I just got done replacing my head gasket, timing belt, waterpump and I put on a 2.5" catback exhaust. My car is running too lean now. I took it to autozone and there were 2 codes that it is running too lean and my AF is also displaying lean. After the car warms up, it begins to idle too low and almost stalls. Before i did the engine work it was not running lean. Any ideas on what this could be? I was thinking it could be a 02 sensor but im not sure which one it would be. Any help is appreciated.

upstream 02 sensor. i dont think the downstream one does anything other than emmisions crap. let someone else who knows more answer this though, lol.

if the front o2 goes bad, the car runs rich.

its not running rich tough, its running wayyy to lean.

Sounds like your cam timing is off...check your timing marks again..could be your o2 also, but check both

I just realized my egr is loose on the half that connects to the TB. Its blowing out alot of air. Could that have anything to do with it?

The gas in my tank is about 2-2.5 months old. Would that make it run lean? Also how do I check if its a bad 02 sensor?

From what you've listed it could be the following. The EGR pipe being disconnected. Cam timing could be off. O2 ... maybe but the symptoms don't sound right for that. Another sensor that plays with the fuel possibly.

Anyway, you probably want to check the o2 first. It's the easiest. Find someone with a working o2 and plug it in. Only way to know for sure. Some cars just won't throw and o2 code (mine didn't).

2nd. Check the cam timing. It sounds like it could be off but it's a pain to check the timing marks since you can't see the crank sprocket timing mark with out removing the crank pulley.

Other stuff. Get a logger and check to see what the other sensors are displaying. Make sure they are in the proper ranges.

How exact do the camshafts marks have to be aligned? When I installed it they looked good. So you dont think it would have anything to do with old gas or an old fuel filter?