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Discussion Starter #1 (Edited)
I just bought my 05 Cobalt from a friend a few weeks ago and I actually knew it had this problem when I got it, but that didn't stop me from buying it.

It's pretty loud when driving presumably due to this leak. I can also smell the exhaust inside the car sometimes. When I put my hand where the exhaust manifold connects to the downpipe, I can feel exhaust coming out when it's running. I took the heat shield off to inspect the manifold and I couldn't find any cracks in it so I don't believe it's the manifold itself.

My first question is would it be easy to replace the gasket where the manifold connects to the downpipe? I found this on Amazon, pretty sure this is the gasket for it: https://www.amazon.com/Walker-31643-Exhaust-Gasket/dp/B001AGJYGY/ref=cm_cd_al_qh_dp_t

It looks like it would just require removing the three bolts and replacing the gasket but I'm new to all this so I just want to make sure I can do this before taking it apart and then finding out there's something I missed.

My second question is would a leak like this cause O2 sensor codes? I don't have the exact codes on hand right now, but I believe it has an O2 sensor code. At first I was thinking about getting the sensor(s) replaced but I was reading and it looks like this could be caused by the leak and not by the sensors themselves.

And one more question... I keep seeing things about torque specs for bolts. How do I ensure I tighten a bolt to the correct torque? I have basic tools like sockets and stuff but I don't know how I would tell what the torque the bolt is. Sorry about the newb question haha.

-Nathan

Edit: I just found out about the bolt torque spec stuff -- have to get a torque wrench! (youtube.com/watch?v=5XzKvEnk1FY)
 

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Sometimes I use a sheet of paper to see if there is a sizeable exhaust leak coming out, since it should make the paper flap.

Yes, that is the correct downpipe to manifold gasket.

A leak shouldn't cause any kind of O2 sensor code unless it's absolutely massive. Worth double checking what the actual code is and posting it here before doing anything.

You'll need a torque wrench to ensure bolts are torqued to spec. Just make sure to get one that isn't a piece of shit (look at reviews).
 

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Discussion Starter #3
Sometimes I use a sheet of paper to see if there is a sizeable exhaust leak coming out, since it should make the paper flap.

Yes, that is the correct downpipe to manifold gasket.

A leak shouldn't cause any kind of O2 sensor code unless it's absolutely massive. Worth double checking what the actual code is and posting it here before doing anything.

You'll need a torque wrench to ensure bolts are torqued to spec. Just make sure to get one that isn't a piece of shit (look at reviews).

Thank you! I will get a torque wrench and that gasket and do it then. Do you know where I would find out the torque spec for that part of the exhaust?

And okay, I thought (and was hoping) it could be caused by the exhaust leak. There is a good amount of air coming out of the leak, but it isn't absolutely massive. I will find out what the code is and post it here.
 

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A lot of times that flange on the downpipe rots off and won't allow you to bolt it back up. That's why I left mine alone.
 

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A lot of times that flange on the downpipe rots off and won't allow you to bolt it back up. That's why I left mine alone.
Perfect time to upgrade the downpipe!

That said - mine never rusted off prior to removing it.
 

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yeah you might want to check the actual flexpipe for damage as well, the original accordion style likes to rot out and might not be noticeable at first. those are fairly cheap and easy to replace but if your going to go that route you might as well get a new downpipe so if you want to upgrade later part of the job is already done, ZZP makes a good replacement flexpipe and great exhaust kits.
 

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Discussion Starter #7
yeah you might want to check the actual flexpipe for damage as well, the original accordion style likes to rot out and might not be noticeable at first. those are fairly cheap and easy to replace but if your going to go that route you might as well get a new downpipe so if you want to upgrade later part of the job is already done, ZZP makes a good replacement flexpipe and great exhaust kits.
Would replacing the downpipe be super expensive? Because I don't have enough money for big changes at the moment, unfortunately. That would be a nice upgrade eventually though.

If the flex pipe is broken, I'll just get the flex pipe replacement and have a muffler shop weld it on for me, then I won't have to bother with replacing the gasket myself (getting the rusted bolts out would probably be the hardest part).
 

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If your car is a manual, you can run an LSJ (SS/SC) downpipe, which is a little bit of an increase - but at a budget-friendly price, since they're widely available used.
 

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as long as you use a little heat and maybe some PB blaster you should be fine with the exhaust bolts, mine were pretty bad and they came loose just fine. like MP81 said its not hard to find one used, you should be ok.
 

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Discussion Starter #10 (Edited)
A leak shouldn't cause any kind of O2 sensor code unless it's absolutely massive. Worth double checking what the actual code is and posting it here before doing anything.
Hey, I found out the error codes:

P0135 - O2 Sensor Heater Circuit

P0449 - Evaporative Emission System Vent Valve/Solenoid Circuit

(Freeze Frame 2AA1)

http://imgur.com/mgA1wuI

So I guess this means the O2 sensor probably needs to be replaced? And I wonder what is causing the other code?

I haven't fixed the leak yet, but I checked again and there is quite a lot of air coming out of it, all around the flange connection (doesn't seem to be the flexpipe). It's as if there isn't a gasket there. I need to fix that.
 

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Yep - just a bad O2 heater, so new sensor will be required.
 

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The heater is in the sensor, not a separate component.
 

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The sensor still works but doesn't kick in as quickly as it should. Will work fine when warmed up. Heaters in my experience have been the main fault of 4 wire 02 sensors.
 

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Yup, you'll just get a code.
 
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