New World DW60 dishwasher knowledge?

I have a 5 year old New World dishwasher that has only been used used once weekly on average. The pump seems to be working but the detergent tablet does not dissolve and the dishes don't get clean although there seems to be steam present when door is opened. It is giving an error message E3 which is described in the manual as a temperature sensor fault.
I would like to know if this means the thermostat and where it is situated in the machine. Also could I attempt the repair myself as the parts cost about £10. I am reluctant to pay for a repair man because of its age (other parts may fail) and it would cost nearly as much as a new machine. Any advice would be greatly appreciated.