Refit the panels, re-fill the coolant then follow the easy bleed procedure under 'Filling without vacuum filling unit:' here. It's not the cheapest but I was in a hurry. I got a HAAS sensor from EuroCarparts (PN 224110291) at £16. This video shows you where the coolant reservoir, hoses and connections on your 2011 528i Base are located and the steps needed to fix minor leaks. Otherwise, your 528i may have a radiator leak. Wear goggles and keep your mouth shut as coolant will pour out. However, a coolant leak (aka an antifreeze leak) is most likely to occur where the hoses connect to the engine or radiator. The pulling takes much more effort than the wiring connector but persevere, it will come out. There's two lugs on the side of the sensor so squeeze and pull out the sensor. Push that down lightly and pull out the connector. There's a spring on the top of the senor which retains the wiring connector.

Assuming you've crawled under from the front of the car and on your back with your head in line with the right hand ramp, you should be able to reach up with your left arm and feel the sensor, cable and (perhaps) the leaking coolant. It's not essential but use an inspection mirror as shown in the PDF to have a look around to get your bearings. I got a bag of 10 for £4 from eBay, item number 171366334901. For a more accurate estimate based on your location, use our Fair Price Estimator below. This range does not include taxes and fees, and does not factor in your unique location. Labor costs are estimated between 44 and 56. You may find some are corroded so buy some new ones to replace. The average cost for a BMW X5 Coolant Leak Diagnosis is between 44 and 56. All you need for the under-body panels is an 8mm socket to remove the self-tappers (BMW 07149126886).

Some of the low profile plastic ramps out there won't be enough clearance.

Bmw coolant leak fix