98 Ford Expedition P0155 Code Persists After Multiple Repairs

I'm still struggling to resolve the P0155 diagnostic trouble code on my 1998 Ford Expedition. I've already replaced the O2 sensor, fixed an exhaust leak, cleaned the EGR tubes and valve, replaced one faulty injector, changed spark plugs and wires, installed a new valve gasket, and even replaced the Powertrain Control Module (PCM). Despite these efforts, the P0155 code remains unresolved. UPDATE: The issue was identified as a physical problem — one of the pins in the female connector of the sensor harness had become lodged too far inside, resulting in an intermittent connection. Once this mechanical issue was corrected, the P0155 code cleared.