What condition causes the CEL to blink versus being constant with a P0171 (bank 1 lean) code?

 

Blinking means misfire. At least bad enough misfire to cause catalyst damage.

 

It is the Danger Will Robinson of MIL `s

 

Generally, in OBD ll, a steady light is an indication that although there is a fault of some sort, immediate attention is not required.

A flashing light means that the fault is so dire, that the machine should not be operated further until repair has been made, or serious damage may occur.

 

Usually a blinking CEL is something that will effect emissions CAL spec, usually a severe misfire that can cause un-burnt fuel to enter the catalyst and cause a melt down of the catalyst
.