Published by Rims & Tires
Compiled & checked by the site's publisher from manufacturer, NHTSA & USTMA data · Updated 2026-08-12
P0420 is the OBD-II code for Catalyst System Efficiency Below Threshold (Bank 1). In plain terms: the computer compared the oxygen sensor in front of the catalytic converter to the one behind it, and the two readings looked too much alike. A healthy converter changes the exhaust passing through it, so those two sensors should disagree. When they stop disagreeing, the computer decides the converter isn't cleaning the exhaust well enough and turns on the check-engine light. That does not automatically mean you need a converter. It means something in that measurement is wrong, and the difference between the cheap answer and the expensive one is several thousand dollars.
What P0420 actually measures
Your exhaust has two oxygen sensors per bank: one upstream of the catalytic converter, one downstream. The upstream sensor is the one the engine uses to trim the fuel mixture, so its signal swings up and down constantly. The converter's job is to store and release oxygen as it burns off leftover hydrocarbons, which smooths that signal out. So the downstream sensor should read almost flat compared to the upstream one.
When the downstream sensor starts mimicking the upstream sensor's swings, the computer reads that as a converter that has stopped storing oxygen, and it sets P0420. Bank 1 is the side of the engine with cylinder number one. On a four-cylinder there's only one bank. On a V6 or V8 there are two, which is why you may also see P0430 for the other side.
This is worth understanding because it explains the whole diagnostic problem: P0420 is an inference, not a measurement of the converter itself. Anything that makes the two sensors read alike, including a bad downstream sensor or an exhaust leak letting fresh air in, produces the same code.
What causes a P0420 code?
A genuinely failed catalytic converter is the most common cause, and on a high-mileage vehicle it's the likeliest one. Converters wear out. They also die early when something upstream poisons them: an engine burning oil or coolant, a long-running misfire dumping raw fuel into the exhaust, or leaded or contaminated fuel.
The cheaper causes are real and they get missed. A failing downstream oxygen sensor reports a converter problem that doesn't exist. An exhaust leak anywhere between the engine and the rear sensor lets outside air in and skews the reading. A stored misfire code, bad fuel trims, or a leaking injector will set P0420 as a downstream symptom of a completely different fault. So will an aftermarket converter that doesn't have enough precious-metal loading to satisfy a particular manufacturer's threshold, which is a common outcome when a cheap converter goes on a Toyota or a Honda.
The order matters. Any shop that reads P0420 and quotes a converter without first checking for other stored codes, testing the downstream sensor, and looking for exhaust leaks is guessing with your money.
West Georgia Local
Find the right shop near you
Tell us your vehicle and what you need — we'll match you with a vetted West Georgia shop. Free, no obligation.
What does it cost to fix a P0420 code?
It depends entirely on which of those causes it is, and the spread is enormous. RepairPal's national estimate for a catalytic converter replacement is $2,301 to $2,473, with the parts making up nearly all of it. AutoZone's guide puts direct-fit replacement converters anywhere from $300 to $2,500 depending on the vehicle, with installation labor running roughly $70 to $130 an hour on top. An oxygen sensor is a different world: RepairPal's national average for oxygen sensor replacement is $454 to $550, and on many common vehicles it lands well under that.
An exhaust leak repair is usually the cheapest outcome of all, often just a gasket, a clamp, or a weld. That's why the diagnosis is worth paying for. RepairPal's national average for check-engine diagnosis and testing is $122 to $233, and most West Georgia shops credit that fee toward the repair if they do the work. Spending $150 to find out you need a $300 sensor instead of a $2,300 converter is the best return in this entire guide.
One more cost factor specific to converters: emissions-legal parts. A federal EPA-compliant direct-fit converter costs less than a CARB-compliant one, and an OEM converter costs more than either. On some vehicles an inexpensive aftermarket unit will set P0420 again within a few thousand miles, which turns a bargain into paying twice.
Can you keep driving with a P0420 code?
In most cases yes, at least in the short term. A steady check-engine light with P0420 and no drivability symptoms is not the same emergency as a flashing light, which signals an active misfire that can destroy a converter in minutes. If your light is flashing, that's a different problem and a more urgent one.
That said, ignoring P0420 has two real costs. A converter that's breaking down internally can shed substrate and restrict the exhaust, which shows up as lost power, poor fuel economy, and a car that feels like it hits a wall on the highway. And whatever poisoned the converter in the first place, usually oil consumption or a misfire, is still doing damage while you wait.
There's a Georgia-specific reason not to sit on it, too. Douglas and Paulding counties both require an annual emissions test, and a 1996-or-newer vehicle with an active check-engine light fails on the spot because the test reads the OBD-II system rather than the tailpipe. Carroll, Haralson, and Heard counties don't run the test, so drivers there have more calendar room, but the repair doesn't get cheaper by waiting.
Will clearing the code or an additive fix it?
