The gas furnace: Lennox G24M 3/4-100A-7
SN:6398B17474

In T-start, If heat and cool off, fan in on position, furnace indoor blower will run; Also try heat with fan on, blower also run;

however switch T-start to cool, regardless fan in on or auto, when outside condenser is started, the furnace blower will be immediately stopped.

ACBCool-NEUTRALS(any pin) = 0 VAC
Y-C = 24 VAC
R-C =24 VAC

It looks K3 relay in circuit board is dead.

From the diagram, K3 also controls ACBHeat; In my case, when fan is on, heat mode, the blower is started. I don't test fan in auto when heat mode case my house is hot.

Is it possible that my heat is also broken in auto fan mode?

Can I change the K3 relay only in circuit board?

Thanks

Usually the entire circuit board is replaced. I am not saying it cannot be done IF you can find a compatible relay but HVAC service persons just replace the board due to liability issues. I am sure there is a relay available that might work even if you have to use a remote add a relay type device and solder the wire extensions to the board. Been there done that but never on a customers equipment.

Thanks, sounds reasonable. I will see how I can proceed after my testing on heat/fan-auto is done.