1996 2WD 6cyl Taco - Exhaust Fumes in Cab

I'm getting exhaust fumes in my cab and I think I need an O2 sensor (ck engine light has been on for a few months). Could the bad sensor be causing this or is it likely I have an exhaust leak? I don't see any holes or cracks in the headers or exhaust pipes but it does seem a tad louder than it was a few months ago. Still not very loud but I hear a touch of engine noise that wasn't there a while ago.

So I'm wondering - O2 sensor, exhaust or maybe both?

Thanks!

 

I should have posted later because I just got the codes and I guess it's the EGR sensor and not the O2 sensor. Can that result in an exhaust smell? Also, part of it could be a gas smell, so I guess this could mean I need plugs and wires too.