Trying to add a 'C' wire to my thermostat
When working in the blower compartment note that furnace power is shut off automatically by a safety switch. You can override the switch by pressing it in, and must do so in order to take meaningful voltage measurements with the compartment door open. When bypassing the safety switch take care so that if the blower motor were to turn on any long hair, clothing, or other items would not be sucked in and cause injury.
There's a legend in the schematic that says B/C is common (a box at the right-hand edge, middle section). The schematic also shows the 24 volt transformer TNS connects to the red and blue wires in that white 12-position connector (positions 7 and 11 according to the pinout diagram at lower-left).
The easy test is to measure voltage from R to B/C -- if it's around 28 then yes, B/C is probably your C terminal and you could use that spare yellow conductor to carry it up to the thermostat.
If you want to be a little more cautious in confirming the idea, measure AC volts between the R terminal and first the red, then the blue wire. You should find 28 volts from R to one of those and 0 volts from R to the other. The finding should be opposite when measuring B/C to the wires: 0 volts to one and 28 volts to the other. This confirms that B/C is common or &quot;C wire.&quot;