The code P0455 is a evap code for large leak. The evap monitor test is run by closing the vent valve and opening the purge valve under precise conditions. A vacuum of up to 25 millibar is pulled and then held. If the 25 millibar can't be reached in a certain time the code P0455 large leak is judged. Possible causes are: large leak and/or malfunctioning tank pressure sensor.

Using the combined codes (the P0170 is a mixture adaption code) suggest a hose being off the purge valve. It is located on the left side of the engine compartment next to the ABS/ASR hydraulic actuator. Right next to it are a couple of switchover valves that change the source to the MAP sensor. I would be sure all these lines are tight.

__________________
Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician