It sounds like you need to change the 02 sensors.
Reduced engine power may be many things but the reduced engine power will also throw a trouble code. Get that information to diagnose your problem. If the check engine light is on steady it is usually OK to drive however if it is flashing you must shut the engine off. A rich running condition will cause your catalytic converter to overheat and melt. www.trailvoy.com All the answers for your Envoy and more!
Bank one fuel system rich.
System to rich (bank 2)
Get Rich Quick - 2004 is rated/received certificates of: Australia:MA
2004
Trouble code P0172 means: System too rich (Bank 1)
New Orleans Rich and Haunted - 2004 TV was released on: USA: 23 October 2004 (ScreamFest)
CMT Total Access Big and Rich - 2004 TV was released on: USA: 3 September 2004
Trouble code P0172 means: System too rich (Bank 1)
Get Rich Quick - 2004 was released on: Australia: 9 July 2004 (Melbourne Underground Film Festival) USA: 1 June 2006 (Westham Films)
Second Verdict - 2004 Commonwealth Vs. Christie Rich - 1.2 was released on: USA: 12 October 2004
Trouble code P0175 means: System too rich (Bank 2)