6 and V8 engines. Everything on this page applies to both.
How urgent is a P0420 code really?
Weeks, not hours, if the light is steady and the car runs fine. Get the diagnosis scheduled rather than someday, because upstream causes like misfires get more expensive with time, and your emissions deadline does not move.
Will P0420 hurt my gas mileage?
Sometimes slightly. The code itself does not change how the engine runs, but the conditions behind it can. A noticeable mileage drop alongside P0420 points at an upstream cause, which is useful diagnostic information to mention at the shop.
Why did the code come back after a new converter?
Either the converter was the wrong grade (common with the cheapest aftermarket units) or the converter was never the root cause and the original problem is still upstream killing the new one. Both are reasons the test-first order exists.