I have a 94 Cobra stock except for Mac H-Pipe with high flow cats. My check engine light began to come on and off about a week ago. I checked for codes and I got back trouble codes 136,172,176,181 all of which indicate a lean condition in both oxygen sensors. I changed both oxygen sensors, reset the computer including the Keep Alive Memory and now I only get codes 172 and 176 which indicate lean condition in right oxygen sensor. The car stills runs rough(backfires, and the tach stutters instead of sweeping when rpm is increased) at times, and the check engine light still comes on. I called my Ford Dealer and they said I might have a vacuum leak. I changed my vacuum hoses and the check engine light still comes on sometimes. I checked for codes again and 172 and 176 still come on. I also changed the Cap and Rotor, which helped a bit becasue my old ones were worn. Also I changed the fuel filter two weeks ago and added fuel injector cleaner six days ago. My question is what can be the cause of my lean condition and poor performance at times? I put the oxygen sensors in four days ago, how long is the break in time for the sensors? Any help is greatly appreciated thanks? 
