Hi,

I have a millivolt Honeywell gas control system for my fireplace. The pilot is on.
The thermopile voltage on the terminals is 663 mV. I checked the burner orifice and it is OK. When I command the mainburner to come on, nothing happens.
Would please advise how to proceed.

Regards,

Some fireplaced also have a thermocouple besides having a thermopile. If so it is possible the thermocouple is defective.

Some fireplaced also have a thermocouple besides having a thermopile. If so it is possible the thermocouple is defective.

I have replaced the thermocouple and cleaned up the thermopile. I am now reading 773 mv.
The main flame is still not coming on!
Valve info: Honeywell millivolt Gas valve; VS8420E or 82L8601

Check to see if that milivoltage is getting to the gas valve. If it is and the valve is not opening then I would suspect the valve. This might not be your fireplace setup exactly but the two items below might help.