x2 what the Warrior said. Crapply plastic impellers. Crack and break often, and then no pumping. Remove Therm and start engine. Should *shoot* water out. (Think garden hose, full-bore) If no flow, renew pump.
Side Note: If you have a '96 to 2002 Volkswagen TDI, you have the same problem coming... China hasn't got the corner of the market on crappy water pumps
--Vince