2002 GMC Sierra 1500 Failed Inspection: OBD2 Codes P0171 and P0174 with Recurring P0327 Knock Sensor Issue
I have a 2002 GMC Sierra 1500 that failed state inspection due to OBD2 fault codes P0171 and P0174. I replaced the intake manifold gasket, but the check engine light came back on after just 50 miles. I then replaced the oxygen sensor, only for the check engine light to return again—this time with code P0327 indicating a knock sensor issue. The vehicle runs perfectly at 227,000 miles: no oil consumption, smooth idle, and consistent performance. Despite this, the recurring codes suggest an underlying problem. Is the knock sensor faulty or is there another cause? Any advice on diagnosing or fixing this would be greatly appreciated.
Which engine configuration do you have? P0171 and P0174 relate to Bank 1 and Bank 2 oxygen sensor readings—these are part of the same trouble tree. For reference: https://www.obd-codes.com/p0171. Start with simple, low-cost diagnostics: inspect for vacuum leaks and verify fuel pressure is within specifications.