The car’s combustion process leaves behind nitrogen oxides (NOx), carbon monoxide (CO), and unburned fuel, which are bad for the environment. The catalytic converter turns them “safer” by reducing NOx to nitrogen and oxidizing the rest into CO2 and water.
The catalytic converter relies on two oxygen sensors (upstream and downstream) for data about the exhaust stream’s oxygen content. They should send different readings. Otherwise, on-board diagnostics (OBD) will log a P0420 code and illuminate the check engine light to indicate an exhaust problem.
Diagnostic trouble code (DTC) P0420 stands for “P0420 Catalyst System Efficiency Below Threshold (Bank 1).”
It indicates that the exhaust system’s oxygen levels are below the required threshold in Bank 1, which can damage the engine’s exhaust and fuel system.
Bank 1 refers to the side of the engine that houses the number one cylinder. The opposite side of the engine is Bank 2.
A catalyst monitor or catalytic converter monitors the converter’s ability to reduce harmful emissions using oxygen. Technically, the sensor that monitors catalyst efficiency is measuring the oxygen storage capacity of the catalyst in that converter.
When the catalytic converter is performing well, it uses oxygen from the exhaust gas to break down pollutants. The PCM compares the output of upstream and downstream oxygen sensors to determine the level of oxygen entering and leaving the converter.
Ideally, there should be little to no unburned oxygen after the process. What this means is that the downstream (catalyst monitoring) sensor should have a very lazy signal compared to the upstream sensor.
The downstream (catalyst monitoring) sensor should have a very lazy signal compared to the upstream sensor.
–Richard McCuistian, ASE Certified Master Automobile Technician
When combustion occurs in each cylinder, pulses of oxygen hit the upstream oxygen sensor. The oxygen sensors generate voltage signals that fluctuate with the pulses.
When the catalytic converter works, the downstream oxygen should detect little to no oxygen. Its voltage signal remains consistent to indicate the converter has depleted the oxygen.
The voltage must be 650 millivolts to indicate a low oxygen level. If the voltage is lower than that points to a high oxygen level, which means the catalytic converter was inefficient.
The catalytic converter has been a required component on all cars since 1975.
Code P0420 applies to all vehicles made after 1996 and some made as early as 1994, as catalytic converter monitoring became standard in cars to comply with the OBD2 element of U.S. National Emissions Standards.
Code P0420 may appear on some Honda vehicles. On a 2013 Honda Accord LX 2.4L, for example, the code appears because of the following conditions:
For this specific vehicle, Honda also notes that when a malfunction is detected during the first drive cycle, a Pending DTC is stored in the PCM memory. If the malfunction returns in the next (second) drive cycle, the MIL comes on and a Confirmed DTC and the freeze data are stored. The MIL is cleared if the malfunction does not return in three consecutive trips in which the diagnostic runs. The MIL, the Pending DTC, the Confirmed DTC, and the freeze data can be cleared with the scan tool Clear command or by disconnecting the battery.
P0420 is a generic powertrain code that various vehicles can log. The process to diagnose might vary depending on make-model specs.
Here are some of the steps mechanics take to diagnose it:
Mechanics might conduct other tests depending on what they find.
If you want to diagnose the code yourself, following these steps is a great start. Research what tools you need to diagnose your car’s catalytic converter, and make sure you have them on hand.
Here are some videos you might want to check out before doing anything.
However, if you’re neither familiar with diagnosing trouble nor confident in DIY-ing the process, it’s better to bring your ride to an auto repair shop near you.
Resolving the P0420 code can be tricky, as the fix depends on the cause. You might have a hard time if you don’t have the right tools and know-how for the job. Under these circumstances, it’s best to let professionals resolve the issue.
But if you’re determined to do it yourself, whether to save on costs or because you trust your repair knowledge, make sure you rely on repair info specific to your vehicle. Detailed guides like those from an ALLDATA subscription or Chilton are great resources.
A catalytic converter can last up to 10 years or more, but they’re designed to last as long as the vehicle.
A catalyst triggers changes in other elements without changing itself.
Modern vehicles have three-way catalysts, which means they process three harmful gases: nitrogen oxide, carbon monoxide, and unburned fuel or hydrocarbons (HC).
Catalytic converters have a fine, honeycomb core with tiny tunnels that are coated with catalysts that detoxify the exhaust gases that pass through them.
These catalysts are usually made of rare and expensive metals, such as palladium, platinum, and rhodium, which is why catalytic converter theft is a problem.
When the catalysts reach operating temperature, they break down the harmful gases.
The catalyst nearest the engine, known as the “light-off catalyst,” separates NOx into its nitrogen and oxygen components. It’s the only catalyst that the PCM monitors.
Another catalyst, usually the one farthest from the engine, adds oxygen to HC and CO to turn them into CO2.
If you’re resolving the P0420 code on your own, don’t replace parts without identifying the issue’s exact cause. Rule out other possible causes of catalyst system inefficiency before replacing your O2 sensors or catalytic converter.
