P0122 and P0108 Fault Codes on 1998 Buick Century 3.1L Engine - Diagnosis and Repair
My 1998 Buick Century with a 3.1L engine recently developed fault codes P0122 (Manifold Absolute Pressure Sensor Range/Performance Problem) and P0108 (Manifold Absolute Pressure Sensor Circuit Low Input). The car was previously running fine, but while driving it suddenly stalled as if a wire had been disconnected. Since then, the vehicle cranks unevenly and shows signs of struggling to start—sometimes appearing as though it's trying to ignite, yet failing to run properly. I've already tested the throttle position sensor (TPS) and manifold absolute pressure (MAP) sensor by disconnecting them individually, but neither action allowed the engine to start. When reconnected, the car only runs very briefly when the gas pedal is fully depressed. I suspect a possible failure in the powertrain control module (PCM), as this issue occurred before with an older OBD-I system. I'm now considering whether the engine has lost compression or if there's a timing chain jump—though that seems unlikely given how abruptly it stopped running. The backfiring during attempts to start further raises concern about internal mechanical failure. I've ruled out exhaust restrictions and am planning to check fuel pressure and verify spark quality. What are common causes of P0122 and P0108 on a 1998 Buick Century? Are there specific diagnostic steps or component replacements that should be tried before disassembling the engine?
To troubleshoot, try disconnecting both the throttle position sensor (TPS) and manifold absolute pressure (MAP) sensor. If the engine starts or runs better after removal, it points to a faulty sensor causing P0122 and P0108 codes.