? regarding unknown OBDII Codes my reader is showing for my 2003 Lincoln TC

What is your question?

I have a lower end OBDII reader but it's showing 2 Codes that are unidentifiable when I look them up on the OBDII reader code section on repair pal. The 2 codes are listed as such - *P1151-Lack of HO2S21 Switch- Sens Indicates Lean & Code 2 is, *P1152-Lack of HO2S21 Switch- Sens Indicates Rich Can anyone explain what these mean? I'm assuming based on some of the letters & words they are both referring to some type of either oxygen or fuel sensor. Code 1 indicating a lean mixture of oxygen or fuel possibly? Code 2 indicating the exact opposite, with a rich mixture? Am I anywhere close to the correct answer? & If not could anyone enlighten me on these 2 code readings & offer an opinion on any type of a solution I could go about to hopefully fix these codes? Any help would be greatly appreciated. Thanks for all the help I've gotten from repair pal affiliates w/ other questions as well. Greatly appreciated guys!