Hitachi wd1300b fault code f6

We have the above model which i understand is a rebadged servis quartz based machine, we stopped using it as the main machine as it kept shutting down with f4 faults when loaded with anything heavier than a set of clothes, however the dryer still worked ok so it was plumbed in the garage as a backup. i decided to put it on a wash cycle with a couple of small towels in and it filled started to tumble and then shut down f6 (heater fault), cancelled program and reset to rinse and fast spin, the machine filled and then f6 again, several resets same ending , some occasionally with motor action some not, can anyone possible shed any light on where to look for the fault, where it might be etc and the servis model it is actually based on for spares etc, i dont particularly want to spend a great deal on it (16.5 years old) and motor getting noisy again (damn thing seems to eat them), all suggestions and help greatfully received

Not sure hw to edit but here goes!!, i checked the door interlock and the microswitch was burnt, cleaned and now ok, checked all heaters and stats for continuity all seem ok, checked pressure switch bottle and cleaned, i put the machine on a rinse/spin cycle it filled and shut down f6 as before, machine will now not start up, all buttons work except start, anyone have any other ideas or is it time to go to the dump and launch it, many thanks.