Help running a C wire from my furnace--no voltage from the C terminal on the circuit board
The furnace control board is in the blower compartment. Nearly all furnaces have a safety switch which cuts power to the furnace and its controls when the blower compartment is opened. You'll have to bypass that safety switch while measuring AC volts between R and C. (While doing this, take the usual precautions to ensure that long hair, clothing, etc would not be pulled into the blower if it should come on unexpectedly.)
For mains-voltage AC it would be a clear violation of electrical code to run one extra conductor separate from the cable. Not so for low voltage like a thermostat, but still it has the potential to be confusing to others later. If it's possible to run one conductor from the furnace to the thermostat, I suggest to run a new 7-conductor cable instead. The work is the same but the result is much more professional.