The oxygen (lambda) sensors wear out and throw codes — know upstream vs downstream before buying the wrong one.
Upstream (pre-cat): measures exhaust oxygen for fuel trim — a lazy or dead one causes poor economy, rough running and fuel-trim codes (P0130-series).
Downstream (post-cat): monitors catalyst efficiency — a fault here usually means P0420, which is more often the cat than the sensor.
Diagnose:
- Read the code and live sensor data on VCDS — a healthy upstream sensor switches quickly; a flat-lining one is dead.
- Match the code to which sensor — don't replace both blindly.
The fix: replace the failed sensor with an OE/genuine unit; lightly anti-seize the threads.
…