The furnace in question is a Trane XR95. I added a natural gas-powered heater yesterday, which required the natural gas to be turned off for approximately 8 hours. By the way, the furnace had been operating without any issues whatsoever since the replacement of a high limit switch about 2 years ago. After turning the gas back on, I checked each natural gas-driven device to make certain everything was in working order.

After following the shutdown/restart procedure for the furnace, the flashing light on the board was flashing as it should. I called for heat (set the thermostat higher) & the flashing light responded accurately as well. But, after 3 failed attempts at firing up the burner, the Trane turns on the fan, but no heat, just cold air & the red light was now flashing twice, pause, twice, pause, etc., (External Lockout)

Like I said, not a single problem since replacing the limit switch 2 years ago. How does a furnace go from 100% functional to "bricking" like it has now from simply turning off the gas for 8 hours? It seems to be cycling the way it should, except for gas getting to the burner. With the bottom panel off during diagnostics & looking through the sight glass, I observe a glow of sorts (a total of 3 times) in the burner area which lasts about 5-10 seconds each, before the fan turns on & I receive the External Lockout Code.

 

The lockout indicates it attempted to ignite 3 times and failed, the reasons why can be numerous.
Upon a call for heat, the sequence of operation is,...

#1 The induced draft motor runs for 30 seconds
#2 The pressure switch senses the induced drafts operation and closes and must remain closed
#3 the ignition will energize for 30 seconds either a spark type or hot surface ignitor
#4 The gas valve will open for 30 seconds and establish a flame
#5 The flame is sensed and after 30 seconds the indoor blower will begin running.

If there is a problem at any stage with any of these sequences the unit will lock out after its 3 attempt to operate , there is no 4th attempt and the unit locks out.
The lock out requires a time delay of several hours or you can cut the power then return the power after 30 seconds, this will reset the circuit board to try 3 more times.
Most units will give a code for the stage or operation which it goes into lock out mode.
By observing the sequence of operation you can determine whats wrong.

If the induced draft motor does not run or close the pressure switch you then you know the induced draft motor is bad if it never runs, the pressure switch never closes no ignition will begin.
If the pressure switch wont close the gas valve will not be energized, if the pressure switch wont stay closed and opens at any time the unit will go into lock out.
The flue could be clogged or the port on the induced draft housing is partially clogged it waver between open and closed, this will create a lock out, If any limit switch opens it will lock out, there are many possibilities for these lock outs, a hot surface ignitor can be burnt out or a flame sensor is dirty, the pressure switch's tubing and its orifice attachments are obstructed this can create the lock out.

The induced draft motor will or wont begin running, it will run 30 seconds to purge the combustion chamber to remove any gas, the induced draft motors operation will cause a pressure differential which is sensed by the pressure switch.

The pressure switch has a rubber tube connected to a port or nipple on the induced drafts housing, the port that the tube connects to on the induced draft motor housing will clog with carbon and requires the tube be removed and a small drill bit of stiff wire ream out the port insuring it is able to sense the induced drafts operation, this port often partially clogs allowing some sensing but not enough to keep the pressure switch closed continuously, this is a problem many techs do not know of and no parts replacing will cure the intermittent locking out or locking out with every attempt.
The tubing itself can collect condensatio n and if it loops the water will collect in the lowest spot and eventually obstruct the pressure switch's ability to prove the induced draft motor is operating by
closing a pressure switch that proves to the the circuit board the induced draft motor is operating, IF the pressure switch closes and does not waver or open,close,open,close the next sequence of operation is the spark ignitor or hot surface ignitor will energize, if the pressure switch never closes or opens even for a second the next sequence of operation will not begin.
This is probably not the job for a home owner unless familiar with mechanical equipment and electrical power.
Typically if the unit can run and light intermittently the pressure tubes orifice on the induced drafts housing is clogged either fully or partially

 

XR-95 is a group..... not a model number. The model number is inside on the sidewall near the burner on an ID tag. It sounds like you have a spark ignition system although most of that series were hot surface. It also sounds to me like the gas line hasn't been purged of air yet. Turn the furnace off to reset the board and try it again. It may take several reset periods to purge all the air out of the line.

 

Thanks to both Keith & Pjmax for their rapid replies. After several resets & subsequent purging of air from the gas line, the component at fault was a faulty gas valve coil. The gas valve consists of a poorly constructed onn/off toggle switch which apparently goes bad quite easily. The only problem for me here in Southwest Missouri is a poorly managed Trane Supply Warehouse which has no stock (in the middle of January!) & the unavailability until January 24th. Unfortunately, other sources can't deliver product until the same day.