My engine light keeps going on,airfilter and sensor filter were replaced,now

The place I go to said the lean code and they changed the air filter and cleaned the sensor and the light came back on they said it was a evac code? After reading the most common complaint about this car,I tried tightening the gas cap and the lights off again,for how long we will see,when I called to cancel the appountment for the smoke test I asked if a vacum leak could of been the gas cap and they said it sure could be,so why would they do a smoke test before trying the gas cap? $ maybe,I will see how long the engine light stays off,iam hoping that was the problem.Would a smoke test have detected the gas cap?