1996 Ford F-150 check engine light.

I own a 1996 Ford F-150 truck, 5.0 litre, w/113k miles. The check engine light just came on recently and I took it to AutoZone and had them put a diagnostic analizer on it and it didnt reveal a specific code, but rather just said that it was a "lean mixture". He concluded it couldve been something as simple as a tank of bad gas in the past and the computer just kept that in its memory. So he reset it and the light went off. But a day later and its back on. The engine does not appear to be running rough or anything. Just wondering what you think this could be. Ive tried un-hooking the battery for a while and letting the computer reset itself, but that didnt work either. Thanks in advance.