What to do if i have a partially gray molar. is this is a major sign for a root canal?

Good indication: Then again, if the tooth has an amalgam filling, it may be stained from that, too. No need to self diagnose these problems.. Let your dentist take a radiograph and be responsible.

Gray molar: Hard to say with out seeing the tooth and what type of restoration if any is in it at the present time. Large silver fillings can make teeth look gray but decay can also do that and the larger the gray area could indicate the larger the decay therfore increasing the chance that a root canal could be necessary.