P0420 means the engine computer compared the oxygen sensors in front of and behind the bank 1 catalytic converter and decided the converter is no longer cleaning the exhaust well enough. The car usually drives normally; what it fails is an emissions test. It is not always the converter, and the converter is the expensive answer, so rule out the cheaper causes first: a misfire or fuel-trim fault (fix any other stored code before this one), an exhaust leak ahead of the rear sensor, and a slow downstream O2 sensor - each usually costs far less than a converter. If the car is within 8 years or 80,000 miles of its first sale, the federal emissions warranty covers the converter.
Also written as PO420 or P420.
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P0420 means the engine computer has decided the catalytic converter on bank 1 is not cleaning the exhaust as well as it should. The computer compares the upstream and downstream oxygen sensors - if their signals look too similar, it concludes the converter has lost efficiency. It is one of the most common check-engine codes, and while the car usually drives normally, it will fail an emissions test until it is fixed.
Yes. P0420 by itself does not change how the car drives or performs - the engine runs the same whether the converter is efficient or not. The only real consequence of waiting is failing an emissions test, so there is no urgency to stop driving. If the converter was damaged by a misfire or a rich condition, get that underlying cause fixed too, or a new converter will fail the same way.
Listed roughly most-likely first. Confirm with a scan tool and the freeze-frame data before replacing parts.
| Fix | DIY parts | At a shop |
|---|---|---|
| Replace the catalytic converter | $150 - $1,200 | $400 - $2,400 |
| Replace the downstream oxygen sensor | $40 - $120 | $150 - $400 |
| Repair an exhaust leak near the converter | $20 - $100 | $150 - $500 |
Ranges are rounded and for the US market only - actual price depends on region, shop labor rates, and the exact make and model.
What a new catalytic converter costs, if testing points to it
These open an Amazon search - add your year, make and model, because most of these parts are fitment-specific.
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P0420 means the catalytic converter on bank 1 is not cleaning the exhaust as well as the PCM expects. The computer compares the upstream oxygen sensor, which swings rapidly as the mixture cycles, against the downstream sensor behind the converter, which should stay relatively flat because a healthy catalyst buffers those swings. When the downstream sensor starts mirroring the upstream one, the catalyst has stopped storing oxygen and P0420 sets. The important thing to understand is that this code reports a measurement, not a diagnosis - a lazy downstream sensor or an exhaust leak produces the same reading as a dead converter, and one of those costs thirty times more to fix.
The most expensive outcome of P0420 is a new catalytic converter, and on a car that is young enough it may not be yours to pay for. The federal emissions warranty is written into the Clean Air Act, and the converter is one of the few parts it covers for the longest term.
Sources: US EPA, Frequent Questions related to Transportation, Air Pollution, and Climate Change: "What is the federally mandated warranty for emission controls on my car?" (page dated June 10, 2026; retrieved 2026-10-13); CCR title 13, section 1968.2 (CARB OBD II requirements): the light-off, permanent fault code and misfire-blink rules in the FAQ below, via Cornell LII (retrieved 2026-10-13)
The single most expensive mistake on this code is buying a catalytic converter first. A large share of P0420 cases are caused by an upstream problem - a misfire, a lean condition, an exhaust leak, or a tired downstream sensor - and replacing the converter without fixing that cause simply destroys the new one too.
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Difficulty: Moderate to Hard. Time: 1 - 4 hours. DIY parts: $60 - $1,200. Shop total: $150 - $2,400.
The diagnosis is very DIY-friendly and worth doing yourself, because it determines whether you are facing a 90 dollar sensor or a 1,500 dollar converter. The converter replacement itself depends entirely on whether the old one unbolts or has to be cut out and welded - a bolt-on direct-fit unit is a driveway job, and anything requiring welding is not.
