I also have a Pentair Minimax model HTR 400 NT TSI NH installed in spring 2005, ie less than 1 year old, so far it's failed 3 times with error message E04 on LED screen which may be same problem as JWB's.
Not happy with this unit, rest of Pentair kit works great.

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in 1 year with same E04 error message, so as well as phoning pool builder I emailed Pentair with my problem. They phoned me 5 days later and said E04 can be many things and very sorry for problem and can they send a specialist repair guy that they use here in Houston.