P0332 Fault on 2001 GMC Z71: Diagnosis, Fuel Efficiency Drop, and Vacuum Leak Suspicions

After my last oil change, my 2001 GMC Z71 started displaying a P033-2 diagnostic code, triggering the 'Service Engine Soon' light. I consulted a certified mechanic who provided only vague feedback—no clear explanation of what caused the fault. Since then, I've researched extensively and found that some users have reported resetting the code temporarily, with no symptoms until they drive under load. I began troubleshooting based on common causes for P0332, which is typically linked to a faulty or damaged knock sensor. My first step was checking for vacuum leaks—especially at the fuel cap—since this can interfere with engine sensor readings. I replaced the fuel cap and reset the code. For a few days, no error appeared until I started driving. The P0332 returned shortly after, suggesting a possible vacuum line leak or sensor issue. I’ve noticed a significant drop in fuel efficiency—now only 6 to 10 MPG—which is concerning given that my vehicle has nearly 160,000 miles. A professional diagnostic scan showed no major engine issues, but the P0332 persists. I’m hesitant to have knock sensors replaced without identifying the root cause, as this would be costly and time-consuming. I’m a veteran driver with limited access to newer tech, so I prefer hands-on troubleshooting over part replacements. I want reliable, factual information on how to diagnose and fix P0332 in a 2001 GMC Z71 without unnecessary expenses or blind repairs. Any insights from experienced owners or mechanics would be greatly appreciated.