i also have a 2008 535 and recently had to replace the electrical waterpump and thermostat. If you are getting the yellow warning light you probably lost the pump and need to replace it. The pump itself is $500, plus around $100 for the thermostat.
Audio: Jehnert Flatline speakers, Mobridge M1000 DA2 prepro, Helix P-DSP, Zapco Z-150.6 amp
Performance: Cobb Pro Tune, AR Downpipes, ER FMIC/CP, DCI, RB turbos, KW V2 coilovers, Fuel-It Stage 2 w Ported fuel rail, Mfactory LSD