I have a trane furnace and I was able to initially light the pilot light but the pilot won't stay lit for more than 30 sec. (approx.). I was able to do this about 2 or 3 times but now I can't even light the pilot anymore. I can't understand what could be wrong; It's not the thermo couple because I already had it checked and they told me it was working.

Piolit assembly probably needs cleaned, or could be a cracked heat exchanger, does the piolt go out when the fan comes on?