All-Go warranties almost everything we sell.  Mechanical parts we have in stock typically have a 90 day warranty (other yards typically have a 30 day warranty), and electrical parts we have in stock typically have a 30 day warranty (other yards may not warranty electrical parts at all).  Special order parts (parts ordered just for you, at your request) usually have a 30 day warranty.  Some things that no salvage yard will warranty include: carburetors, front wheel drive spindle bearings, bad installations, overheated parts, abused parts, or disassembled parts.  No salvage yard warranties labor, fluids,  gaskets, or other installation related costs.
    In general, it's a warranty problem if the part if the part we sold doesn't work as designed.  Of course, we can't guarantee your mechanic told you the right part to buy, or that you'l actually need the part you bought.  If your car is at a competent local repair shop, it's a good idea to let the shop buy the parts.  That way the shop will make sure you're only paying for the parts you need.