2001 Mercury Marquis fails to start when I least expect it; why? on 2001 Mercury Grand Marquis

Tried replacing coils. Motor turns over, but will not start. This will happen about every 3 or 4 days.

Asked by for the 2001 Mercury Grand Marquis
when it doesnt start check for spark and fuel pressure
1 more answer
inspect the crank sensor wiring for any chafed section.And retrieve dtcs.