The P0455 code is for a major leak. Does it interest you that you never have a minor leak?

In testing a fault code one must understand what is being faulted. The system does an evap test usually once a drive cycle (when conditions are right... tank between 1/4 - 3/4, below some altitude, rough road condition gone, etc.). To test the system the ME controller closes the vent valve, preventing the system from breathing through the charcoal cannister, thus sealing the system. It then pulses the purge valve allowing vacuum to pass into the tank where the tank pressure sensor (part of one of the tank senders) reads the value. The purge valve is pulsed till a pressure of about 25mb is arrived at or a time limit is reached. At the moment the pressure is reached the purge valve is closed and the system monitors how much vacuum is lost in 60 secs. It can't loose more than 5mb in 60 seconds.

So, if the pressure isn't reached by the time limit, the large leak code is registered. If the first step is completed and the leak down is greater than 5mb/min then a small leak code is registered.

As it turns out, a large leak is almost never a leak. The most common problem (after missing gas caps) is a purge valve that doesn't operate. This is because the first criteria of reaching a certain vacuum level can't occur if the purge valve doesn't pass the vacuum on. Other possibilities are a vent valve that is totally inoperative or a tank pressure sensor that doesn't register a change (a BS reading will give a circuit code).

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