I have a York Gas Furnace (model# P2MPD14W08001B, 12yrs old) giving out no heat. I did basic troubleshooting and realised system was not igniting; replaced ignitor (changed original 4-5 years ago) - fired up initially and then went out after running for about 30 minutes. I cleaned the flame sensor - fired up initially, ran about 30 minutes, and not again. I reseated the connections,made sure gas supply was on,changed filter, cleaned out dust from the controller, even reinstalled old/replaced ignitor to no avail. The diagnostic light flashes one when system is powered on - I think that is the furnace lock out/chech furnace code.
The thermostat calls for heat, the blower comes on, etc. all the sequences seem to be in order,but system will not fire up.
Help please... its getting cold outside.!

Follow wires from the gas valve all the way back to the board, see if there are any pressure switch's, roll-outs, or limits in series with the gas valve that are open.

I'll suggest you to make it run again (check all safety switches as Northernheat suggests) and stay there until it stops. Pay attention at the moment that this happens. If you do not have any problem with using voltmeters, I'll recommend you to connect one at the gas valve relay and other at the draft inducer. Once it is starting to show symptoms that it is going to stop, see what the meters are reading and what the unit is doing also.

Then post back to see what happened.

I know the whole purpose to DIY is to save money, but it is still not that much to buy some fairly good and cheap voltmeters, they are always helpful.

This is going to sound silly but If the unit runs for 30 minutes it is "ok" for 30 minutes - some safety device it is stoping the unit, and afterwards when you try to start it again, whatever failed, it might still be at a failure state or the unit has locked. Also, maybe try to RESET completely by switching all power off and try again to see if it starts to keep looking for the root cause.

I hope this helps.

Thanks for the feedback.
I checked all the connections to the board and found everything OK.
I had already done a full power down from the circuit breaker, but did again with same results... no go.
Unfortunately, I do not have a voltmeter handy.
Any other suggestions would be appreciated... Thanks.

Fixed...
The pressure switch contacts needed cleaning (dust build up). This was causing the system to be in lockout.
I also found out that my thermostat needs to be replaced.
Thanks.