Shortly after getting an oil change and replacing the air filter I got a check engine light. Went to a shop and diagnostic machine says one of the o2 sensors is bad but the codes will not tell which one. Some $300 for both from the shop. Under $100 online(non oem). Are these easy to get to, any idea if it would be the top one since the air filter was changed then this problem arose? I figure just try to clean it off before buying a new one.