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.

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Styling: M5 body kit, M5 trunk spoiler, LED Angel Eyes, HID fogs, LED side markers, Jehnert Flatline speakers, Mobridge M1000 DA2 prepro, Zapco Z8 DSP, Zapco Z-150.6 amp, iDock on dash
Performance: JB4/G5 ISO flashed on Cobb, Meth Injection, AR Downpipes, ER FMIC and Charge Pipe, DCI, Walbro LPFP, RB turbos. H&R Sport springs/Koni FSD shocks.