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Northstar Engines and System Technical Discussion Discussion, Po138??? in Cadillac Engine Discussion; Hi All, I've got a '97 STS with 122K miles on it. Been running fine for a few years, then ...
  1. #1
    caddynick is offline Cadillac Owners Member
    Join Date
    Nov 2003
    Nevada near Tahoe


    Hi All, I've got a '97 STS with 122K miles on it. Been running fine for a few years, then all of a sudden last week she started misfiring awfully bad. I changed out plugs and wires and still misfiring though not as bad. And now I'm getting code PO138, something to do with O2 sensor. Does anyone know how to test this O2 sensor to see if I need a new one? My real question is where do I go from here? I'm mechanicaly capable and I would like to do it myself if I can.

    Thanks. BTW there's some very knowledgeable people, I've learned a lot just from reading.

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  3. #2
    Ranger's Avatar
    Ranger is online now Cadillac Owners Member
    Automobile(s): White Diamond '03 DHS (with DTS floor shift)
    Join Date
    Jun 2004
    Woodstock Ill.

    Re: Po138???

    I am not positive, but I would guess that sensor needs to be replaced. BTW, it is the pre-cat sensor incase you were not sure which one it was.

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