# Catalytic Failure



## Worthy 2 Keep (Sep 2, 2009)

Looking for advice from a mechanic or any one with similar vehicle experience. I am currently at a loss on where to begin fixing my current issue. I am running a 2004 Chevy Silverado z71ext. cab with the 5.3L engine at 125,000 miles. Other than the normal oil and trans fluid changes, brakes, roters, calipers, and air filter, I haven't made a lot of changes / maintenance to the vehicle. Earlier this year I put on a leveling kit and upgraded to a larger tire. I noticed a slight drop in mpg's but was expecting that and it remained consistent until about three weeks ago. I had just had my emissions testing done in early August for the inspection, and no codes or trouble with the inspection. I began hearing a rattle from under the truck and pinpointed it to the drivers side catalytic converter. Definitely hear the rattle when rubber mallet tapped against cat while engine is off. So this means the catalyst has broken up and not working. The two causes of this I have researched is a physical impact of the cat, which hasn't happened with me in the vehicle and visual shows no physical damage, and thermal shock, in which a hot cat is exposed to an extremely cold environment fast enough to cause the catalysts to fracture, typically happening in snow/ice conditions. I did have to drive through high water after highway driving for 20 minutes on my way to work one morning about 3 weeks ago. The truck is currently sucking down gas and downshifting on every overpass due to the loss in power, and not a single check engine light has come on. I know without a doubt the cat has to be changed. Not sure about all four of the O2 sensors, however I still need to pinpoint the root cause of the cat failure so that it doesn't kill the new cat. My questions are where to start without replacing all of the known causes, mass air sensor, plugs, plug wires, injectors, fuel rail, knock sensor, pcv valve, MAP sensor, ignition coil etc. While I don't mind replacing these things if it fixes the problem I am trying to avoid the "trial and error" approach. I don't have ANY expendable money so the labor would be done by yours truly. Any ideas?


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## snapperlicious (Aug 11, 2009)

Pretty much all of the other things would throw a code if they went bad.


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## Worthy 2 Keep (Sep 2, 2009)

snapperlicious said:


> Pretty much all of the other things would throw a code if they went bad.


So would they show up as the check engine light flashing, or would you only see them when a scanner is used, cause like I said, I have yet to see the MIL blink even once other than the normal start up ignition sequence.


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## capt. david (Dec 29, 2004)

A bad cat should have a code.


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## Newbomb Turk (Sep 16, 2005)

Talk to a tuner in your area about installing an aftermarket exhaust with some high flow cats then get it dyno tuned. They'll be able to see all your engine parameters during the tune. They can turn off the rear O2 sensors.


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## snapperlicious (Aug 11, 2009)

*Yes*



capt. david said:


> A bad cat should have a code.


All of the other things should have a check engine light and code thrown. I'm not sure of gas engines but for a diesel you'll get a code if there isn't enough back pressure.


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## busaro (Dec 29, 2012)

Cat coming apart would def give you code p0420 or p0430 and just cause you here noise in the cat doesn't make the unit bad p0420 catalyst below threshold p0430 catalyst below efficiency threshold so if cat broke up you would get a mil for sure 


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## iamatt (Aug 28, 2012)

Also don't go cheap on the o2 sensor. Bosch over denso. Tried to go cheap on a chevy and the cheap o2 stem was too short and did not get good reading in exhaust stream, went with 80 dollar bosch and was gtg
Should only need to replace the one in the exhaust manifold. The other downstream ones usually do not have affect on his the engine fuels.


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## 4x4tx (Nov 13, 2005)

I'm getting this code p0430 on a 08 Avalanche with 115k miles..is the o2 sensor worth replacing before replacing cat? Anyone have part number and I assume is easy to replace.


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## 4x4tx (Nov 13, 2005)

???


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## gnspeed (Jan 31, 2012)

Replace the cat and dont worry about it.Water shock could have caused it to break free some times they just broke free.SES light flashing means truck is misfiring and will damage cats.The reason why SES light isnt on is because catalyst is still efficient it is just loose in housing causing noise.good luck


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## JFolm (Apr 22, 2012)

gnspeed said:


> Replace the cat and dont worry about it.Water shock could have caused it to break free some times they just broke free.SES light flashing means truck is misfiring and will damage cats.The reason why SES light isnt on is because catalyst is still efficient it is just loose in housing causing noise.good luck


I would go with this. Rear o2 sensors are monitoring cat efficiency.


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## snapperlicious (Aug 11, 2009)

*Yes*



4x4tx said:


> I'm getting this code p0430 on a 08 Avalanche with 115k miles..is the o2 sensor worth replacing before replacing cat? Anyone have part number and I assume is easy to replace.
> 
> Sent from my iPad using Tapatalk


O2 sensors are cheap compared to new cat.


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## capn john (Feb 2, 2008)

O2 sensors rarely go bad, most O2 codes tell you how the motor is running or how the catalytic converters are working. Aftermarket universal catalytic converters don't work, only go with direct fit (Aftermarket direct fit work most of the time). A bad misfire will ruin 
cats and sometimes O2 sensors. Dumping chemicals in the throttle body will kill cats and O2 sensors every time and never clean injectors. If you can properly diagnose your problem it is cheap to fix. If you do not have the proper tools and knowledge it is usually cheaper and less of a headache to take it to a good mechanic and get it right the first time. It costs a lot of money to throw parts at a problem until it is fixed


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## Y-Not (Sep 14, 2012)

Your truck is 12 years old, put a cat on it and drive it like you "stole" it. Cats don't last forever and do go bad in time.


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## fishingcacher (Mar 29, 2008)

Most places will sell you a replacement and give you some credit when you bring in the old cat. Valuable Pt in there.


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## 4x4tx (Nov 13, 2005)

Avalanche is experiencing low power and sputtering off and on now...son said it died on him this week too..first I'd heard of any real issues with it other than light. So guessing cat for sure now?


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## iamatt (Aug 28, 2012)

Cat should be warrantied for 80K Miles.

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## JFolm (Apr 22, 2012)

4x4tx said:


> Avalanche is experiencing low power and sputtering off and on now...son said it died on him this week too..first I'd heard of any real issues with it other than light. So guessing cat for sure now?
> 
> Sent from my iPad using Tapatalk


Have you ever changed the plugs. And yes it's worth it to change a bad front O2 sensor. Those are the most important ones.


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## iamatt (Aug 28, 2012)

JFolm said:


> Have you ever changed the plugs. And yes it's worth it to change a bad front O2 sensor. Those are the most important ones.


Don't. Cheap out. Get Bosch. Been there done that. Front one right off exhaust manifold usually all we ever need to replace.

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## Camarowithbass (Jul 6, 2011)

Had code P0300 on my 6.0L escalade.

I replaced plugs ,wires,map sensor, MAF sensor, all 02 sensors , knock sensors,intake manifold gaskets , and a few other things I can't recall. 

After all of this it still ran like ****, took it to the mechanic and for $160 he fixed me up with a new cat.


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## dannyw (Nov 20, 2011)

Plug in a good code reader and go to the option that tells you sensor outputs. One of them will be bank one Front O2 sensor. It will display a voltage. If the O2 sensor is working correctly it will cycle between .1 to .9 volts. If it stays low, less than .45, it's a bad O2 sensor and it will run rich and cause your problems without throwing a code. It should cycle low then high 40-60 times a minute.


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