The PNP switch is like the "old" neutral safety switch. The PNP lets the Transmission computer know what gear you are in, when the switch fails, the information sent to the computer doesn't correspond, and the check engine light could be turned on depending on the codes being set. Often the up arrow on the dash also flashes, and the car is stuck in "limp home mode" which is a fancy way of saying you will start off in 3rd gear. Sometimes if you ratchet the shifter from Park to L 15-20 times rather rapidly you can clean up the switches internals enough to keep the switch from setting a 313 code (PNP switch). The switch is on top of the transmission, just to the left(cars left) and aft of the transmission dip stick tube. Look where the red, positive lead for the battery goes down and aft and you will be in the area. In the next couple of days there will be a post at Volvospeed.com on how to replace the PNP switch. Try the ratcheting and then use the directions at Volvospeed.com to read and clear the codes and see if it works, all free it and could save you the time and expense of a replacement.