Check Engine Light Issue

I own a 2004 Mach 1 that has a check engine light on constantly. I had Ford replace the computer module in it that was reading a Mazda 1.3L turbo diesel. Ford stated that it had failing o2 sensors, so I replaced all 4 with new Bosche (sp?) o2 sensors. It is still throwing a code so I scanned it and so far I have one code being thrown but it had two different readings. One said o2 sensor slow response driver's side, and the other said System Running Lean (Left Bank). Any ideas on what might be throwing codes?

I purchased it mid-October of this past year. I am still trying to get real plates for the car because of this check engine light. I am the third owner, and, obviously, one of the owners tried to modify the car. The only mods I could tell it had when I got it was BBK shorty headers, straight pipes with a BBK x-pipe, subframe connectors, and after a trip to Ford, a PCM reading a 1.3L Mazda. The only modification I made was getting rid of the old exhaust and putting on a Bassani catback x-pipe exhaust system. Now, I'm just trying to figure out exactly what is causing the check engine light to come on.

Update

I don't remember the numbers. The full descriptions I recieved were System Running Lean (Bank 1 Sensor 1), and Slow Response o2 Sensor. I originally took the car to Planet Ford, but I suspect, with as much time of mine that they wasted, that the technician(s) working on my car had no knowledge of what the hell he was doing. So I took it down to a local performance shop that I trust, and had them do a full inspection. They came up with similar codes and the main technician feels that the o2 sensors are fine, that he thinks maybe its receiving an incorrect reading from the computer. The car was bought with BBK long tube headers, hi-flow cats, BBK X-pipe mid-piece with I guess straight pipes. He said usually cars that they install long tubes on, that if an o2 sensor fouls or acts up that its a rear and not a front sensor. The car is running rich too. Not sure whats up yet, if it's tuning or something needing to be replaced. He took it to a Houston Performance shop and is having people he does business with and that dyno tune cars for them look the car over and see if they can provide some insight. I'll keep you updated.