I recently studied the design changes made on the T series of old British cars (a hobby) made by the MG Company in England from 1936-1955.  Models progressed over that time period including the TA, TB, TC, TD, and TF.  I noted a strange looking two position switch on the dash of the TA and TB.  It was marked M and R.  I found that M means Main and R means Reserve – dual redundant fuel feeds with a human switching mechanism. There was no fuel gauge warning light. While driving a TB I discovered that the engine dies when the fuel level goes below the M fuel feed. I switched from Main to Reserve and drove to get fuel.

In the subsequent models the switch was replaced with a light.  The light indicated when the fuel in the gas tank was low – a diagnostic sensor with a human final element.  Of course, without this “automatic diagnostic” light, a manual periodic proof test could always be done with the wooden dipstick. My dipstick has calibration marks on it to indicate the gallons of fuel. But like all proof tests, one must be reminded to do them. I prefer the diagnostic light backed up by a dipstick proof test over the complexity of the redundant fuel tanks.  

So, it appears as if the engineering design has advanced from redundancy to diagnostics in these old automobiles. This trend has continued from the 1930s design decision to today. The advances in diagnostic capabilities in both mechanical and electronic devices are quite impressive. Many believe that these advanced diagnostics are the future of functional safety.