A school bus with a Cummins ISB diesel engine cranks normally but will not start, and the technician confirms there is no fuel reaching the high-pressure pump. Which check should be performed FIRST?

1. Replace the high-pressure common-rail pump
2. Replace all fuel injectors as a set
3. Increase the rail pressure relief valve setting
4. Inspect the low-pressure (lift/transfer) supply circuit and primary fuel filter for restriction