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Was the unit sitting dormant for a while?
Are you using it on natural gas also?
but it always fails after blower starts?
usually faults related to when blower start (after) are heat exchanger related...
A good test would be to disable blower...see if it runs..then start blower and see if it kicks off
can you get to the ignition points and clean with steel wool? especially the flame sensor?
depends if it is gasket or crack....but I am hoping you just have a dirty flame sensor...a dirty sensor only allows 3 seconds of gas/flame...then shuts down.
I would agree if it were more erratic...but with the set timing it appears sensor related
yep...3-4 seconds has to do with the flame safety.
Just rate service to move off active list and then you have 30 days of follow up.
Thanks for your patience,
can you tell me what the gas pressure is that you have entering?