error code F 04 on a ecomax 824 E boiler?

F04 is an NTC fault (could be the wiring to it), but most likely the NTC itself, specifically NTC2.
The NTCs 1&2 are located top left of the main heat exchanger, NTC2 being the one on the right.

If you're getting someone in or trying to buy the NTC (and for future reference), you should mention that your Ecomax is an 824/1e, not to be confused with the more common and later 824/2e.
The two boilers have very different NTCs and you could easily get the wrong one as they are not interchangable.
You could also identify it as 'bevel fronted' Ecomax, this would also help engineers and parts suppliers in fault finding and future parts replacement.

Carefree
January 2011