HT4100 4.1, 4.5, 4.9 Discussion, which O2 sensor for 85 DeVille/HT4100 in Cadillac Engine Discussion; [cross-posting since I got no response from DeVille forum]
My O2 sensor is lazy (but not failing) so I'm going ...
[cross-posting since I got no response from DeVille forum]
My O2 sensor is lazy (but not failing) so I'm going to replace it while I'm swapping out EGR valve. However I'm confused about the right one for my 85 DeVille w/ HT4100. Delco and Rock Auto show three different OEM options -- AFS20 (part #25162693), AFS21 (#25166816) and AFS22 (#25162753) -- but they all look and read the same to me. Meanwhile, alldatadiy gives a GM part #25165119 which Rock Auto says is AFS23 (something else entirely). The FSM does not specify a part type, and it doesn't look like I can get the old sensor out without lifting and disassembly the wheel well cover.
I talked to one of my mechanics about this. He called his Delco rep and that guy said AFS20 and AFS21 were interchangeable (although he did not know the difference between them), and that AFS22 was discontinued (it's still listed as available in many places including the acdelco.com online catalog). My guy also looked in his shop software, which said that the original was GM part number 25162693, which rockauto says is the AFS20 (the one with the "OEM" tag in the description). So going from all of that, AFS20 appears to be the choice, while AFS21 should also work.
I'm a bit curious what the difference between them might be but I guess it's not really important. When I was trying to figure this out with Google, I saw that some Buick Grand National models used the AFS21 (the GNX), so it may be a bit more sensitive. OTOH it may just have a longer pigtail or something.