Troubleshooting millivolt gas furnace help request.

mattmia2

Since you didn't show your work for the t-stat cable, remember there are 2 wires in the cable so the length of the wire is twice the length of the cable. A dc milliammeter in series is the way to go to measure the current, a clamp on meter is going to be wildly inaccurate at the tiny current you're dealing with.

109A_5

DC is like AC, that you only use one wire. When you get into very low currents the clamp-on method may get erratic. Try it, it won't hurt anything, then you will know. With DC it should read negative it you have it clamped over the wire the wrong way, but it really does not matter much in this case.

I would imagine the current through the thermostat circuit is probably very low, if I had to guess it is under 50 mA, it may be much lower like under 10mA. How much power is in a pilot flame ?

The meter's resolution looks reasonable for the price range. The range value numbers listed are probably the maximum value measured in that range.

With the 400mV range, +/- 1% rdg +/- 2, 1% of 0.4 VDC (400mV) is 0.004 (or 4 mV), +/- 4 mV added to +/- 2 display counts, so the way I see it, your measurement it could be off by 6mV in either direction from the actual voltage.