For example, some car owners make the mistake of replacing a perfectly fine catalytic converter because of wiring or software issues. Not only did they waste money replacing functional parts, but they also didn’t resolve the trouble code.
To resolve this trouble code, you must determine its underlying cause and address it.
Yes, it’s one of the issues that can make the catalytic converter inefficient. A catalyst monitor uses signals from the downstream O2 sensor to determine catalyst efficiency. It uses signals from the upstream O2 sensor as a reference point.
If any or both of these oxygen sensors fail, they can trigger the P0420 code.
The P0420 code means “Three-Way Catalyst Malfunction” in new Nissan models instead of “Catalyst System Efficiency Below Threshold (Bank 1).”
Either way, the code indicates a decline in the catalytic converter’s performance.
One way to determine whether faulty O2 sensors triggered this trouble code is to force the air/fuel mixture to too rich and lean conditions.
To turn the mixture rich, add propane to the engine’s air intake and raise RPMs to keep the engine from stalling. Monitor the oxygen sensors with a scan tool while doing that. The sensors must detect the changes caused by the propane and adjust their signals accordingly.
To force the mixture into lean condition, disconnect a large vacuum hose. Track whether the oxygen sensors’ signals are responding to the changes.
If the oxygen sensors’ readings change accordingly, then they’re working as they should.
Defective parts like an inefficient catalytic converter, inoperative oxygen sensor, or a bad spark plug can easily cause the P0420 code. The last thing you want is to fail your emissions test as a result, and driving around with a rotten egg smell won’t be pleasant in the least. As such, it’s best to resolve the code as soon as possible. If you need to replace any of these parts, you can find what you need at CarParts.com.
CarParts.com offers a wide selection of parts available at different price points to suit your budget. Our easy-to-navigate website allows you to place your order with just a few easy clicks. You won’t even need to leave the comforts of home. We can guarantee fast shipping, with orders arriving in as few as two business days.
Check out our selection and order your new catalytic converter and other parts on CarParts.com today.
Any information provided on this Website is for informational purposes only and is not intended to replace consultation with a professional mechanic. The accuracy and timeliness of the information may change from the time of publication.
View Comments
I scan Diagnostic on my car with FIXD. My car recently just lost power while driving like it was running out of gas. But I wasn't. I tried adding gas it doesn't work. It still sound like Im out of gas. So when I used FIXD P0420 popped up. Could this be the reason my car just turned off on me. And What is the best thing to do if I cant get it to a license mechanic?
Can the Catalytic Converter Cause This?
I was thinking just replacing everything that a P0420 can be connecting with for I wont have a future problem.
Hello Lisa,
It's possible that the catalytic converter is clogged, creating excessive exhaust backpressure (causing poor engine performance) and setting code P0420. It's also possible that there's another problem that's causing the engine to run poorly and, as a result, damaging the converter.
Unless you have sufficient automotive knowledge, I highly suggest you take your car to a mechanic for a professional diagnosis. Catalytic converters aren't cheap, and you don't want to replace yours only to find out there's another problem.
I recently had put my sister van diagnosed and p0420 had popped up on the diagnosis along with other codes and I was wondering if I get rid of the cat and straight pipe would I have the same problems or should I just replace the cat with another one
I recently had my sister van diagnosed and
p0420 had shown up and I was wondering if I was to get rid of the cat and straight pipe would this create more problems for me or would I be best to replace the cat
Hi, I have an 03 Tundra 3.4l v6, no matter how many miles I drive I can't seem to set my catalyst monitor for smog check. No engine code, temp on my front cat inlet 350F outlet 520F, the rear cat inlet and outlet are the same at 320F, any idea is appreciated.
I had a P0420 "Catalyst Insufficiency below threshold" and a P2238 Upstream Air Fuel Sensor Error, " Pumping Current Low... ". The Check Engine light was cycling on/off about every 50 miles.
The car has 170,000 miles with original CAT and sensors.
I replaced the upstream Air Fuel Ratio sensor and it cleared the P2238 Sensor error.
(1) Does the scanner usually point to the specific faulty sensor? i.e., If the scanner points only to the upstream sensor, does that usually mean the downstream sensor is still good, otherwise it would have reported the error?
(2) About how many miles must I drive before the Engine Control Module resets and performs a calculation and tests whether the catalyst system efficiency is working?
Hello,
What is the year, make, and model of your vehicle?
I have a 2017 Chevy sonic and I've sunk about 3 grand into it to get rid of this code. And it still won't go away.
