The standing pilot light is on but the burners won't ignite. The fan runs. Could a bad thermocouple cause this. I know that there are many other problems possible but this is the cheapest place to start.

If the pilot light is burning the thermocouple probably is OK,does it have a good blue flame? can you hear the gas valve opening, is gas going to the burners and the flame just not reaching or close enough

This is my sons furnace and when I looked at it today, the pilot light is a strong blue flame. The burners light, stay on for about a minute, go out, then come back on in a minute or so, with the blower limit switch clicking the burners on and off.

OK,, more info is allways a help,, kill the power to furnace,, remove and clean the flame senser with steel wool or fine sandpaper ,reinstall and turn back on the power,, let us know if this helps,,