Trouble Code P0171 on your Pontiac Bonneville indicates your Pontiac's engine is running lean in both Bank 1. Bank 1 is the side of the engine where the number one (1) cylinder is located. the oxygen sensor on bank 1 has noticed the engine is running lean. Running "lean" means either inadequate fuel is getting to the combustion chambers, or too much "unmetered" air is being mixed in with the fuel.
First you'll need to look into things which would cause unmetered air to enter the intake manifold, such as cracked or disconnected vacuum hoses.
Second you should inspect things which would cause inadequate fuel delivery such as emission sensors which may be reporting false values to the ECU (engine control computer). A good example of an emission component which could be defective and causing inadequate fuel delivery is the MAF (Mass Air Flow Sensor). A good example of a mechanical component would be a dirty fuel filter or weak fuel pump.
There is always the possibility of the Bank 1 Oxygen Sensor itself being defective and not sending out the correct fuel mixture voltage, however it is recommended to suspect a defective oxygen sensor after inspecting the vacuum lines and the MAF.
