2000 Ford Windstar

I just bought a 2000 Ford Windstar and drove it 2 days and it drove great but then it stalled while idling.I took it to my mechanic and had it scanned,it came up as fuel is too lean.I changed fuel filter and air filter and it still has the same codes coming up.Do you have any thoughts on what it might be? The help sure would be appreciated. Thank you.

An engine will be lean (air/fuel mix) when extra UNMEASURED air gets into the engine intake. This is usually called a vacuum leak. Vacuum leaks are what your mechanic should be looking for.
On the other hand, the problem can be too little fuel being injected into the engine. Too little fuel can be from low fuel flow/pressure, or the engine computer commanding too little fuel for an amount of air.