Engine Warning Light on Toyota Corolla 10/07 ZRE152R

Question

I have had the Engine Malfunction light come on about 2 weeks ago and Code 59 Running too lean comes on when its checked. The car is not running lean in fact its running rich using more fuel than normal, I have an exhaust smell and black soot around my exhaust. I have taken it to my mechanic who advised I run premimum Unleaded fuel through as it might be bad fuel. That didnt work. I took the car to Toyota who said the air flow sensor was dirty, cleaned air flow sensor that didn't work. They checked for any vacumn leaks, none present. They removed the fuel filter and looked ok. They then told me it was a blocked Cat Convertor. I took it to get the cat convertor repaired. They inspected the Cat Convertor and said it was not blocked. Toyota say if its a faulty sensor their machines will tell them that. So what do I do next, how do I find out what's wrong with my car? My question is, can it be a faulty or dirty sensor and it not come up on the machines? If my Cat covertor is not blocked could there still be an issue with it that is causing this light to come on. Where to start, if I'm going to start spending money on my car, what do I replace first?? My car has 130000 KLM on it, and has always been serviced. In the last service, my mechanic forgot to replace the oil engine cap. I returned the car to the mechanic several weeks latter when I discovered it, he cleaned the vechine topped up the oil. The engine light came on a few days latter, could this have anything to do with it??