The 24v starts from R terminal on board
Goes to R terminal on thermostat
The thermostat acts as a traffic cop...sending back on whatever wire for a given function.
In this case it sends back on green for fan...and Y for cooling.
We know it is sending back on G as the blower starts
We know we have bypassed the thermostat by twisting r,g,y, together which eliminates stat as suspect.
Our only loose end is Y signal between thermostat and circuit board.
Now to narrow down further..
furnace door switch taped down.
Thermostat r,g,y, twisted together
with meter at furnace....measure c to r......c to g.....c to y.
Tell me those readings and we have a final area to deal with