I have a coherent model 210 head and meter that is no-workey. I disassembled the head to check for loose wires etc. Everything looks intact. The thermopile looks good with no burns or discolorations.

I tested the head on the mV setting of my scope with a ~4 watts in. No voltage generated over the noise.

What could cause a failure of the thermopile.

Worth trying to replace?