Originally Posted by SwissCheese

As someone else stated, it stands for "Check Engine Light". If you're just sticking with the catback exhaust, then you shouldn't have to worry about it. The problem comes with when you replace the catalytic converters with either High-Flow Catalytic Converters or Test Pipes.

There are sensors before and after the catalytic converters that measure how well the catalytic converters are filtering the air. When you replace the cats with High-Flow Cats or Test Pipes, they air doesn't get filtered nearly as much (or at all), so the computer thinks something is wrong and triggers the CEL.

Using power tools to remove the old piping without removing the sensors can ruin the sensors, also triggering a CEL.