Honeywell F 300 electronic air purifier (by 3 users)

there's usually a troubleshooting guide stuck on the inside of the door.  it suggests a systematic flowchart for diagnosing the problem.
you should be able to isolate it to either the power supply (means replacement), or one of the cells.
on mine, one of the many little vanes got a bent during cleaning, and was shorting out.  (it doesn't take much.)  a quick visual and very little judiciously applied force straightened it out and fixed the problem.
i've also found that the unit will crackle a lot after a cleaning because of drops of water remaining in the cells, so i usually reassemble everything, but leave the unit turned off for an hour or two, which lets the blower on the furnace dry the cells for me.