Age, heat, "wear"? Could just be crappy and old.
You can use two bank one sensors - Bosch # 17098, available from many places online. The rear bank sensor, 17102 is much more expensive and harder to find.
E92 335i/6MT Dinan Stage 3: intercooler, oil cooler, intake, flash, exhaust, LW dual-mass flywheel.
Vishnu PWM methanol custom trunk mount, Diffsonline 3-clutch Salisbury custom diff, Ohlins Road and Track coilovers, Vorshlag camber plates, much more...
E28 M5 w/ buncha stuff, R171 SLK350