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Thread: cat question

  1. #1

    Default cat question

    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. #2

    Post cat question

    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. #3
    Jr. Engineer
    Join Date
    Jan 2009
    Location
    Rochester, MN
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    231

    Default

    Quote Originally Posted by silveradotrailblazer View Post
    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.

    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.
    99 Silverado Z71 4.8
    2" Suspension lift (Add-a-leafs and leveling keys)
    Long tube headers with 3" offroad Y-Pipe
    Volant Cold Air Intake with Ram Air Intake Tube
    Jet Mass Air Flow Sensor
    Diablosport Predator Tuner

    Previous Truck:
    97 Suburban K1500
    3" Body lift
    2" Suspension Lift (Add-a-leafs and cranked up torsion bars)
    35" Yokohama M/T's
    K&N Intake
    Airraid Throttle Body Spacer
    Hypertech Programmer
    Dual Flowmasters

  4. #4

    Default

    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. #5

    Default

    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.
    Scott

    ΜΟΛΩΝ ΛΑΒΕ

    '94 Chevy Suburban K1500
    3" exhaust w/ aero turbine non-baffled muffler
    CFM Technologies TB spacer
    CB w/ 44" whip antenna


  6. #6
    Jr. Mechanic thatjerk01's Avatar
    Join Date
    Feb 2009
    Location
    currently arizona soon to be idaho
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    84

    Default

    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.
    Casey Clark
    Currently Arizona
    Soon Idaho
    1996 CHEVY K1500 Z71
    Front end collision
    145000 miles about


  7. #7

    Default

    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. #8

    Default

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

  9. #9
    Legend cascott325's Avatar
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    Aug 2008
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    Lawrenceburg, Kentucky
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    Default

    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.
    2000 silverado with a 5.3l v8, shorty headers, upgraded plugs, 10 mil wires, magnaflow high flow cats, flowmaster 80 series muffler(2 in 2 out) w/ 18 inch stainless silverline tips. i have a k&n CIA. its a 4x4 z71 offroad package. Ls model.
    extended cab 160k miles, truxedo low profile bed cover, diablo sport predator tuner.
    reman tranny @ 130k miles
    Chris

  10. #10

    Default

    do high flow cats still maintain the required back pressure?

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