Missing O2 sensor?

[benntn]

Should my 84 GMC with a 305 4 bbl have an O2 sensor? I can't find where one was if it did. I've ran my hand all along the exhaust from the manifolds to the cat and there is no hole or O2 sensor. My truck has failed emissions test and I'm been trying to figure out why. I've only had it over a year and it past when I first bought it. I've not made any changes either. I've actually not driving it very much. The exhaust is stock and has no holes. Its a little lose aft the muffler but not enough to leak. The second time I had it tested, they said it wouldn't even read??? The first test the hydrocarbons where a little high so I started studying it and I changed the EGR value and replaced a few vacuum hoses before my second test. So after it failed again, I thought i would check the O2 senor next but to my surprise there isn't one. The cat is there but what the heck no O2? Anyway the exhaust appears to be all stock and doesn't look like anyone has ever messed with it expect from the cat and the muffle on back. Any ideas? Thanks.