P0420 How To Diagnose A Bad Catalytic Converter -EricTheCarGuy

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O2 Sensor Video: https://www.youtube.com/watch?v=NIzcztO-CCs

Discussion about this video: http://www.ericthecarguy.com/forum/18-The-EricTheCarGuy-Video-Forum/42514-p0420-how-to-diagnose-a-bad-converter#42610

I know this video will not only save people some money but also I think it will solve a lot of confusion concerning the operation of the catalytic converter. These tests work as you can see in the video. So the next time you have a P0420 or someone tells you your catalytic converter is bad you’ll know what to do to find out if that is indeed the case.

Stay dirty

ETCG

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26 thoughts on “P0420 How To Diagnose A Bad Catalytic Converter -EricTheCarGuy

  1. Hi Eric, I hope you still read old videos comments as I have an interesting story on a catalytic converter: My car is a 1994 Autobianchi Y10, OBD1 single point fuel injected, bought new back then.
    The cat is still the oem one but it stopped working many years ago. The car was starting to burn oil and I diagnosed bad valve seals. I decided to do a head job, so replaced the seals, lapped valves, machined the surface for the new head gasket and finally put back the head AND (this is important for the story) I changed the paper seals for the throttle body. Started the car and immediately something was wrong, it runned very rough and had problems to stay at levelled idle.
    I noticed that the engine was running hot, I took my temperature gun to check exhaust temperatures and they were very hot, even at the muffler. I decided to verify temperature before the cat, it was around 220 degrees C (428 F) and 400+ C (750+ F) after the cat.
    So.. the cat was working! I then discovered that one of the paper gaskets for the throttle body was the wrong one so it had a bad vacuum leak.
    I replaced the gasket, solved the problem with the engine but.. cat stopped working again, now I have barely 180 C (356 F) before cat, and around 150 C (300 F) after cat.
    It looks like that the exhaust is not getting at the right temperature to make the cat working properly.

  2. Very good video! My son has the same code, and I’m trying to figure it out. I’m changing the 2 downstream O2 sensors to see if one or 2 might be lazy (wishful thinking, I know) now I’ll have to bring my infrared thermometer out and do the test! Also, I’ll monitor the voltage swings as well. Thanks for the video, you just enlightened me!

  3. I have a 2012 v6 mustang that has a exhaust delete on it and I’m getting a p0420 code. The car sounds really loud but good not to loud, but I’m not sure if it’s a bad cat, a sensor, or something else. I can see the cats are on there but I’m also wonder if the guy before me gutted the cat. My idle also bounces, not by much maybe buy 50 or 200 but not much.

  4. you explain stuff in a way i can understand i like that and thank you there needs to be more people out there like you i am subscribing to your channel i hope to see more from you.

  5. I just got this code on my Toyota corolla 2001 I don't know much about cars and I don't have the funds to be able to pay someone else to do it your technique is awesome. my car has been idling rough for some time now I just didn't know how to fix it and then today my engine light came on I'm really hoping its not the converter and its just the o2 sensor sure a lot cheaper.

  6. What does it mean with downstream O2 sensor stays at 0.04V with 2500rpm and jumps to 0.6 V when let off the gas pedal? Thanks.

  7. Dude! You are a marksman!😃 On all aspects of the word. Pinpointed the problem …but also that steady hand aiming the laser between that tiny space with almost No wobble while lying on the ground side ways… that’s that Sniper-Stuff right there…👌🏾😎

  8. On my scan tool, S1 goes from .00000 to .65 or so back and forth. S2 stays around .76. I had codes P0171 and P0420. I cleaned the maf sensor and reset the codes. P0171 never came back but P0420 did. Any suggestions?

  9. Thanks Eric…. good information…I like the laser test. I am interested how you got the live data. Does a FIXD tester give me this info? If you know….. Appreciate what you do for the car community. I have a cat that pops. No codes… However, maybe performance off. But could be spaark plugs, air filter, fuel filter. At 100K time to change them all I think.

  10. it's not always the Catalytic Converter when a p0420 is thrown…very commonly it's a vacuum leak at the intake, especially if it's the plastic type. simply replace your intake gasket, as well as the throttle body gasket. these gaskets go bad over time and will trick the o2s because unmetered air is getting past them and you end up getting a false positive reading for a bad Cat…just do some homework and look closely for vacuum leaks before wasting money on something that isn't actually bad at all. Cats very rarely go bad and o2s will usually throw their own code if they're failing, so simply replace the gaskets I mentioned and see if that solves your issue…you're welcome!!!

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