If your Check Engine light is constantly trying to get your attention, one possible problem could be a bad O2 sensor. If you do have a failed oxygen sensor, follow these steps to replace it.

Diagnostics An oxygen sensor that’s going bad or has failed altogether won’t always throw a check engine light or trip a diagnostic trouble code. Sometimes if there’s an internal problem ...

Or, you can just replace your O2 sensor (s) at regular intervals, to try and prevent that minor drop in mpg sometimes caused by a "lazy" high-mileage sensor. Simple Replacement ...

We drill holes in an exhaust header to demonstrate how even the tiniest exhaust leak can affect O2 sensor accuracy and yield unreliable tuning data.

The readings of the O2 sensor could also be thrown off if there is a leak in the exhaust which could allow harmful fumes to pass through the catalytic converter and result in a failed emission test.