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There are drawbacks to just firing a parts cannon at a problem. The first is obviously the cost of the parts and labor to install. The second is the possibility of introducing another problem because of installing the wrong part or installing incorrectly.
These furnaces are designed to operate at gas pressures between 11 to max of 14 inches in a water column. That converts to about 0.4-0.5 PSI, a very low pressure. Unless you have a manometer to measure pressures in this range, I would highly recommend not adjusting the regulator. It's true that pressures may need adjustment at high altitudes sometimes.
There are 2 blowers on the furnace; one is the main circulating blower for the warm air, the other is a draft induction blower that supplies air for combustion. The latter is the air stream the sail switch is located in. If the draft induction blower starts upon a call for heat, the thermostat is good. The blower starts and runs for about 15 seconds to purge any unburnt fuel that might be present. Prior to this purge the control board checks for the sail switch to be open. After the purge cycle, the control board checks for the sail switch to be closed which proves you have combustion air. Once combustion air is proven, the igniter is energized and the gas valve opens. After another time delay of 5-7 seconds the control board checks the flame sensor to prove the burner is lit. If it doesn't sense flame at that time, it will shut down. Once flame is sensed, the ignitor will turn off, the flame should be self sustaining at this point. It will then try 2 more times to restrike before shutting down for a period of time.
If the burner does fire, the sail switch contacts must remain closed throughout the burn, if you should lose it, the furnace will shut down. If it fires initially, the sail switch is good. It is possible the contacts will open during the burn, but not very likely.
The next step in the sequence if the burner fires and sustains flame would be the circulation blower starting. I don't remember if it's a time delay or maybe an internal temp sensor in the furnace. The reason for this delay is to minimize the initial blast of cold air when the blower starts. The high limit switch is the safety that will shut the furnace down should the circulation blower fail and the heat exchanger over heats.
From the symptoms you have described, it sounds very much like the flame sensor. These are pretty sensitive and the mounting location is critical too. If you are getting the purge cycle on the induction blower and the burner is actually lighting and shutting down in a few seconds, I'm pretty sure that's the problem. It's possible it's dirty; I would be very careful trying to clean it. Maybe just a Q-tip dipped in alcohol.
I believe the flame sensor is sold along with the ignitor on the same module if you choose to replace it.
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