2003 GMC Sierra P0455 EVAP System Large Leak Diagnosis and Solutions
I recently purchased a 2003 GMC Sierra with a 5.3L V8 engine, which has accumulated approximately 87,000 miles. Recently, the check engine light came on, and the diagnostic code P045-5 (EVAP System Large Leak) was detected. Initially, I ignored the code and simply erased it from the system. However, upon a second inspection, the same P0455 code reappeared. I replaced the gas cap with a new one and ensured it was securely tightened. Four days later, the check engine light returned with the identical P0455 code. I have not detected any gasoline odor when standing near the vehicle, and there are no noticeable drivability issues. The purge flow valve on the manifold has been replaced with a brand-new unit sourced directly from a dealership. Given this, I am seeking expert advice—any insights into possible causes of this persistent P0455 code would be greatly appreciated.
Consider inspecting the vacuum hoses—cracks or leaks in any hose could cause a drop in system pressure. A professional smoke test is typically used to detect such issues, though it can be costly and may not always be available at every garage.