Coolant Sensor Leak
After replacing my lower radiator hose only to find the leak was coming from the coolant sensor, I went out and got a 10 pack of VITON (high temp / coolant resistant) O-rings from Macro Rubber . They were only $10 for the 10 pack. I believe Viton may be a more stable rubber compound than Buna N for this fairly challenging O-ring application. Needs to withstand hot, cold, vibration and chemicals. The VITON O-ring is a 8mm ID x 3mm cross section. Better than the $40 sensor from BMW and just as effective if your sensor works, but is leaking.