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Lambda sensor confusion

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Hi everyone,

Hoping for some wisdom regarding the above. I'm helping a friend (who isn't really a car person) to fix his 2001 Subaru Outback 2.5. We've ascertained that it needs a new lambda sensor, but I'm struggling to find a replacement.

I can see there are two sensors going into the section of the exhaust that contains two catalytic converters. There is one that goes in end-on, and seems to run pre-cat. And there's one that goes in from the top, after the first cat. It's this one that is the issue - the car hesitates and splutters a lot, and won't rev, unless you unplug that sensor.

My confusion relates to the following...

  1. The sensor that needs to be replaced is a four-wire sensor, but when I look on eBay for matching parts I only get three-wire sensors come back. What's all that about?
  2. Does anyone know what other sensors (i.e. from other cars) are a match, technically? I found one intended for various Fords and Volvos that looks identical apart from the plug.
  3. The only Subaru sensor I could find that has the right connector is this one - https://www.ebay.co.uk/itm/142804741377 - but that is not listed as matching this car, and is also described as a front sensor - presume ours is a rear sensor?

I'll try and add some pics here so you can see what we've got. I'm pretty certain it's original as it was a one-owner car from an old couple, and doesn't seem to have been fiddled with.

Would be very grateful for any advice you can offer!

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