Ford Expedition 98 - Help with P0135, P0455, and P0401 OBD2 Codes

Model: 98 Exp Fault Code: P0135 Posted: 2006-04-24 20:42

I own a 1998 Ford Expedition. The engine was previously driven for 190,000 miles on the old unit. Now I'm experiencing check engine light issues with three diagnostic trouble codes: P0135 (O2 Sensor Circuit Malfunction), P0455 (Evaporative Emission System Leak Detected), and P0401 (Exhaust Gas Recirculation Flow Too Low). I've already replaced the oxygen sensor, gas cap, cleaned the throttle body thoroughly, removed carbon buildup from intake ports, and replaced the DEP (diesel exhaust particulate) sensor last year. Additionally, I replaced the EGR valve. Despite these efforts, the codes persist and I'm unable to pass a vehicle inspection for Bike Week. Any advice or troubleshooting steps would be greatly appreciated.