Suburban Furnace

[LAarchitect]

My heater started blowing cold air. I noticed the ignition doesn't ignite. I ordered the closest I could get based on the model number but after I removed the electrodes, I noticed the tip of the electrodes were black and covered with soot. I cleaned them and put them back in and bingo...the heater started to work perfectly.
The same thing happened after a while and I noticed it happened right after I ran out of propane.
Question: Do electrodes get covered by soot because the heater ran without propane? Due to constant sparking!? If not, any idea why this happened?

Thank you all in advance.

[A W Warn]

It is burning too rich, fuel/air ratio is off, so it is a good possibility that something is blocking the intake of combustion air or exhaust. Look further back inside the furnace. Most likely wasp nest or dirt dauber nest are inside. I used a brush on a flexible rod a couple times per year to keep spider webs out. Though, a year or so ago I used a coat hanger wire and a leaf blower to clean mine when a dirt dauber took up residence. Since I installed bug screens on furnace and water heater I have not had to do any bug removal.

Also, I've heard of a few trailer's propane lines getting oil in them, from the oil additive used as odorant in propane. There are a couple threads on this forum.