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My car has started throwing the p2a03 code.
I have checked for exhaust leaks but no visible ones. My upstream wires are intact as well.
I do get an occasional p0300 random miss code sometimes as well as a p0420(Catless)

The car is running good and getting 19.2mpg av29 mph

I did have TBs removed for cleaning and DID NOT replace gaskets, and used my hand to TQ down. The intake resonator tube was missing a hose clamp that I replaced yesterday.

any thoughts? could it be a vacuum leak?

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I'm going to double check and see if the exhaust manifold needs to be tightened. I'm in school, so it may take longer.

Edit Update: Had my dad stick a leaf blower in the tailpipe and I sprayed water+soap around the manifold. bubbles formed around the header to cat section. No gasket in either of the collectors! going to put some in tomorrow. Hopefully, this is the solution.