Oxygen sensor failure: check-engine light and sudden fuel economy drop

Drivers searching “oxygen sensor replacement cost” or “lambda sensor symptoms” often see a steady check-engine lamp and worse mpg. Upstream sensors guide fuel trim; downstream ones monitor the catalyst.

Common clues

• Stored fuel-trim or O2 circuit codes
• Rotten-egg smell if the cat is overworked
• Rough idle after cold start
• Failed emissions readiness monitors

Don’t guess the catalytic converter first

A dead upstream sensor can damage a cat over time, but many cars only need the sensor and a wiring check. An OBD live-data session tells you which bank and which sensor.

Book engine diagnostics on CarFix and bring any code printout you already have.