A P0420 Code Costs $110 to $1,680, or Free Under Warranty

Two owners get the same P0420 on the same day. One drives a 2023 F-150 hybrid, and Ford says that truck may need new software before anyone touches a part. The other drives a high mile sedan with a worn converter and a $395 to $1,680 job ahead.

I'm Anthony, lead technician at Supercanic, the shop in San Jacinto and Hemet that runs MechanicRank, where we list 48,054 shops and mechanics. A code reader tells both owners the same thing. What you should do next depends on your car's age, where you live and how it runs. In some cases the right answer is to leave it alone for now.

What P0420 Means

Your converter sits between two oxygen sensors. The front one swings fast as the computer adjusts the fuel mix. A good converter smooths that out, so the rear sensor stays much steadier. Our oxygen sensor lesson covers how they work.

Kelley Blue Book says the computer sets the code when the two sensors start to read alike. Bank 1 is the side of the engine with cylinder one. Identifix notes that P0430 is the same fault on bank 2, and that inline engines normally use only P0420.

The code doesn't mean your converter is empty. California's OBD II rule sets the fault line by emissions. On most 2009 and newer LEV II cars, one of California's emission classes, the fault has to set when NOx passes 1.75 times the car's standard. On older LEV I cars, the rule also points at hydrocarbon conversion falling below 50%. So a P0420 can mean a tired converter that still does part of its job.

What Each P0420 Fix Costs

These come from our own repair cost guides. Each one adds the part price to the book labor time at $95 to $140 an hour, for a midsize sedan or small SUV.

• One ignition coil$100 to $390
• Thermostat$105 to $400
• One oxygen sensor$110 to $460
• Full set of spark plugs$115 to $440
• Exhaust leak$125 to $600
• Catalytic converter$395 to $1,680
• Exhaust manifold$440 to $1,700

MechanicRank repair cost guides as of October 2026, midsize car, national labor rate of $95 to $140 an hour, quality aftermarket parts, before sales tax. Not a quote for your car.

The title's range is the sensor at the low end and the converter at the top. Coils and plugs show up because a misfire can set off P0420 or kill the converter. On many newer compact cars the converter is built into the manifold, and our manifold guide says that part alone can run $500 to $1,200.

Identifix publishes higher national averages, at $454 to $550 for an oxygen sensor and $2,301 to $2,473 for a converter. Our ranges price an independent shop and an aftermarket part. In California, New York, Colorado and Maine, that part has to meet stricter rules and costs more. Our converter cost by state page has the detail.

The table pairs each fix with the clue that points to it, at the $120 to $175 an hour our guides use for California.

MechanicRank repair cost guides as of October 2026, at a California labor rate of $120 to $175 an hour. Converter range is before the higher price of a CARB approved part. Trucks, hybrids and European cars cost more.

Six Cars Where the Carmaker Calls for Software

Many owners never hear about this check. Carmakers sometimes admit the test itself is too touchy, or that the computer is running the converter too hot. We pulled these bulletins from the files carmakers send to NHTSA. They're examples, not a full list. Each one covers only the cars it names.

Read from the carmaker bulletins linked in each row, except the GM row, which comes from Identifix. Call a dealer with your VIN to see if one applies to your car.

Software isn't free labor. Ford allows up to 3.1 hours for diagnosis, the update and the drive cycles, and says normal warranty rules apply. Honda books 0.9 hours for its update and test drive. As a worked example, that's about $85 to $125 at our national labor range if you're out of warranty, and dealers usually charge more. That still beats a converter by a lot.

Note the Hyundai row. Software doesn't always save the part. That bulletin pairs the update with a new converter part on a P0420. The software is there to keep the new part from running too hot.

Check the 8 Year Warranty Before Anything Else

Federal rule 40 CFR 85.2103 gives catalytic converters and related parts 8 years or 80,000 miles on light duty vehicles. The emission control module gets the same term. EPA's emissions warranty fact sheet splits it into two paths.

The defect path covers a part that fails from a defect in materials or workmanship. The performance path covers a car that fails an approved emissions test, when your local rules require the repair. EPA says the maker can only deny a claim inside those limits if evidence shows poor care or misuse caused the failure. Our converter guide warns that a converter killed by a neglected misfire is the classic denial. The same fact sheet says other emission parts, oxygen sensors included, get 2 years or 24,000 miles.

So go to a dealer first if your car is under those limits, and bring your service records. If they say no, EPA says to ask for the reason in writing. It also says a California car may have added coverage, so check your warranty booklet.

Fix It or Live With It

Most P0420 pages say fix it now and stop there. I don't think that's the whole answer. A steady P0420 with no other codes on a car that runs well isn't an emergency. Whether you can wait depends on which of these you are.

