I have a 15 year old 70000 BTU high efficiency gas furnace, and it is cycling every 2-3 min and the house is COLD (about 10 degrees lower than comfortable)

The small exhaust fan comes on, the igniter turns on, the pilot turns on, the pilot sensor heats up and then the main gas goes on and burns, then the main fan comes on and then a minute later it shuts off and 1-2 min after it starts the same cycle over again.

The temp limit system does not reach max temp, and it does not look or feel like the system is remotely overheating.

What could be wrong?