I am a Mechanic from Ont.canada. I am working on a 2004 Chev Epica with aprox. 99 000kms with the complaint of "lack of power" Aprevious attempt to repair the issue was preformed by doing a tune up. The plugs (NGK), coil boots, fuel and air filters, bank2 sen1 02 sensor were replaced. Launch scanner revield missfires on #'s 3, 4,& 5 cyls and a PO158 voltage high on bank 1 sen2 O2. if this code is set the missfire disappears. if the codes are cleared, the missfire redevelops untill the code sets again...??????? What is the probable cause