P0420 Exhaust Leak Bubbles Between Cats 1 and 2.

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Greetings. '96 LX. Popped a P0420. Crammed a leaf blower up the tailpipe. :cool: Defintely have a leak BETWEEN cats 1 & 2 at the gasket. Does a P0420 indicate a leak BEFORE cat 1 or can a leak between cats 1 and 2 trigger that code? I certainly wasn't going to run out and buy a new catalytic converter or 02 sensor before checking for leaks after popping this code. I’ve got 2 videos below. One shows the bubbling and the second one shows my live O2 sensor voltage readings. I'll be honest, I'm not sure what I'm supposed to be seeing on the voltage readings except that they should not be the same. The O2 sensor video is of the engine revving lightly up and down while in Park. Thanks!



 

cruiserdan

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P0420 indicates catalyst inefficiency. It's comparing what's going in to what's coming out. Fix the leak first then see what you get.
 

cruiserdan

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I had a set of high-performance aftermarket cats on my SRT8 Challenger and It kept tossing cat codes. It ended up being a couple pinhole leaks in the welds. It doesn't take much.
 
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Wow. Good to know.

I had a set of high-performance aftermarket cats on my SRT8 Challenger and It kept tossing cat codes. It ended up being a couple pinhole leaks in the welds. It doesn't take much.
 

cruiserdan

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Do check for leaks upstream of the front O2 as well, just to be safe. Especially at the top of the "Y".
 
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Yep...no other leaks showing front to back except for between the cats. The Y-union was my first guess but looks good. Could be a tiny pinhole like you said though. I'm showing 90917-06076 for the gasket between the cats...I think.

Do check for leaks upstream of the front O2 as well, just to be safe. Especially at the top of the "Y".
 

cruiserdan

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90917-06043. The 06076 is behind the second cat. Check that joint as well since it's upstream of the rear sensor.

You want to be sure there are NO leaks in between the sensors.
 
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Excellent. Thanks, Dan!
 
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Posting this pic here for the visual learners in the future. ;)

Cat gaskets between and after the 2nd cat.png
 
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So a leak upstream or near any oxygen sensor is going to cause it to read leaner than normal, as outside oxygen will get pulled into the exhaust through the leak and cause the sensor reading to be skewed.

Your video shows the rear O2 sensor sweeping between at least .2 and .8 volts. I would not suspect the sensor is bad since it is working through it's normal range. However, the rear O2 is mirroring the front O2 sensor. This means the catalytic converters are not doing anything to the exhaust. This can be normal if they're still cold when you recorded. However, if they're doing that while cruising for 10 to 15 minutes, it's not looking good for the cat. Fix those leaks first, and repost a recording of the sensor outputs when they're hot.
 
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Yep, that was a cold engine.
 
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New gasket in and no leaks via the leaf blower/soapy water method. I'll clear the P0420 and test today. I love working on this rig...my first non-rust belt 80 or 100 out of 4. Exhaust flange bolts/nuts off in 5 minutes.

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I've a re-occuring leak at top of Y-pipe and flange and that will throw a 420. I keep patching it but will be replacing with complete new exhaust system including headers
 
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I went for a short drive and no code yet (may not have driven it long enough to set code-no code pending on my reader either) but the voltage on the O2's seem to be following each other. It may be the cat(s). We'll see.
 
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Well, shoot. No CEL yet but I went for a 35-minute drive last night and I do have a "pending" P0420 on my OBD Fusion app which means it will set during one of my future drive cycles. I'll check for more leaks but that leak between the cats was relative large and it wasn't the P0420 problem so I'm thinking I likely just have a bad downstream cat. Not gonna buy one quite yet though until I observe things a bit longer.
 
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No it’s not. I’m tempted to try a flange-style O2 sensor extender. If that fixes it, bad downstream cat. I’m curious if this one would work. Good chance the O2 sensor flange size is the same so I may try it.

 
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Well folks, I just needed to turn up my leaf blower up to full throttle to find the leak at the Y-pipe. I know how to weld—is there anyway to get in there and take care of that leak or do you have to pull the manifold off?

Thanks!

429CFE5D-93E6-4029-8144-565325045488.jpeg
 

cruiserdan

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That is a very tough place to fix. The only chance to get it is with the pipe on the bench. That material is very difficult to weld.

That is if it's a stock pipe.
 
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Figured as much! Thanks. I just got back from a 50 mile round-trip in 90° weather and it did not kick a check engine light so I’m just gonna keep watching things for now. The code is pending though.
 

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