Hi My XV 90 Trane Furnace is only blowing cold air. There is a red light flashing on the unit. Any ideas?

Below is the heating sequence of you unit for the heating cycle. If you will read #8 you will see that you should hear some sparking when you turn the thermostat to heat and turn the target tenpature above the room tempature. This sparking should go on for a few seconds to a minute or so, then you should see a pilot flame. After the pilot lights it then should be another minute or so and the burners will light.

So listen and look for the spark, if you hear the spark but it never lights the pilot I would then like for you to use a fireplace match or a bar-b-que lighter and try to light the pilot where the spark is sparking. If the pilot lights that means the spark is not arcing where the gas is coming out of the pilot. So try to clean this area, it could be covered with dust or corrision.

Let me know what happens.

Heating:
5−Heating demand initiates at W1 in the thermostat and energizes
Relay K13. Heating demand also passes through
High temperature limit S10 to combustion air prove
Switch S18.
6−Relay K13 closes to energize combustion air blower B6.
When the combustion air blower nears full speed, prove
Switch S18 closes. Heating demand continues through
S18 and through flame rollout switch S47 to energize
Ignition control A3.
7−Ignition control A3 then waits 30 to 40 seconds to allow
Combustion air blower B6 time to draw exhaust gas from
Combustion chamber and to introduce fresh air. Combustion
Air blower B6 operates throughout the heating
Cycle.
8−After the ignition control delay, A3 activates gas valve
GV1 and the spark electrode. When flame is sensed by
The flame sensor ,the spark electrode stops. If flame is
Not sensed after the first trial for ignition, controller A3 repeats
Steps 7 and 8 up to two more times (depending on
Controller make) before locking out. If the control locks
Out, it can be reset by breaking and remaking thermostat
Demand.
9− Blower delay relay K25 de−energizes blower B3 after
150 second delay.