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This is obviously an EVAP system problem. I have checked the system for leaks. All seems good, fuel cap seems okay. Checked for vacuum at the canister purge valve with engine running. No vacuum through the valve. Unplugged the purge valve input connector at the harness. No voltage between the two sockets. No battery voltage on either socket and no ground on either socket. I am not sure where to go from here. Is a "down stream sensor" telling it not to open up? If so which sensor and were would it be located on my car? Could there be a bad connection or broken wire? If so, where do I start looking?

Vehicle: 2008 Honda, Civic sedan, 1.8 L, 4 cylinder

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P0455 Evaporative emission (EVAP)Large leak detected

P0497 Evaporative emission (EVAP) Low purge flow deteted

The purge valve should have battery voltage at one side, and when it is commanded on there is a ground command from the PCM.When the EVAP system see a low purge flow it considers this a EVAP large leak.Both these code are not relating to a open or a shorted circuit with the sensor, if there was  a power problem you would not have these code, it did a evap inspection and failed the system, For now check the fuse number 3 , There is a TSB about P0456 , it may relate to the condition that you have encountered, whenever P0455 occurs P0456 should be present TSB # 08-013 this is about the purge valve not sealing properly if it does not seal proerly not only do you get small leak you alos get no purge or low purge flow.

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by (13.4k points)

P0455 Evaporative emission (EVAP)Large leak detected

P0497 Evaporative emission (EVAP) Low purge flow deteted

The purge valve should have battery voltage at one side, and when it is commanded on there is a ground command from the PCM.When the EVAP system see a low purge flow it considers this a EVAP large leak.Both these code are not relating to a open or a shorted circuit with the sensor, if there was  a power problem you would not have these code, it did a evap inspection and failed the system, For now check the fuse number 3 , There is a TSB about P0456 , it may relate to the condition that you have encountered, whenever P0455 occurs P0456 should be present TSB # 08-013 this is about the purge valve not sealing properly if it does not seal proerly not only do you get small leak you alos get no purge or low purge flow.

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