P0420 code: fix it or call a pro?

Code P0420 means the car thinks its catalytic converter isn't cleaning the exhaust well enough. The full name is "catalyst system efficiency below threshold, bank 1." It doesn't always mean the converter is bad. A worn sensor, a leak or an engine problem can set it too.

A P0420 code is usually not an emergency: with a steady light and nothing else wrong, it's usually OK to drive gently to a shop. Get it checked soon, because Kelley Blue Book warns that driving on it can do more damage to the converter. It is an emergency if the light flashes, the oil or temperature light comes on, or the car loses power badly. Then pull over and turn off the engine.

A P0420 code can turn off on its own, but the problem behind it usually stays. New York's inspection program says the light can go off after a few drives with no fault, but the code stays stored. The car only runs the converter test in certain kinds of driving. So a converter on the edge can pass one day and fail the next. A code that comes and goes is still worth a check.

No, clearing a P0420 code doesn't fix it. Clearing it turns the light off, but the car keeps checking. If the problem is still there, the code comes back.

Yes, a car with P0420 and the check engine light on will usually fail an emissions test in states that plug into the car's computer. New Hampshire's DMV says the car is rejected if the light is on when it's tested. Clearing the code doesn't get around it. Texas says the car's self-checks reset to "not ready," and Ohio says they take about two or three days of driving to finish.

Yes, a bad oxygen sensor or an exhaust leak can set P0420 even when the converter is fine. Kelley Blue Book lists both, along with a misfiring engine. Worn spark plugs can cause misfires, and RepairPal says misfires and ignition problems should be fixed first. That's why a good shop tests these before it replaces a converter.

P0420 by itself usually doesn't change how the car drives, RepairPal says. But a badly clogged converter can choke the engine and cause a lack of power, and O'Reilly says the trapped exhaust can even make it stall. Kelley Blue Book says it could keep you from going over 30 to 40 mph. If that happens, pull over, turn off the engine and call a shop.

P0420 and P0430 mean the same thing on different sides of the engine. P0420 is bank 1, and P0430 is bank 2, on cars with a V6 or V8.

Sometimes the fix is cheap, but a bottle of additive usually isn't it. If it's a loose sensor plug, a leak or a sensor, the fix costs much less than a converter. Sea Foam's own site says its products won't clean catalytic converters. In 2014, an advertising review board found Cataclean's claim that it cleans converters wasn't backed up. Gadgets that trick the sensor aren't a fix, and the EPA says tampering with emission controls breaks the Clean Air Act.

Catalytic converters on 1995 and newer cars are covered for defects for 8 years or 80,000 miles, whichever comes first, the EPA says. Ask a dealer for your car's brand.

Call a dealer for your car's brand first if the car is under 8 years old and under 80,000 miles. The converter may be covered by the federal emissions warranty. If not, call an auto repair shop. Ask them to test the oxygen sensors and look for exhaust leaks before they replace the converter.

Fixing P0420 can cost a few hundred dollars or over $2,000. It depends on the cause. An oxygen sensor runs about $440–$523, and an exhaust leak about $417–$596, according to RepairPal. A new converter averages $1,511 in CarMD's 2026 index, and RepairPal lists $2,280–$2,480.