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P0420 Looks Like I Need New Cats.
post Jul 8 2013, 10:23 PM
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ISD 4/12/05 just out of 8 year emission warranty. 73000 miles. Any way to help?
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post May 28 2014, 01:31 PM
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Our family has always had high mileage cars, we've found a few techniques to cleaning cat converters...

First off, you need to check the oxygen sensor, sometimes the sensor is bad and just needs to be replaced.
Pull the sensor out, put it in a vice, and hook it up to a multimeter, get a propane torch (or some sort of flaming torch device) handy.
****CAUTION**** do not let the leads of the multimeter touch each other, it can cause damage to the oxygen sensor when you start testing. ****CAUTION****
Light up the torch and set it to low heat. Move the flame over the oxygen sensor, wait for the oxygen sensor to heat up a bit (oxygen sensors wont work correctly till 600F), look at the voltage.
The voltage should increase and then level out at .6 or .7ish. (you should look this number up to make sure, or maybe someone can verify).
Move the flame away and the number should go back to 0v.
Flame back to the sensor it should go up to .6/.7v
It should only take a few seconds for the voltage to increase/decrease, anything slower can cause a code. If you're voltage is not correct or it takes too long then your oxygen sensor is probably shot.
I have managed to save a few of these by cranking up the heat on the torch and blasting the oxygen sensor with a high dose of heat to burn off residue, but I can't guarantee this will work.

So lets assume the oxygen sensor is good. There are a few engine cleaning tricks that can help with this as well.
First is the old fashion sea foam through the intake. I'm assuming that you have an 05 suburban based on the ISD that you posted (although I have no clue what ISD actually stands for).
If thats the case, than on the back cylinder on the driver side (I think its cylinder 8) there is the PCV valve. Grab a can of sea foam from oriellys and a dixie cup and pour half of the sea foam in the cup.
Start up the truck and let it warm up a bit. I usually have a helper sit in the truck and start it & kill it for me.
Slowly & steadily let the engine suck the seafoam out of the cup (i like plastic cups cause I can bend them to get the last bits). I like to keep my thumb over the hose to help moderate the sucking.
Once the cup of seafoam has been sucked into the vehicle, turn it off.
Pour the other half of the can in your gas tank (sometimes I do the whole can, though most prefer to do half, if I feel the truck is older or has lotsa gunk built up I will go against advice and do the whole can i have done this before with the 5.3s, they are big, tough, engines)
Wait 5-10 minutes and turn it back on. If you've never done this before its going to make a crapload of smoke and lights appear on the truck. These lights will go away and is normal. If you have a GF/Wife you can send her to the grocery store for milk and watch the hilarity ensue.

If that doesn't clear it then you can increase the octane of the gas to help clean it out.
Wait till the truck is low on gas and fill up with premium. I like to use shell for this because from my understanding they use additives in their fuel that burns at a higher temperature. *Note that I do not use shell on a day-to-day basis*
The active ingredient in octane boosters (seafoam, techron, the little button you press at walmart's filling station) is typically derived from Isopropyl alcohol. Obviously some of the cleaners (like seafoam) have other additives, but when it comes to octane boost they use isopropyl alcohol. Why? Cause its cheap, stable, readily available, and safe. When you get to the higher grade stuff (Ether derivations) they use other chemicals, but the price and danger levels sky rockets from there.
Regardless, swing by walmart/CVS/walgreens and buy what equals out to a gallon of the stuff, exact amount begins to become more of an artform than a science. Like the seafoam, if the vehicle is older (IE dirty converter)or I have a suspicion that the converter is clogged than I will typically put more in. At 1 gallon of isopropyl you're diluting your tank to a ratio of 32:1.
Now we get use a technique from the italians - "The Italian Tune Up." Get on the street and redline it.
The extra heat generated from the higher octane fuels will help to heat the catalytic converter up nice and hot. This will in turn burn off some of the buildup from over the years (in a dirty converter situation).
The extra pressure from the fuels will push out any clogged catalytic converter honeycombs (they are honeycomb shaped) that has happened due to bad fuel or what may you (this is a clogged converter).

If all else fails, there as been a proven method of simply soaking the converter overnight in laundry detergent. While I have never done the science behind it is solid. Detergent breaks down oil based fluids and lets you scrub the converter clean. It's not like the converter pumps into the engine so any left over residue will simply be burned out on the next trip. You can search youtube for more information on this. If you try this lemme know how it works.

I have managed to save all but 2 converters through the above mentioned tricks. One we cut out and welded in a new one on (it was our previous 250k miles suburban, it ended up going till 335k before we sold it).
The other is in my little brothers tacoma, I still need to play with it some but he decided that the best place for his passenger door was attached to the front of another driver's grill. So I'm not fixing his truck till he's done repairing his side work tongue.gif (gotta learn the lessons somehow!) It's worth noting that he decided to top his tank off with diesel one day - there may be a reason for his shot converter.
Point in case - most pre 150k converter error codes are dirty or abused (do you use E85? change oil routinely? etc.) or a false signal (oxygen sensor).

Best of luck and let us know what you find.
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Gary P
post May 29 2014, 03:59 AM
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LOL, Rogue - Your write-up was very well done, and had me laughing when I got to the Italian tune-up part. hysterical.gif
You have excellent writing skills. You should be an educator of some kind, as I'm sure you'd hold everyones attention. thumbsup.png

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