Probably an injector solenoid gone bad, or, possibly the connection thru the VC gasket. Either way you need to pull the valve cover to check.
Disconnect each injector in the bank effected (should be 4, 5, and 6) and ohm them out. Should be less than 1 ohm and greater than 0 resistance, look for the odd ball.
Disconnect injector harness outside the VC and ohm out each wire to check for continuity and resistance. Should be less than 1 ohm and greater than 0 resistance.