Assessing the need for built in sensors to detect failures on running products.

When assessing the detection on a DFMEA all the guidelines I have read say the detection must occur before the product goes to production i.e. during the prototyping and validation phase.

Previous colleagues at my company also included automatic detection of failures that are built into the product as part of the detection rating. An example of a built in automatic detection feature is the oil sensor on your car that detects low oil and turns on the oil warning light.

Clearly these features should only be built in as a last resort and the failure mode and occurrence should be reduced as a first priority when possible. When you can’t completely reduce your risk factor to an acceptable level, and you need to put an on-board sensor in the product, where is the appropriate place to assess and document automatic detection features when conducting the risk analysis during the design phase?