Presumably the OAT sensor varies resistance with temperature. If the resistance normally decreases with increasing temperature, I'd be looking for a short between the sensor wires, or maybe a short between one wire and airframe. If the resistance normally increases with increasing temperature, look for a broken sensor wire, including a pin pushing back out of a connector so it doesn't make contact.
If you can measure the resistance at a couple of temperatures (cold water and warm 40C/100F water, Dynon should be able to tell you if the sensor is OK. You'll need to give the sensor part #, or at least the system it's working into.
Peter
