Here's a better link to the service manual so you can
see the schematic and troubleshooting flowchart:
http://dms.hvacpartners.com/docs/100...00/SP04-62.pdf

good..

see Fig 12.
Note #5 in the lower right corner.

This wire must be connected to furnace sheetmetal for the flamesensor to work.

THe note is for the return wire of the gas valve.

I'm not sure what they mean exaclty, but something there needs to get to furnace sheetmetal ground for the flame senor to work.

Mark