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?
So now you have the oven thermostat saying one thing and a thermometer saying something else. As other answers have said, that could be because you didn't wait long enough for the oven to heat evenly, or that the thermometer is not in a good location. If neither of those applies, the only conclusion you can draw is that one of them is wrong. In order to conclude that the oven is off, you need at least two thermometers. Of course, if you do that, you may find that all three disagree.