it can also be caused by a o2 snsor issue....The exhaust crossover needs to be removed to do a oil pan...I would have your mechanic verify that all o2 sensors are plugged in and that he installed the air box assembly corectly
....I know you think you're asking a simple question, but "lean" and "rich" codes have the longest possible causes list of ANY of the fault codes. The conditions under which it is lean can be used to steer the diagnostic direction. When those codes set, there was also some "freeze frame" data that went with one of them. Now, maybe the tool being used won't show it to you, depending on the quality of the tool. But that FF data is helpful. A lean condition at idle only, and not higher RPMs is a possible indication of a vacuum leak. A lean condition at higher RPMs but not at idle is an indication of a fuel delivery problem like pump and filter. A lean condition at idle that suddenly swings to a rich condition at higher RPMs is an indication of a contaminated MAF sensor. However, it it extremely common for the plastic upper intake seals to leak vacuum. It is also very common for the IMRC shaft seals in the lower intake to leak vacuum. The upper seals aren't that big of a deal, but the IMRC shaft seals in the lower require a complete new lower intake manifold (the actual aluminum manifold) to correct. Ford also has a reprogramming (reflash) that helps. Often, a new set of upper seals plus the reflash will "hide" the IMRC shaft seal leakage. But eventually the lower manifold gets do worn out that the reflash won't hide it and you'll need a new lower intake manifold. That is just one common issue may not be your problem. You will need to take the vehicle to a shop that can monitor fuel trim PIDs on the scan tool. They can spray the intake seals and watch the reaction in the trim PIDs to determine any vacuum leak. Another common issue is when someone replaces the air filter, and the plastic PCV hose pulls out of the snorkel on the back side (nearest the master cylinder)... that causes a lean code at idle due to unmetered air entering the engine. I'd check that PCV hose first and if that is ok, take it to a shop. Also, the PCV hose that goes to the PCV valve (in front).... make sure oil hasn't gotten to the rubber part where it slides onto the intake, swollen it and causing it to leak vacuum. Those are a couple of DIY'r repairs I can think of... the rest really need a tech with proper test equipment to figure out.