If it never gets above 70, your main thermostat is indeed goosed. The sensor could also do with replacing as they've revised it a few times over the years. Although the sensor costs almost as much, if not more than the thermostat.....but whilst it's in bits and all that!
If the fans come on after start-up, check you haven't got climate control on 'Auto'. Switch it to 'Eco' and they should turn off.