The only way to do it is get the factory service manual and follow the flow charts to diagnose the problem. I would guess the problem with the O2 sensor is causing the lean code. The O2 is critical to the fuel mixture and without it working right you'll have problems. If you are a little bit mechanically inclined the manuals will quickly pay for themselves. If not, then find a good mechanic other than the dealer. They will probably still need the manuals unless they know the supra inside out. Good luck.