87 E 150 fault codes # 12 & 41

#12: Cannot control rpm during self test high rpm check - idle too low. #41: EGO (oxygen sensor) indicates system lean (right side)

You did not specify the engine or injection system, so it's hard to give specific advice how to fix those. For the first one you probably need to check your idle speed controller. If that's a multiport injection system, there will be an idle air bypass valve somewhere. It may need to be cleaned or replaced. Or perhaps it's as simple as a bad electrical connection. If the engine appears to idle properly, I would not spend too much effort on this one. There are many reasons for a lean indication. Vacuum leak would be one. Bad fuel pressure regulator could be another. A defective oxygen sensor is possible, but not likely, so don't rush to replace it. If you have a DVM, you can check the sensor by forcing a rich condition and confirming that the output climbs to around 1V. The easiest way to force a rich condition is to disconnect the ACT (air charge temperature sensor) -- this fools the computer to think that the engine is breathing arctic air. You will find the ACT threaded into the intake manifold. Don't confuse it with the ECT (coolant temperature sensor), though they look identical.