I have a lennox G26Q-50 unit. The question is on the AC side and the blower not operating and tripping the low voltage breaker on the unit.

1. the blower works with interior fan switch on.
2. furnace works and interior fan works.
3. Turn the thermostate to Cool and the fan does not work/trips low voltage breaker.
4. Turn the thermostate to Cool and fan setting to On and/ trips low voltage breaker.

Is the problem in the circuit board / transformer?

Possible a shorted contactor outside in the condenser unit,, I`m not a Lennox man but this can happen in all brands