obsessed212

First off. my car is a stock 2006 ss. has 65k miles.

When i bought the car a few months ago it had the p171 lean code. i took it to the local garage and they couldnt find any leaks in vaccum lines. the gave me the car back after cleaning the maf and told me to see if that fixed it. drove it 40 miles and the cel came back on. i took the car up to the shop the next morning and they kept it all day. I go to pick it up and they say the dipstick tube came out of the engine, that caused unregistered air to come into the engine causeing the lean code. They tightened it up and its never gotten loose again. I dont believe that happened, i think they ripped me off

secondly, a few weeks after i get a cel again. take it to advance and they say p135, i checked and i had all symptoms of a bad o2 so i replaced it with a bosh oem fit sensor. 2 more weeks go by and its running fine, i run low on gas (i run 93 octane) and i put some gas i had sitting in a can at home (87 octane) i drove 1 mile and my cel came back on. the o2 had went out again same p135 code. i replaced it. Had tons of carbon on it. I waited a few days and its been fine. Seafomed it and not alot came out but smoked for a lil bit. Its about time for my 2 weeks to pass. i can hear a small tick in my exhaust. could my cat be stopped up causeing my car to have o2 problems? the local dealer told me to remove my cat, see if my performance changed for the better and if so they would replace it.

so....
if my problem is a faulty o2, what caused it? carbon? what caused the carbon? help!