• PO420 Catalyst System Efficiency Below Threshold
  • PO420 Catalyst System Efficiency Below Threshold
  • PO420 Catalyst System Efficiency Below Threshold
  • PO420 Catalyst System Efficiency Below Threshold

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  1. #31
    Member vdogamr's Avatar
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    I had to have my Cat replaced on my 88 shortly after buying it. I had one symptom: No power. I couldn't make it up a hill or achive more than 40mph on a flat surface. Had it towed to the mechanic. Now this was because my Cat clogged my entire exahust system. I was told by my mechanic that all 3 cats needed to be replaced and they had thrown chunks into the muffler. I put on a whole new Flowmaster exhaust system (only one Cat ) for much money.

    When my CRV needed its Cat replaced it also had a noticable loss of power, but not undrivable. The tow truck guy gave me this quick and easy test (probably not fool proof): In neutral, give the engine a quick high rev and have someone else feel for a lot of exhaust gas comming out. If not then your exhaust system is probably clogged, ie your Cat is probably bad. Again I could not drive my car so your situation is different.

    I doubt this all helps as much as I hope it will. But it may be helpful to know that a Cat is about $550 +- $100. It was for both cars. Even though in the end I think my new exhaust system was $1800+install.
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    When I bought a fixer-upper because I thought it would be fun, I had no idea how much fun it would take.

  2. #32
    Member LLC5's Avatar
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    Quote Originally Posted by dogfish246 View Post
    OK, would there be any tell tale signs that it is the cat converter? I dont smell sulfer, and the light has not come on for a while now.

    I would hate to replace a cat! But if I did, do you replace both at the same time, or only the broken one? Is there anything tricky about changing a cat?

    Thanks for all of the help!


    The only signs of a low effeciency cat are a check engine light on OBD 2, or a less than 100 degree F difference between the inlet of the cat and the outlet, which is how you checked an OBD 1 or earlier system (OBD 1 had no secondary O2 sensor to monitor the cat). Other signs of a bad cat are noises (buzzing, jingling, etc) or poor performance when they physically fall apart internally, but low effeciency is not detected visually unless it is combined with physical damage. You only need to replace the one cat as the other one is on a different bank of cylinders. There is nothing tricky about replacing one, they are usually bolted on, but some do require welding.
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  3. #33
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    Thanks Everyone!

    I guess I will wait for the check engine light to come back on since the Vette is performing beautifully right now! Then I will replace the cat!

  4. #34
    Member LLC5's Avatar
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    Quote Originally Posted by dogfish246 View Post
    Thanks Everyone!

    I guess I will wait for the check engine light to come back on since the Vette is performing beautifully right now! Then I will replace the cat!


    As a side note, if you do replace the cat be aware that a-mkt cats are generally not as effecient as OEM units and can set codes, especially high flow cats. Also, cats can be damaged by cylinder misfires, coolant from bad head gaskets, and incorrect A/F ratios, so make sure your vehicle is operating properly before replacing an expensive converter. I am not saying that your vehicle has any of these problems as usually other OBD 2 codes would be set, just to be aware of them. If you do buy an a-mkt cat, as they are usually a lot less money than OEM, shop around and make sure it comes with a strong warranty. Good luck with it.
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  5. #35
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    Quote Originally Posted by LLC5 View Post
    As a side note, if you do replace the cat be aware that a-mkt cats are generally not as effecient as OEM units and can set codes, especially high flow cats. Also, cats can be damaged by cylinder misfires, coolant from bad head gaskets, and incorrect A/F ratios, so make sure your vehicle is operating properly before replacing an expensive converter. I am not saying that your vehicle has any of these problems as usually other OBD 2 codes would be set, just to be aware of them. If you do buy an a-mkt cat, as they are usually a lot less money than OEM, shop around and make sure it comes with a strong warranty. Good luck with it.
    Thanks, that is good advice! Just the advice im looking for!

  6. #36
    Gone but not forgotten Remo's Avatar
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    Default Rear 02 Sensors

    The rear 02 sensors throwing codes is a common problem especially with the C5 guys who have installed long tube headers with high flow cats. The most popular resolution is to get the car retuned and intall a set of 02 simulators on the rear sensors.
    Remo

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  7. #37
    Technical Writer for Internet & Print Media
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    "LLC5's" posts two aboove this one is very important information about cat issues.

    As for the one by "Remo" above, I think all it does is confuse the issue, but I'll be a "nice" guy and ask "Remo" how does that apply to an LT1 with no headers and stock cats which has to pass an IM240 exhaust emissions test?

  8. #38
    Gone but not forgotten Remo's Avatar
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    Quote Originally Posted by Hib Halverson View Post
    "LLC5's" posts two aboove this one is very important information about cat issues.

    As for the one by "Remo" above, I think all it does is confuse the issue, but I'll be a "nice" guy and ask "Remo" how does that apply to an LT1 with no headers and stock cats which has to pass an IM240 exhaust emissions test?
    Good point Hib.....I'll step out.
    Remo

  9. #39
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    No, that was helpful. Maybe I will check out this path too. If I ever get headers and a new exhaust without cats, this is another path to check out. Then I could use an O2 simulator... But then I would have to find someone to get my car "inspected"!

    Probably will just get a new cat when the time comes, but all insight is welcome, as long as it is friendly!

    Thanks Guys!

  10. #40
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    Quote Originally Posted by Remo View Post
    Good point Hib.....I'll step out.
    Remo
    No...

    Don't "step out".

    Let's all try and help the ole "dogfish" with his particular problem.

    As for the part about headers...if you go that route, what Remo says becomes pertinent, however, before you put on headers make sure whatever emissions test you have in NJ cause problems for a car with headers.

    In fact, if there is a state exhaust emissions test in Jersey, I'd be interested to know how it works.

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