Monessen Millivolt Gas Valve Troubleshooting

Tis the season. I think I've had a service call on every millivolt system in my rentals.
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I am debugging a Monessen millivolt gas valve. The pilot works fine but the main burner does not come on by the wall mounted thermostat or the manual on/off switch. Jumping between the TP/TH and TH does NOT turn the main burner on. Below are my tests:

The thermopile is generating 480 mv when heated by the pilot, measured at the terminals on the gas valve.
When I hold the thermopile over the flame of the kitchen stove it generates 735 mv.
When I turn on the burner switch the voltage drops to 191 mv, measured at the gas valve.
Resistance from the Electro Magnet Unit (EMU) connection point to TP is 3.4 mΩ.
Resistance from the EMU to TP/TH terminal is 3.0 mΩ
If I tap the gas valve smartly with the plastic handle of a screwdriver the main burner will ignite and stay lit for at least a few minutes.

Is there any other test I should perform or is it time for a new gas valve?

 

Thermopile.... 480mv at idle
191 mv with burner engaged.
Is that 191mv measured at the same location you measured 480mv.

That's low.... a typical drop with burner on is usually +/-50mv.
Decent pilot flame ?

From Monessen....
If the meter reads greater than 175 mV and the burner does not light, replace defective valve

Make sure you have the voltage on Tp and Th (actual valve coil)
I'd consider 175 mv to be a little low.

 

Thank you, I should have done the obvious and go to their website. I guess I'm mentally pushing back the chance of a bad gas valve because I replaced this one last November. But, even relatively new parts can fail. It's never a good sign if percussive therapy helps.

Yes, both thermopile voltages were taken at the same place. The gas valve has both screw and spade terminals. The thermopile is connected to the screws and I tested on the spades and screw heads.