what does it do? how do i know i need one? how much does it cost to buy/install? can i do it myself?

 

I bought my cap and rotor for about 20$$. My mechanic did it for me real quick.

Idk how difficult it is but doesn't look he had any difficulty doin it

 

The rotor spins with the engine and distributes spark to the appropriate cylinder. It should be replaced with the cap and wires or plugs. Look for pitting, scoring or worn traces on it. It's about $5. Yes, you should be able to do it yourself. Replace the cap as well, don't use dielectric grease on the rotor.