If you look where the flame comes from, you will notice the spark probe and another probe. Both are in the flame path. Press in on the start button while the gas is off and watch which probe sparks. The other one is the thermocouple. It will secure into to a small brass fitting under the heater frame. This is very easy to remove and replace. There may be a friction clip holding the thermocouple in at the flame end but screwed at the piping end. You can find a new thermocouple at better hardware or farm stores. Bring it with you to size it. It's OK to have one slightly longer but you will have to tuck the extra length into a very small space.