My 10 years old trane gas furnace would not blow out warm air, only cold air. I checked gas is on and electric switch is on. Every thing seems normal. It may be the ignition problem. How can I find out?
Your help is greatly appreciated

Bass

It could be many things. Is the thermostat turned on to heat? Is the temperature setting high enough to have the heat come on?

Check sequence of operations in the sticky at https://www.askmehelpdesk.com/heating-air-conditioning/heating-cooling-maintenance-repair-troubleshooting-frequently-asked-questions-58313.html

Usually the controls wouldn't let the blower come on unless you had flame. Could be a bad high limit switch or temperature sensor.

Usually. When the fan runs non stop. On a call for heat you have an open limit in the circuit somewhere.