Clearing the code with a scan tool turns the light off. If the underlying condition is still there, the computer runs its catalyst monitor again over the next few drive cycles and the light comes back, usually within a week of normal driving. Clearing it also wipes the readiness monitors, which means the car will fail an emissions test for not being ready even before it fails for the code itself. Don't clear the code the day before an emissions appointment.
Fuel-system and catalytic converter cleaners are a mixed bag. If the real problem is carbon fouling or a converter that's mildly contaminated, a quality cleaner and a long highway drive occasionally clears it. If the converter's substrate is melted, cracked, or physically broken down, no bottle fixes that. Treat an additive as a $20 experiment, not a repair plan, and give it a couple of full drive cycles before you decide.
The one genuinely useful free step: most parts stores will read the codes for you at no charge. That tells you whether P0420 is alone or sitting alongside a misfire or fuel-trim code, which changes the diagnosis completely.
What to ask a shop before you approve a converter
Ask three questions. Were there any other stored codes, especially misfire (P0300 series) or fuel-trim (P0171 or P0174) codes? Was the downstream oxygen sensor tested with live data rather than assumed good? Was the exhaust checked for leaks ahead of the rear sensor? A shop that has done all three and still points at the converter has earned the sale.
Then ask what converter they're quoting. OEM, CARB-compliant aftermarket, and federal-only aftermarket are three different parts at three different prices with three different odds of setting the code again. Get the warranty on the part in writing; federal law requires manufacturers to warranty converters for a defined period, and a reputable shop will tell you exactly what applies to yours.
RimsAndTires doesn't perform repairs. It's a free directory connecting West Georgia drivers to independent shops that do. If you're looking for someone in Carroll, Douglas, Paulding, Haralson, or Heard County to actually diagnose this instead of throwing a converter at it, tell us what your car is doing and we'll match you with a shop.
P0420 causes and what each typically costs to fix
| Likely cause | Typical cost | How a shop confirms it |
|---|---|---|
| Exhaust leak before the rear O2 sensor | Often under $300 (clamp, gasket, or weld) | Smoke or pressure test of the exhaust, visual inspection |
| Failed downstream oxygen sensor | $454–$550 national average (RepairPal); less on many vehicles | Live sensor data compared against the upstream sensor |
| Misfire or fuel-trim fault poisoning the converter | Varies widely — plugs, coils, or injectors | Other stored codes, fuel-trim readings, misfire counters |
| Failed catalytic converter | $2,301–$2,473 national average (RepairPal); parts alone $300–$2,500 (AutoZone) | Other causes ruled out first, plus converter temperature or back-pressure testing |
Frequently asked
What does the P0420 code mean?
It means Catalyst System Efficiency Below Threshold on Bank 1. The computer compared the oxygen sensors before and after the catalytic converter, found their readings too similar, and concluded the converter isn't cleaning the exhaust to spec. It's an inference from sensor data, not a direct test of the converter, which is why other faults can trigger it.
How much does it cost to fix a P0420 code?
Anywhere from under $300 to over $2,400 depending on the actual cause. RepairPal's national average for a catalytic converter replacement is $2,301 to $2,473, while an oxygen sensor replacement averages $454 to $550 and an exhaust leak repair is often cheaper still. Pay for the diagnosis before you pay for the part.
Does P0420 always mean I need a new catalytic converter?
No. A failed converter is the most common cause, especially on higher-mileage vehicles, but a bad downstream oxygen sensor, an exhaust leak ahead of that sensor, or an unresolved misfire will all set the same code. Those are much cheaper fixes and they get missed when a shop quotes off the code alone.
Can I drive with a P0420 code?
Usually yes in the short term, as long as the light is steady rather than flashing and the car drives normally. A flashing check-engine light is a different and more urgent problem. Long term, a breaking-down converter can restrict the exhaust and cost you power and fuel economy, and whatever damaged it is still doing damage.
Will a P0420 code fail an emissions test in Georgia?
In the counties that test, yes. An active check-engine light fails a 1996-or-newer vehicle on the spot because the test reads the OBD-II system. Of the counties this directory covers, Douglas and Paulding require the annual emissions test; Carroll, Haralson, and Heard don't.
Will clearing the code make it go away?
Only until the computer re-runs its catalyst monitor, which usually takes a few days of normal driving. Clearing also resets the readiness monitors, so the car will fail an emissions test for being not ready even before it fails for the code. Don't clear it right before a test appointment.
Why did P0420 come back after I replaced the catalytic converter?
Two common reasons. Either the converter wasn't the problem and the real fault, a sensor, a leak, or a misfire, is still there, or the replacement converter doesn't have enough catalyst loading to satisfy your vehicle's threshold. Inexpensive aftermarket converters set the code again on some vehicles, particularly Toyotas and Hondas.
Keep reading
Last updated 2026-08-12. General guidance only — confirm specifics with a local shop for your exact vehicle.