Double lambda sensor failure?

[mowog01]

Hi all,

Hoping for some sound advice as I'm pretty stumped.

I'm working on a family member's car, a 2002 Outback 2.5 that failed its MOT on emissions (lambda and CO). It was pretty bad to drive, and often wouldn't rev properly. When I unplugged the rear lambda sensor it started to run OK, so I figured that was probably the issue.

Anyway, to cut a long story short, I ended up replacing the whole exhaust because the rest of it was rusty as hell, including a new cat and rear lambda sensor. The sensor I used was a Bosch 'universal' one, though it appears identical the the one that came off the car, just without the Subaru connector on the end.

On restarting the car it seemed to run worse than ever, alternately revving to 2k and then almost dying completely. Once again I tried disconnecting lambda sensors, only this time I find that unplugging the front sensor makes the issue go away.

• Has anyone ever heard of a double lambda sensor failure?
• Is it possible that the act of undoing the front one from the rusty old cat (it was pretty hard to get out) could have made it faulty?
• Am I just chasing my tail here, and actually the fault could be something else entirely?

Can't really justify the expense of paying a garage to fix it as the car is worth considerably less than a grand, so if anyone has any suggestions i'd be very grateful!