On A 2001 Cadillac Deville, What Does Error Code Po174 And Po171 Mean?

It means that both bank one and bank two o2 sensors are registering a lean condition.(To much air to fuel)
most Common Problems: In this order
Plugged Fuel Filter or weak Fuel Pump
Vacuum Leaks (Intake Manifold Gaskets, vacuum hoses, PCV hoses etc.)
Mass Airflow Sensor (MAF)
Plugged or dirty Fuel Injectors
PCM software needs to be updated