My check engine light is on...problem=TDC sensor!????

I'm reading through my haynes and it says the top dead center(TDC) sensor determines the ignition timing at startup when the engine is cranking. Now, I remember the check engine light came on while I was busy driving and not at startup. The ECM uses the TDC data and the CYL sensor data to determine the duration during which fuel is injected. Is this a major problem...is it easy to fix myself?? How much are they?? Any help greatly appreciated!