My own shop advice, not a legal opinion. Rules vary by state, and some states also check the light at a safety inspection.

KBB's state by state guide, dated July 2025, lists 21 states with no emissions testing, among them Florida, Michigan, Tennessee and Washington. Many others test only in some counties. Texas tests in 17 of 254 counties. Arizona tests around Phoenix and Tucson. Check with your state before you count on it.

If you're in that last row, the conditions matter. The light is already on, so a new fault won't warn you with a new light. Buy a basic code reader and scan the car every month or so. Watch for a new code, a burning smell or a loss of power. Fix oil burning and coolant loss, since our converter guide lists both as converter killers.

Be clear about the trade too. Your car is putting out more pollution than it should. You'll also have to deal with the code if you move to a test area, and any buyer with a scan tool will see it. KBB warns that ongoing driving can damage the converter further. Waiting buys you time to plan, not a free pass.

California Help If You Can't Afford It

If your car failed its biennial Smog Check, the state's Consumer Assistance Program may help. It pays up to $1,450 toward emissions repairs on a 1996 or newer car. You pay 20% of a total up to $1,812.50, plus the $8.25 certificate fee. Household income has to be at or below 225% of the federal poverty level.

Don't take the car in early. BAR says to wait for your eligibility letter before any repair starts.

How a Good Shop Proves the Converter Is Dead

A shop that says you need a converter should be able to show its work. These checks follow the steps in the Identifix guide.

• Read the freeze frame and fuel trims. Our fuel trims lesson shows what those numbers mean. A rich or lean engine gets fixed first.
• Clear misfires. Check plugs, coils and injectors. Our P0300 misfire page prices those jobs.
• Test both oxygen sensors. Add propane to force rich, or open a vacuum hose to force lean. Both sensors should respond.
• Hunt for leaks. Look over the manifolds and pipes, and smoke test if needed.
• Check the converter itself. A back pressure gauge finds a clog. A temperature reading on the housing should show the outlet hotter than the inlet.
• Check software and drive cycles. Look for bulletins, then run the right drive cycle to confirm the fix.

What Our Own Jobs Show

Supercanic keeps a public study of what customers actually paid. It covers 953 paid jobs from August 2025 to October 2026, most of them at the customer's home or work. The full table is on our mobile mechanic prices page. Exhaust work isn't its own line in that study, so we won't guess at a figure for it.

Supercanic paid jobs, August 2025 to October 2026, before sales tax. Most were in Riverside and San Bernardino counties. Middle half is the 25th to 75th percentile.

A $150 visit is small next to a $395 to $1,680 part you may not need. Shops that post a check engine diagnostic price on their own websites post a median of $100, with the middle half between $80 and $150. Our check engine light diagnostic guide has more. A P0420 can take longer than a basic scan, so ask what the fee includes.

Old Advice That Stuck

Put a spacer on the rear sensor. A spacer's whole job is to fool the sensor. Federal law bars installing a part when a principal effect is to bypass or defeat emissions equipment. It also bars knowingly removing that equipment, which covers a straight pipe.

Pour in a bottle of cleaner. None of the five carmaker bulletins we read above calls for one. We have no test data to say it works, so we won't sell you on it. If it's your last try before a smog retest, know that you're taking a chance.

Replace the rear sensor and see. KBB warns that swapping sensors without a full diagnosis may not fix it. Test it first.

Buy the cheapest converter that fits. Our converter guide warns that lightly loaded parts can bring P0420 back within a year or two on some cars, Subarus and luxury makes included. A cheap part can cost you the job twice.

Before You Say Yes to a Converter

Run this list with whoever quotes you.

• Warranty. What's the car's age and mileage against 8 years and 80,000 miles?
• Bulletins. Did anyone check for a software bulletin on this VIN?
• Proof. What ruled out the sensors, leaks and misfires? Ask to see the readings.
• The cause. What killed the old converter, and is that fixed? Identifix says most converters last up to 10 years or longer.
• The part. Which converter is it, and is it legal in your state?
• Written estimate. The FTC says it should name the problem, the parts and the labor.
• California drivers. The Bureau of Automotive Repair says you must OK the work before it starts and again before any added repairs.

Find a Shop for the Job

MechanicRank lists 48,054 repair shops and mechanics in 6,649 cities. Every listing shows the shop's phone, reviews and hours, and the phone rings the shop direct. Search your ZIP code and ask for a P0420 diagnosis that tests the sensors and checks for leaks before anyone quotes a converter.

At Supercanic, our own shop, labor is $150 an hour, with a 90 day or 5,000 mile labor warranty.

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