Repair misfires and mixture faults immediately. Catalytic converters almost never die of old age. They die because raw fuel from a misfire or a rich condition superheats and melts the substrate inside. The 60 dollar coil you ignored is what turns into a 1,500 dollar converter. Log this car in ServiceSage and keep plugs, coils, and filters on schedule - the maintenance that keeps a converter alive long past the point most fail. Track this maintenance free.
Typical US independent-shop repair cost for P0420 runs about $150 - $2,400, depending on the root cause. Common fixes: Replace the catalytic converter (about $400 - $2,400 at a shop); Replace the downstream oxygen sensor (about $150 - $400 at a shop); Repair an exhaust leak near the converter (about $150 - $500 at a shop).
Sometimes the light goes out on its own, but a P0420 caused by a worn converter does not stay gone. Under California's OBD II rule (13 CCR 1968.2) the computer turns the light off after three driving cycles in a row in which the catalyst monitor runs and passes - which happens when the real cause was an exhaust leak or misfire that has since been repaired. Clearing the code with a scan tool or by disconnecting the battery is different: on many 2010-2011 and all 2012 and newer models the same rule has the computer keep a permanent copy of the code, and only the catalyst monitor running and passing erases it. So clearing P0420 just before an emissions test does not hide it.
Bank 1 is whichever side of the engine holds cylinder 1. On an inline four-cylinder there is only one bank, so P0420 is about the one converter the computer monitors between its front (sensor 1) and rear (sensor 2) oxygen sensors. On a V6 or V8, P0420 is the converter on the cylinder 1 side and P0430 is the other one. Which physical side holds cylinder 1 is not the same on every engine, so check the cylinder numbering for yours before you buy a converter or a sensor for "bank 1".
Yes, as long as P0420 is the only code and the light is steady - a converter that has only lost efficiency does not change how the engine runs. Two things to watch while you put it off. First, a light that is already on cannot warn you about a second fault, so read the codes again every few weeks: a misfire or lean code hiding behind P0420 is what ruins a converter, including a new one. Second, if the light starts blinking, ease off and get it checked that day - California's OBD II rule (13 CCR 1968.2) makes the light blink once a second during a misfire bad enough to damage the converter, even when it is already lit for something else.
Often, if the car is young enough. The EPA says the Clean Air Act makes manufacturers cover the catalytic converter for 8 years or 80,000 miles, whichever comes first, counted from the sale to the original owner - and the warranty carries over to used cars. The manufacturer can deny the claim if the failure came from improper use or maintenance rather than a defect, so fix any misfire first and keep the record. Check your warranty booklet for any longer coverage your state or model has.
Sometimes, and only in a narrow case. A fuel-and-exhaust additive can help a converter that is borderline because of carbon deposits, which is why it is a reasonable cheap first try on a car that otherwise runs well. It cannot repair a converter whose substrate has melted, cracked or been coated by oil or coolant, and it does nothing for an exhaust leak or a failing downstream O2 sensor.
Related trouble codes: P0430 - Catalyst System Efficiency Below Threshold (Bank 2), P0141 - O2 Sensor Heater Circuit Malfunction (Bank 1, Sensor 2), P0133 - O2 Sensor Circuit Slow Response (Bank 1, Sensor 1), P0300 - Random / Multiple Cylinder Misfire Detected
Honda: Many four-cylinder Hondas have the primary catalytic converter built into the exhaust manifold, close to the engine. When P0420 points at that converter, the part is the manifold-and-cat assembly, which changes both the price and the labor - confirm which converter the code refers to before ordering.
Subaru: P0420 is widely reported on older Subaru boxer engines. Because the engine is flat, check both front-pipe joints for exhaust leaks and confirm the downstream sensor is healthy - a leak ahead of the sensor mimics a failed converter.
Toyota: Toyotas use an air-fuel ratio sensor upstream and a conventional oxygen sensor downstream. Use a scan tool that graphs both, and look for oil consumption on high-mileage engines - oil burning is a documented way to wear out a converter early.