Blowing out moisture from gas line to furnace

Uh, WHUT?

I've serviced units the piping of which had been in place for 20-30 years, and had never been opened to "drain moisture." Unless you have a poor gas supplier, there should not BE much if any moisture in there

Do you actually GET moisture out of the trap?

Otherwise, this might be due to another problem. What kind of furnace, and what sort of ignition does it use?

That is, older standing pilot?

Hot surface, spark ignition?

Is if forced draft, IE has a draft motor?

All that "stuff" can complicate what goes on.

On the newer electronic stuff, the flame sensor is almost always what is known as "flame rectification." this involves a VERY tiny current of sometimes only 1 or 2 microamps. If the sensor probe is improperly located, corroded, and or has deposits on the ceramic insulator, or maybe the probe has a poor connection to the wire lead, they can be "fussy."

Many (most) newer furnaces have a diagnostics "flasher" on the board, which can help. Otherwise, you need to learn (or have someone) who can diagnose if the firing cycle is OK