I have a 95 GT and have been getting a check engine light intermitantly ever since I had a rebuilt engine installed. The codes are 172 and 176, (left bank lean), and (right bank lean). It occures randomly and will stay on at idle, but I have only ever seen it come on at power, never comes on at idle. It may come on for a minute or two and go off for a minute or two, totaly randome. The engine is a stock rebuild except for a Ford Motorsport E303 Cam (I think, whichever is the mildest of the 303 series). It has headers, H pipe exhaust, cold air intake, no cats, and the smog pump has been removed, all of this was done well before the engine change. So far, I have changed the O2 sensors, didn't fix. I checked the vacuume and it is 21psi at idle, no problems there. I cleaned the MAF sensor with CRC MAF sensor cleaner, no change. I do not have a fuel pressure guage, but I capped the vacuume on the fuel pressure regulator which should have raised the pressure by 5-8 psi, no change. When the engine was installed I put in a new fuel pump, new stock injectors, new fuel filter and new idle air controller. I pulled some of the plugs and they have a light grey color, which if i remember right is good. The car runs strong and has smooth idle, other than the light I can't tell anything is wrong. It does hesitate a little when pulling out, if you don't let it warm up in the morning, but it did that before the rebuild. I just don't want to damage the engine if it is running lean. I can't think of what else to check, at this point I am stumped. All suggestions welcome.