Not essential - it forms part of the emission control system. The charcoal in the canister absorbs fumes from the fuel tank (instead of them being vented to atmosphere) and then, when the engine is up and running, the ECU opens a purge valve which draws the absorbed fumes out of the charcoal and into the air intake to be burnt in the engine. The charcoal never really 'wears out' and the canister is replaced as a preventative measure really. Never seen one where the contents can be changed though - they're normally sealed units!JeremyD said:Going through the service maintenance schedule in rave number 23 relates to replacing the charcoal canister, is this essential does everyone do it? Must I just change the contents whats the story here?