Code 21 O2 Sensor

I have a code 21 error. It could be either the upstream or downstream oxygen sensor. Is one more likely to fail than the other? Coincidentally this occurred right after Dodge replaced the head gasket. Is it possible oil (there was a lot of it) could have gotten into the upstream sensor? Local Dodge dealer says it is only a coincidence that it went bad around that time. Bottom line, the sensors are pricey, but it is probably cheaper to replace both rather than have them replaced by Dodge. Which one is more likely to fail? I would try that one first. Thanks!

from what i've heard its usually the upstream sensor (the one close to the exhaust manifold). should be around 40-50$ for a bousche replacement and fairly easy to replace. the only problem I had was getting a wrench around the damn thing-its in an awkward spot

I had the same dilemma, I took a gamble on the upstream, and, jackpot, I guessed right. Go for the upstream...