I have a 2007 Scion TC (Base). I started having misfires, first cylinder 4 then cylinder 2. In checking various posts I swapped coils and spark plugs, error codes didn't change. Then, based on other recommendation in this forum I had my valve cover gasket replaced. For a few days everything seemed normal then once again I got misfires on cylinder 4. I would reset the code but every couple of days it would again misfire. Last Saturday I replaced the spark plugs. Though they had only 50K miles they looked pretty bad. Put my car through a stress test, everything seemed fine. The following day on my to Church again, misfires (oil around the spark plug). Cleared the codes. Yesterday after coming home from errands, I checked my oil. It was a QT low so I added 1/2 QT. This morning on the way to the gym, again misfires the engine was cold but when it warmed up misfires stopped. What I don't know is what brand of gasket the mechanic installed nor whether or not he properly torqued the valve cover.

Any ideas would be greatly appreciated.

 

I had something similar before. I would swap the coil and the P0171 CEL would go to P0172. It ended up being the harnesses (the old sockets). They get brittle and can lead to faulty connections. It's worth a look and was only a $10 fix. I didn't solder new harnesses, I just de-pinned and swapped out the sockets.
Do them (the wires) one at a time so you don't mix the wires up.