HEPA Filter Replacement Instructions Once you have replaced your Dyson Pure Cool DP04 purifier HEPA filter, press and hold the Night mode button on the remote control for 5 seconds, the purifiers display will then count down from 5, once completed, the HEPA Filter Life settings have been reset and you are now able to operate the purifier again. Active Carbon Filter Replacement Instructions Once you have replaced your Dyson Pure Cool DP04 purifier Active Carbon filter, press and hold the Oscillation (< >) button on the remote control for 5 seconds, the purifiers display will then count down from 5, once completed, the Active Carbon Filter Life settings have been reset and you are now able to operate the purifier again.