The HPOP sensor is located on the High-Pressure Oil Pump (HPOP) itself, which is situated on the front of the engine, near the top. It's a relatively small sensor, and you'll likely need to consult a repair manual or a detailed diagram to pinpoint its exact location. The access might be somewhat limited.
Important Note: Working on the HPOP system requires a good understanding of diesel fuel systems and safety precautions. High-pressure oil can cause serious injury. If you're not comfortable working on this system, it's best to take your truck to a qualified mechanic.
