If you install a de-cat or a high-flow sports catalytic converter on your vehicle, you may get a check engine light. This is normally because the sensor is sensing the increased exhaust gas flow and interpreting it as a malfunction, thus lighting up the check engine light.
This lambda sensor extension known as a MIL cheater elbow may address the problem quickly, but getting the car remapped is recommended to guarantee that the problem is resolved.
Usually, it is recommended to get the car remapped if you install the decat, as its likely to trigger the check engine light.
From anecdotal reports it works some of the time and some of the time it doesn't. It's generally used with a sports cat that can sometimes trigger the MIL.