As a tech we never have to or check that voltage. You need to check the DC micro amps. The flame sensor needs to be perfectly clean and if you got oil or anything on it from your fingers then clean it with clean steel wool or #800 emory paper. You also need to clean the face of the burner it sits in front of down to bare metal or the metal in front of the sensor which ever is closest.

It works with DC flame rectification and current passes thru the flame and converts to DC volts / current and then finds it way back to the board thru grounding. Make sure the board is properly grounded also. You can check from C on the terminal board to bare metal and should have 0 or low ohms. Make sure the flame sensor wire is not burnt/bare or damaged and shorting to ground.

 

Thank you for the response. My understanding is that DC flame rectification relies on the flame sensor line starting with ~90v AC from the controller board, which allows for a certain level rectification when flame hits the rod. So if the flame sensor line doesn't start with the appropriate AC voltage, you won't achieve the expected rectification, even if the flame sensor is working. As I've tried two new and clean flame sensors now, I don't believe the flame sensor is the culprit.

Assuming the flame sensor is working, and we're still not seeing DC flame rectification, any suggestions on what the next thing to troubleshoot would be?

BTW, I did check the board for grounding per your suggestion. It showed very low ohms, around 1 or 2.

 

If the flame is hitting it correctly it should produce a minimum amount of current and that is where a micro amp reading is necessary. Usually when it is below .5 ma then it runs for 15 seconds and lockouts on low flame signal error code. If you are getting no ma then I think the board is shot or your flame sensor wire is shorting to ground. Never had that happen but I have seen some black and burnt and bad looking wires. If the flame sensor wire goes into a molex connector pull it and make sure all the pins are forward and seating properly. You may be able to buy a basic Klein meter at HDepot for under $100 that does micro amps.

Post a pic of the sensor and burner firing or video here.

 

I got the Klein meter to test the micro amps. The reading showed a range of .1 to .5 micro amps. When measuring the voltage on this line, it showed ~4v (similar to my previous measurement).

I also checked the flame sensor line to the molex connector. I pulled out the molex connector to confirm the pins look OK. I also tested the continuity of the flame sensor line from the molex connector to the flame sensor. Continuity tested OK, and doesn't look like it's shorting to ground.

 

Try cleaning the face of the burner - a small wire brush may allow you to do it without pulling the burners.

 

Try clean the face of the burner in front of the sensor BUT it needs to be clean down to bare metal so I would remove it, OR if the distance to it is greater than that of the furnace wall metal in front of it clean that metal. My Sister had a York and that distance was real close and when I cleaned it I got a better reading. I would clean both. Rust and corrosion work like insulation and even though it may not look bad it is. .5 ma is usually minimum and .1 definitely is too low. Some of those units are pretty finicky.

 

I was able to finally get it working by replacing the controller board. My original controller board was only showing 4V on the Flame Probe line. The new controller board is showing at least 40V, and flame rectification is working as expected.