JTS are know to have Lambda issues. there are 4 in total.
Bank 1: Cyl 1 & Cyl 2
Bank 2: Cyl 3 & Cyl 4
Sensor 1: Pre-cat, the one easily visible when you open the bonnet [ more expensive ]
Sensor 2: Post-cat, under the car. [ cheaper ]
So the Lambda sensor that should have been replace, as you look into the engine bay, is the RHS pre-cat, closest to the battery. Also is should be Bosch , any other manufacturer will cause issue.
Can you confirm that it was a Bosch, RHS pre-cat sensor that was replaced.