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cat question

Discussion in 'Chevy Silverado Forum (GMC Sierra)' started by bad5300, Feb 20, 2009.

  1. bad5300

    bad5300 New Member

    Next week me and my buddy (a well-known mechanic) are gonna change out the stock exhaust on my 2001 Z71, 5.3 etc....I was considering going no cat with a flowmaster 40. Will cutting the cat off set off any codes, such as O2 sensors?
  2. silveradotrailblazer

    silveradotrailblazer Epic Member 5+ Years ROTM Winner 5000 Posts

    Yes, the rear 02s will throw codes because they see a dirty exhaust. I run high flow cats on my 05 Silverado and they threw codes. Used my Diablo programmer to delete codes. You must still have rear 02s intalled and plugged in. Programmer just tricks trucks computer. Ive been told that if you remove cats, you should have a custom tune. Diablo or there dealers can do this for you.
  3. mjalstad

    mjalstad Rockstar 100 Posts


    You still need to have the rear O2s plugged in? Are you sure? I just installed headers with an offroad Y-pipe. Don't have it running yet but I was thinking I could just turn off the DTC for the rear O2s and that would be fine.
  4. bucknasty

    bucknasty New Member

    I cut my exhaust completly apart and still had the rear o2 sensor plugged in and it did NOT trip the computer. the front o2 sensor is the only one that actually messes with the motor. the rear sensor will probably bring up the check engine light if your burning blue smoke or something. Id say cut them off increase power and you can always delete the code later...
  5. ct9a

    ct9a Epic Member 5+ Years 1000 Posts

    From what I understand of how OBD II works, the ECU will actually take readings from both sensors. As long as the rear sensor shows lower than the front (but not turned off completely) it will not throw a code. I have seen O2 sensor adapters that will fake out the ECU for under $50 out there. Keep in mind, my information is based on experience with imports, so it may be different.
  6. thatjerk01

    thatjerk01 Rockstar

    the thing with removing cats depends on where u live if u have to do emissions and inspection u legally have to have cats or its a huge fine. it takes the reading from the front usually.
  7. tbplus10

    tbplus10 Epic Member Staff Member 5+ Years 5000 Posts Platinum Contributor

    If you live in North America you cant legally remove any emmissions equipment whether you have emissions testing in your area or not, this isnt a state or local law, it's federal. No matter where you live if you drive the vehicle in an area that has emissions testing even though it may not be required in your particular area the vehicle must comply with local emission levels where its being operated (same rules apply for operating and safety equipment, lights, horn, brakes, etc.).
    You can be ticketed and fined, punishment for emissions tampering can run as high 5 yrs in prison and $10K. Most major cities have mobile sniffers that are parked along the roadways reading emissions on passing vehicles and taking pictures of their plates, you will get a letter in the mail if you get sniffed by one of these sites.
    They've recently added regulation to impound and crush gross polluters that arent in compliance.
    A properly maintained cat will not hurt performance on modern engines, they actually help to develop and maintain better performance while keeping the air a little cleaner for the rest of us.
    Dont get me wrong I'm not against modifying a vehicle for better performance, just do it the right way. On a newer computer controlled vehicle chopping the cats off isnt the right way.
  8. bad5300

    bad5300 New Member

    Alright i think I got my answer...thanks for all the help!!!
  9. cascott325

    cascott325 Epic Member 5+ Years 1000 Posts

    bad5300, where you at in ky?
    cutting the cats off on a new vehicle as stated before, will lower your back pressure causing a loss in torque and hp. i am running true duals with high flow cats and still have all my o2 sensors in. my check engine light doesnt come on.
  10. yotchie

    yotchie Rockstar 100 Posts

    do high flow cats still maintain the required back pressure?

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