P0420 How To Diagnose A Bad Catalytic Converter -EricTheCarGuy

Omg. Finally someone who knows what they are taking about, and clearly explains what he is doing. Best part is that he shows what a bad cat looks like, and what a good system should look like on the OBD. Very helpful. Thanks

This is how all YouTube videos should be. Clear, intelligent, demonstrative. Rockstar quality.

Hi Eric! It's Shep in Florida.(Retired tech) 3 years ago I replaced the engine (2.2 4cyl) in my 2000 S10 with a new (not reman) engine from GM. I replaced the fuel injectors and pressure regulator (Sorenson). o2 sensors (AC Delco). And cat (Walker). 11 months later ap0420 code appeared.The truck runs like new (it should with a new engine). I have no vac or exhaust leaks (vac at 18 in' average and rock steady) No misfires and fuel pressure is normal. NO other codes,and gas mileage is good. AutoZone replaced the cat at no charge. Last week the p0420 code appeared again.I ran a live data test. The U/S sensor switched as expected. The post sensor switched between .11-.95 volts. AutoZone replaced the cat again and  the post sensor now reads .73-.78 at highway speeds and .44-.47 at idle and right around .50-.54 volts at 35-45 mph. My GM tech friends say I need to install a factory cat ($1000-$2000).My auto tech instructor (the 1 who taught me 30 years ago and is still teaching) and my independent tech friends say that's nuts and a Walker cat ($179) or any cat should be fine. Any ideas? I was always taught that cats don't just die,they're murdered. Florida has no emission testing (except for Miami) but a mil drives me nuts. Thanx and G-d bless.

All of these are great tips, but let me tell you my experience. I have a 2013 Nissan Altima S with the 2.5 4 cyl. For several months I was plagued with the P0420 code. My mechanic tried all the usual diagnostic techniques, checking o2 sensors, cleaning the catalytic converter, even going as far as gutting the rear catalytic converter to relieve a suspected blockage. The problem not only persisted, it got worse. Instead of coming on every 150 miles or so, the Check Engine light came back on every 50 miles. I have probably spent around $500 trying to get the stupid light to stay off. You know what finally solved the problem ? As per routine maintenance, I removed and cleaned (I have a K&N) my air filter. I cleaned and re-oiled it according to K&N's instructions and re-installed it. NO MORE CHECK ENGINE LIGHT. After all the hours and all the money needlessly spent, the solution was to clean my freakin' air filter ! Something so basic and elemental, I didn't even think of it. The lesson here ? Check the most basic, cheapest stuff first. You may think you have a special car with a problem so unique, no mechanic has ever seen it before. In all likelihood, its something simpler than you or your mechanic realize.

You, sir are a national treasure. Thank you for so much great information! I've used your videos many times to handle work I would've otherwise had to pay someone to do. Thank you, thank you, thank you.

Learnt this: Upstream O2 sensor switches voltage all over. Downstream O2 sensor voltage should be more steady.

I had this code on one bank (V6 engine) for close to a year, all O2 sensors checked out fine...also started getting lean/rich codes which made me check for vacuum leaks(all good) & clean my idle control valve because the car started shutting down when idling, no change then decided it must be a bad MAF. bought a cheaper Chinese model for $45 on eBay vs OEM pricing...cleared codes and drove for a couple of days or 70-90 miles...rescanned and everything was good to go including smog readiness. Just passed smog...converter was fine but bad MAF was sending false air readings to the ECU, thus flowing more fuel which cooled the converter aka not warmed up enough (below threshold). Fuel economy was not good...but now i can drive 1/4 tank to and from work for 3 days on a v6.

Great video! I really appreciate you taking the time to document and explain various diagnostic and repair procedures. I thought I might point out something. Not a point of criticism, just trying to help a fellow gear head out. At 7:29 you talk about trying to "shoot the laser between the heat shields." The laser is just a guide for you to know where you are aiming. The infrared sensor below the laser returns the temperature value. Also, the sensor measures a circular area, the further away you are from the target the larger the area. The value returned will be a rough average of that area. It is important to note that the laser will point above, not directly at, the center of this area. On a high quality IR Thermometer you can expect the circle to be about 10:1, distance to diameter (aka distance to spot or D:S ratio). In other words, at 18" distance to target the circular area you are measuring will be approximately 1.8" in diameter. Check your users manual for both the off set distance of the laser center line to the IR center line and the distance to diameter ratio.

Great information. Thanks for the details on how to track down the source of the P0420 code, because obviously I don't want to have to replace my cat if it's just an O2 sensor.

