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2009 Silverado 1500 4.8L 2WD
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Discussion Starter · #1 ·
I’m new on the forum, but not new to the brands. However, this is my sister’s truck I’m trying to help with. 2009 Silverado 1500 4.8L. She got it from someone, drove it about 500 miles and the codes came on for both catalytic converter banks below efficiency and lean condition on bank 1.

There was some rattling so I thought maybe cats blown out, so we ordered an eBay cheapo set of front cats. When we went to install them, we had hell and found out someone else had been playing around in there, probably chasing this same problem. There was a bolt in the exhaust that had busted some of the honeycomb out from one cat, both exhaust manifolds had been removed - we had to replace one due to not being able to get broken studs out. I had to ghetto custom the cheap front pipe into the stock pipe where the 3rd catalytic converter is which was still in seemingly good shape. Put it all back together and the lean condition code went away due to having all the bolts on the flanges now, but within 50 miles the cat efficiency codes popped back up. So we went ahead and ordered fresh A/F sensors and downstream o2 sensors and put those in yesterday. No change. Tried putting in the old anti foulers to see if maybe the new cats were just not cutting it and no change.

I have cleaned the MAF, made sure air filter was clean, can’t find any obvious vacuum leaks. I have read the live data on the sensors and the A/F reads what I believe is in spec and the o2 is pegged at 99.2 i believe which I think is normal if memory serves me correct.

So what am I missing here? Where should I go next? We need to get this engine light turned off and get it to pass smog and get registered.

Thanks in advance for your knowledge.
 

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2004 Sierra 3500 6.0L, auto, extended cab, cab&chassis, upgraded to 4wd
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Google for the specific codes you are currently getting, and go through the diagnostic procedure for them. They never are just "replace part X".

And given someone has mucked around with the system, there is also the possibility they've flashed the ecm with something non-stock, and that could be causing problems.
 

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2009 Silverado 1500 4.8L 2WD
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Discussion Starter · #3 ·
Google for the specific codes you are currently getting, and go through the diagnostic procedure for them. They never are just "replace part X".

And given someone has mucked around with the system, there is also the possibility they've flashed the ecm with something non-stock, and that could be causing problems.
Yeah I'm familiar with how to generically diagnose codes. Based on symptoms is why I replaced what I did. P0420 and P0430. Catalyst efficiency on both banks. There was initially a lean code due to missing collector bolts but that's fixed now with the new manifold and front pipes matching up.
As for the flashing, it isn't out of the realm of possibility I guess, but I don't know what they would have flashed on that would only give me codes for Catalyst efficiency and no lean or rich codes too.

I may zip tie the downstream sensors out to the frame to read clean air to see if anything changes.

Any other ideas?
 

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Cheap cats don't even last a year. New cats will usually throw a code once or twice in the first 50 miles or so as they wear in.
 

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2004 Sierra 3500 6.0L, auto, extended cab, cab&chassis, upgraded to 4wd
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cat codes are also not depending on rich/lean (or necessarily vice versa)...

And the 99.2 out of 100 and constant reading perhaps hints at either a sensor/wiring/connection problem somewhere.
 

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2009 Silverado 1500 4.8L 2WD
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Discussion Starter · #6 ·
Cheap cats don't even last a year. New cats will usually throw a code once or twice in the first 50 miles or so as they wear in.
Well be that as it may, I’m not going to spend thousands on OEM cats. Could replace the cheap ones once every 6 months for almost a decade before it would equal the money they want for OEM. I think we’ve put a couple hundred miles on them now testing and trying to figure out the issues, so I don’t know that the 50 mile bit is applicable here but I do appreciate you chiming in.

cat codes are also not depending on rich/lean (or necessarily vice versa)...

And the 99.2 out of 100 and constant reading perhaps hints at either a sensor/wiring/connection problem somewhere.
True enough, but the lean code we had was due to the exhaust leak and has since been fixed and is a non issue now.

I’m curious about your comment on the STFT 99.2% constant not being correct. I’ve read in multiple places that is normal, which admittedly doesn’t make much sense to me, but generally it isn’t echoed unless there is some truth to it. Any chance you could point me to the right place for info on what the sensors should test out at? I’m not ruling out a wiring issue either as this whole truck has wires that aren’t in looms, indicating someone else was probably chasing an electrical gremlin here too.


I tied up the sensors to fresh air to see if we got any change in codes and sure enough it threw codes for stuck lean, efficiency, etc. So sensors out wasn’t the answer, but unfortunately doesn’t point me to the right questions either.
 

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2004 Sierra 3500 6.0L, auto, extended cab, cab&chassis, upgraded to 4wd
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STFT has nothing to do with the downstream o2 sensors, and should be changing a lot with the engine running.

What sensors did you pull out?
 

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A new OE y-pipe with cats is less than $1500; Magnaflow cats and y-pipe is about a $1000 list price. To each, his own.
 
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Anytime you get a 99.2 for stft, that is a reading for rear stft that the vehicle doesn't have and the scantool doesn't know any better. Garbage data. So if you see 99.2 rear stft, your rear O2 sensors are strictly used for catalyst monitoring and have 0 input for fuel control.

Cheapo cats are for the people who don't care about check engine lights but their cats were stolen, damaged, restricted, etc. If you need the light out, then you're probably not going to be happy with them.
 

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2009 Silverado 1500 4.8L 2WD
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Discussion Starter · #10 ·
Anytime you get a 99.2 for stft, that is a reading for rear stft that the vehicle doesn't have and the scantool doesn't know any better. Garbage data. So if you see 99.2 rear stft, your rear O2 sensors are strictly used for catalyst monitoring and have 0 input for fuel control.

Cheapo cats are for the people who don't care about check engine lights but their cats were stolen, damaged, restricted, etc. If you need the light out, then you're probably not going to be happy with them.
Thanks for the info on the rear sensors. That makes more sense.

I have a hard time believing that cheap cats won't work. This isn't a Lamborghini. I've installed cheap cats on every honda, Chevy, dodge, Nissan, Lexus, Ford, etc that I've ever worked on where they've been plugged or stolen and they all work just fine. Is the 4.8L really so finicky that it requires oem only? This whole truck is worth maybe $7k max so to spend over a grand on a part that does nothing beneficial other than getting it through inspection is insanity.
 

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You don't think the Chinese Communists would rip you off?

Don't blame the car, it's probably the parts you are using.

In my mind, it's never a good idea to tie maintenance and repair costs with the value of the vehicle.
 

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1999 Silverado ECSB 6.0
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I have only had to install cats on a car one time. I used a walker cat on a 2007 Audi A4. Aftermarket cats always suck ass. Best to delete or go OEM
 
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