Ahhh ok, quad driver 4 relates to the boost control solenoid, transmission ttc lockup solenoidor the evap purge solenoid. The majority of the time it appears its usually the bcs that goes bad. That write-up I reffered to gives detailed step by step instructions for diagnosing a code 1640. And just a word of warning, do no go with the Bosch o2 sensor. From what I've read on this site the bosch is too slow to respond and will give lazy readings. Denso seems to be the preferred choice of the board. Good luck!