A school bus diesel has reduced power and a noticeable lack of throttle response, with a clean exhaust and normal rail pressure. The technician confirms low boost. The MOST appropriate next step is to:

1. Replace the ECM and clear codes
2. Inspect the air induction and turbo system for restrictions, boost leaks, a sticking VGT, or a charge air cooler leak
3. Replace all injectors
4. Flush the cooling system