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?)
You also have to consider how long the oven had to warm up (when the light goes off it may not be evenly warmed up, though this should be better in a fan oven) and whether you had to open the door an let cold air in to read the thermometer.