I have a 2006 2.5 frontier. I got it from Carmax in 2015. It had 105000 miles when I got it. Pretty clean and not many problems in six years, fan clutch, Oil cooler gasket, Crank position sensor, Plugs and a coil ; (take off the intake to change plugs? realy?). But still stuff I was able to do myself. I assume most of the emissions are original though. It now has 168000 on it. I had a p0420 last week. I thought because I really haven't worked on many cars in the last 20 years this emissions problem maybe a little over my head. After a lot of reading and watching YouTube I saw there are a lot of things that can be the source of this code. So before I take it in for the first time I want to try some things. I took the following steps. As a matter of maintenance I replaced both o2/ fuel air mix sensors. No fix. Next I replaced the (stuck open) PCV with an Echlin valve from Napa. No fix. Next I bought an Ames 12:1 laser thermometer at harbor freight and measured the first cat at 300F on the inlet and 495F on the outlet while @ 2000 RPM. Converter good I am told. I did not look at the second cat. Next I bought some spray (in house brand carb cleaner) at ORellys and cleaned the Throttle body butterfly and inlet, while on the car. No fix. OPPS, the engine would not Idle after that and kept pulse revving between 1000 and 1500 RPM. I had to go through Throttle body learn procedure after that. Back to normal. Dodged a bullet on that one but still a p0420. Next I bought some CRC Mass Air Flow spay cleaner from ORellys and cleaned the sensor. Cleared the light and for the first time the light stayed off for about 35 minutes under hwy and intown driving. I Thought maybe I was on to something but no fix. The light came back on. Next I taped my wife's blow dryer to the tail pipe and spayed some soapy water on the first cat outlet flange at the connection. Found a leak but burned up the blow dryer. Its a Mahle gasket. Looks good, like the one that was in there. BTW I used PB blaster that I got at Walmart on the exhaust studs and nuts. I spayed them again when I got back, the exhaust was hot and I spayed them again after it cooled down. I cleaned the studs with an end brush on a drill before I started to loosen them. Kept my fingers crossed and lucked out with no problems. Done, ready to go. But for the first time I couldn't clear the engine light. I must have tried 10 times. I Measured the battery at 12.50v. I have been nursing this 6 year old Walmart Everstart Maxx through this winter with My Harbor freight Cen-Tech trickle charger. It just takes longer and longer to take a charge. Today I just don't want to wait 2 days for this battery to charge up past 13v. I doesn't seem to hold either. Its been a good battery but I have milked it. Off to Walmart for another Everstart Maxx. Hope this one is as good as the last one. Replacing the battery cleared the codes or at least turned of the light. We drove the car for an hour Hwy and intown. No light so far. My wife said " what if it was just the battery all the time?". She's probably just mad about her blow dryer. Just kidding, she's all in because she drives the truck to work everyday. I'm pretty sure it was the leaking exhaust gasket. If I was smarter I would have started there. Walker exhaust guys have some pretty good tips I think and they say that's a good place to start. Hopefully I am not speaking to soon an the light stays off. Hope this was helpful. I have gained so much from listing to all of you. thanks. My turn to help.
Hi Geo,
Removing the battery will clear the codes and reset the onboard monitors. The operating conditions will need to be correct before the engine computer runs the catalyst monitor again. So, unfortunately, you won't know whether the gasket fixed the problem until the catalyst monitor runs.
Most code readers and scan tools provide monitor information, so if you have a code reader, you might want to check the status of the cat's monitor.
Most of the time (but not always), a bad catalytic converter is the cause of P0420. If the code came on shortly after you fixed a misfire (the bad coil (s) you spoke of) it's even more likely that the cat is to blame for the code. An engine misfire can quickly damage a cat.
Hello. My Caliber 2010 has the P0420 code. I do not feel any problem in the car, maybe it is taking a little bit more gas. I cleared the code and after 1 week it appeard again. What could be the problem? the Cat, O2 sensor? A leak? Thanks.
Hello,
The most common cause of P0420 is a faulty catalytic converter. But there are other possibilities, as well (e.g., a faulty O2 sensor). You'll want to have a professional diagnose the code firsthand for a definitive answer.
Hi! I have a 2014 Chevy Cruze. When the code originally popped up, they switched out my catalytic converter. Drove it for a few cycles and the light came back on so I brought it back to the mechanic and told them to put my old one back on. Went to a different mechanic and he fixed some stuff and the code went away for almost 6months. But it’s back… do you have any other recommendations? Just try getting the O2 sensors replaced? Thanks!
Hello,
Do you know what parts the second mechanic replaced? Also, your car has two catalytic converters (one integrated into the exhaust manifold and one underneath the vehicle). Have both been replaced somewhere along the way?
No, they haven’t. The one underneath is the one they replaced and it didn’t fix the problem. The second mechanic replaced the mass air flow sensor and a valve cover gasket.
There's a good chance the warm-up catalytic converter (the one built into the exhaust manifold) is bad. I would have a mechanic look into that. The MAF sensor and valve cover gasket should have nothing to do with code P0420. A decent mechanic should be able to properly diagnose the code for you.
I had a P0420 code pop up an my Mechanic put a cleaner in it my gas tank an it went off however I do need a fuel pump do u think that part of P0420? And why it popped up?
Hi Kita,
It's possible. If the fuel pump is causing an engine misfire, that misfire could have damaged the catalytic converter.