Set a digital multimeter to read DC volts in the range of millivolt or less than one volt
remove the thermopile wiring connector from the gas control
attach the red positive lead to the red thermopile wire
attach the black common lead to the white thermopile wire
set your gas control to the pilot lighting setting and light the pilot
when lit, the voltage will start to climb, continue to hold down the gas controls thermostat Knob until maximum voltage is reached, this may take up to three minutes.
this test can be run both with and without the thermal switch to determine problems with the thermal switch
a healthy thermopile in an open circuit test will read anywhere from 650 to 850 millivolts, replace if under 350 millivolts. Some gas valve can operate as low as 300 millivolts.
Other Things that could cause low voltage:
plugged pilot tube
the pilot is weak
improperly placement of the thermopile in the pilot flame
