The oxygen (lambda) sensors wear out and throw codes — and I make sure I know upstream vs downstream before I buy the wrong one.
Upstream (pre-cat): reads exhaust oxygen for fuel trim — a lazy or dead one gives poor economy, rough running and fuel-trim codes (P0130-series).
Downstream (post-cat): watches catalyst efficiency — a fault here usually means P0420, which is more often the [[catalytic-converter-gti|cat]
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