Water in drain pan is suffocating the oxygen required to keep the burner lit. Pilot lights without a problem, but the burner does extinguish itself after a few seconds. Source of this water could be a tank leak or possibly air conditioning condensate draining backwards into the tank into the pan due to the connection of PVC drain piping and slope. There is not enough clearance between the tank and the edge of the pan to sponge out the water. Suggested to owner to replace this 14 year old tank and provided quotes. If a hole is drilled at the side bottom of the pan, the water will drain out and the burner will stay lit, but the water that leaves the pan will most likely will not flow to the floor drain due to the slope of the concrete.