Closed circuit testing is the preferred method for testing the thermocouple. Following the lighting instruction label on the heater, proceed to light the pilot and allow to operate for three minutes. If the pilot will not stay lit, hold the pilot button (located on the combination thermostat/gas valve) down during this test.
Using a multimeter capable of measuring millivolts, connect one lead using an alligator clip to the copper sheath of the thermocouple. Use the second lead of the multimeter to probe the top terminal located at the back of the combination thermostat/gas valve.
If meter reads 10 millivolts or higher, the thermocouple is O.K.. If reading is below 10 millivolts, replace the thermocouple.