I have a Lennox Model #G12Q3-110-1 and the heat works but the a/c doesn't. Last winter nothing was coming on and I found that the transformer looked bad and burnt. I replace the transformer the heat worked but the a/c still doesn't. Any suggestions?

Do you mean the outdoor unit doesn't run? If so, start by checking the fuses and/or circuit breaker for the outdoor unit. There may be fuses in the outdoor disconnect, if present.

[QUOTE=KC13;2802136]Do you mean the outdoor unit doesn't run? If so, start by checking the fuses and/or circuit breaker for the outdoor unit. There may be fuses in the outdoor disconnect, if present.[/QUOTE

Neither the outside or inside unit comes on... the fan won't even come on when I switch the thermostat to "ON" from "AUTO". Any help?

Sounds like there is a short or shunt in the control circuit to the outdoor unit. The transformer you previously replaced may have been overloaded and failed when you switched on the a/c for the first time this season. Check the low-voltage wire to the outdoor unit for damage & check the contactor coil for a shunted or shorted condition.

Sounds like there is a short or shunt in the control circuit to the outdoor unit. The transformer you previously replaced may have been overloaded and failed when you switched on the a/c for the first time this season. Check the low-voltage wire to the outdoor unit for damage & check the contactor coil for a shunted or shorted condition.

Would that cause the indoor fan motor not to run when I switch the thermostat from "Auto" to "On"?

If the transformer failed again, the system would have no control power. Re-read my last post.