In an electric fan oven when cooking a specific recipe at a given temperature should that oven reach that temperature given the 20deg. difference between that and a conventional oven? I bought an oven thermometer placed it in the Fan oven and set the oven at 170deg C. the thermometer only reached 140deg. C. is my oven faulty?
Yes, it sounds like your oven is faulty. Or at least that the thermostat is wildly off. It's not uncommon for thermostats to be 10-20°C off, but 30°C seems like too much.
(On the other hand, how much do you trust your thermometer?)