no rpm and no codes

car had has problem with no start one day would start the next.found no spark replaced coil pack car started . left ok week later called no start again.car will not restart checked for codes no codes but have found i have no rpm reffernce.does anyone know if the car gets rpm reff from crank sensor or cam sensor?sorry car is a 00 2.0 elantra replaced crank sensor car is runnig fine now thanks for looking