My 2004 GMC Envoy service engine light is on for 2 days now. I took it to Auto zone and received the code PO172 (rich code in bank 1). The reduced engine power mode light also came on but turned off. The car has been jerking a bit when you accelerate. I've been reading up on similar cases and it looks like it may be the O2 sensor. Where exactly is this located and if it easy to change? I changed the air filter but that is about it. I want to know where I should start before I take it in and spend hundreds of dollars. Thanks!