And where are they located. I had a diagnostic test done at the local Cherolet stealership and they said I needed a O2 sensor replaced in Bank 1. I took it to Pep Boys and they supposedly replaced it for half the price of the dealer ($125) Now my check engine light is back on for an O2 sensor again. What are the chances it's a different one or Pep Boys replaced the wrong one? It's has 50,700 miles on it. I'm asking because if it has 3 O2 sensors, for the $255 the dealer wants for one, I can just have Pepboys do two for that price. ActuallyI can do the one on the drivers side above the spark plugs. Hopefully there is only 3 total.