737-800 Windscreen Heaters
 

B737-800 diverted to port with engineering facilities due to a failure of windscreen heat/temp sensor?

Is it correct that an aircraft must have at least 3-4 sensors working to fly? How many are fitted to the aircraft?

Is this a common component to fail, and what is the likelyhood of 2+ sensors failing on one flight?

i.e If an aircraft can fly with one sensor not working, but requires divert due to failure, does this suggest that the aircraft was initially flying with an allowable fault?

Any thoughts?