This was really informative and professional. You seem like a mechanic and youtuber who cares about their work. Thanks for the information, this will definitely help me with my maxima I've been having issues with.

I got an abandoned 01 Xterra from a tow-yard for $550 because of the P0420 code in Colorado where they do emission test. After using an Auterra to diagnose the issue at the lot and the Autel tool I use at work once I purchased the vehicle, I was able to identify a failed front cat as well as an intermittent failure on the front O2 sensor. $100 catalytic converter, $100 Bosch 02 sensor and $150 in labor later and it's running like a champ once again. I tried the $20 eBay "Chinesium" O2 sensor, which lasted two weeks before burning out again, so best to get OE quality parts. It was funny because once everything was bolted up and I fired it up and revved it all the sooty black water got pushed out the pipe. Soon after the rear cat temperatures and O2 values began looking right, so I lucked out with it only being half-plugged (due to a leaky valve cover gasket that was dripping oil onto the front cat). No leaks or issues ever since. Best of all, no engine money light. This video came in quite handy through that ordeal several months ago, so thanks for your help. This was the first vehicle I ever had to do emissions work on, and the last time I worked on a KA24DE it was in a 92 that didn't have hardly any emission devices. A lot has changed in nine years, thankfully it wasn't that hard to mend the issue. The hardest part was all the rusted and heat-welded bolts that had to be cut off with a angle grinder. I must have wasted two weeks trying to PB-blaster soak them and hitting it with the breaker bar for nothing. The ones that gave usually just ripped the heads off the bolts, so just cut it off and save time.

I end up on your videos often and it resolves my problem or otherwise educates me. Thank you for what you do.

No, cats work like I explained in the video and that is their main purpose. They also help with HC and CO but those are secondary effects.

have to be careful with a lazer style temp gage, most people think that where the lazer hits is where the temp is being read. Actually the area being read is larger than just wat the lazer hits and the farther you get away the larger the area being read. Think of it more like a flashlight not a lazer. if you want to pin point you got to get super close. Most of this should be covered in manual.

Good job on showing what a good cat does not only a bad cat

Hi Eric, I really love your videos, as I've been searching them for an answer as to why my Mazda Protege 2000 loses power when it warms up. It runs great though, smooth idle and purrs nicely, just has no pull and goes so slow. After watching your videos I now know it's nothing to do with my exhaust obstruction, thanks. So now to watch more of your videos to find out what the issue is.

Eric you gotta open your own mechanic school. Teach people man you know your stuff. Keep up the good work! Love the videos.

Eric, Thank you explaining this! I known for a long time that laser temp trick, but I never used it or seem the temps from it. This is going to be very helpful this weekend!

I add a “gallon” of lacquer thinner to between 5-8 gallons of gasoline. Before oil changes is my rule of thumb , to keep the cats cleaner and clog free. I do it the smarter way. I put a 5 gallon can with fresh fuel in the trunk with the gallon of lacquer thinner. Then with a half tank or less I head to the interstate. At a rest stop or before entering the highway I pure that gallon of lacquer thinner into my gas tank with the engine at operating temperature. I drive with the overdrive “off” until the fuel is nearly depleted. I carry the 5 gallon can with me just in case I need it. I have done this a few times now, and I have not needed it. No sputtering, no hesitation with my engine, 4.6L Mustang 2007. All catalytic converters get dirty then they can get clogged. When they get clogged 2 things could happen; 1. The exhaust pressure could blow the clog out of the system. 2. The exhaust pressure blows the clog along with the internal catalyst materials out of the system. Causing a catastrophic failure.

As has been mentioned by Eric, you need to know about what you are working on. The three-way-catalytic converter (TWC) is part of a whole engine fuel/exhaust system design. With a proper functioning engine system, today's TWC are designed to operate for the life of the car and meet emissions requirements. The best way to test a TWC is with an exhaust gas analyzer (which are expensive) and proper testing methods of which there are several. If a TWC is determined to be defective, the cause of the failure must first be determined. Not to do effective diagnosis and repair of the root cause of TWC failure can result in replacement failure soon after. California emissions testing is super tough and replacement TWC must meet those requirements. Even replacing a TWC in California must meet strict requirements. In some areas of the U.S. to even replace a TWC requires a special certification. Go to any auto parts web site (try rockauto.com) and notice most of their TWC offerings cannot be sold in California. OEMs are often what is needed. For a comprehensive discussion on engine emissions design and catalytic converter info and testing go to this link. This information is also quite helpful in avoiding a TWC failure in the first place. http://www.lbcc.edu/attc/documents/BAR-2011-CAT-testing-methods.pdf