What is a CEL fix?
Note: This article only applies to cars MY01> - you won't get any warning lights / error codes when decating anything earlier than 2001.
Your ECU has one or more sensors to detect how efficent your catalytic converter(s) is working. If the sensor records too high a temperature for an extended period of time then it will raise a fault illuminating the Check Engine Light (CEL) to tell you your catalytic converter is no longer working as efficiently as it should - great given that you've just removed it!
A CEL fix is something you would do after fitting a de-cat uppipe or test-pipe. To stop your ECU from raising a CEL, you have the option of mechanical or electronic CEL fixes. Otherwise you will end up with the CEL code P0546 = Exhaust Gas Temperature Sensor Circuit Malfunction (High Input) or the rear CEL code P0420 = Catalyst System Efficiency Below Threshold. This code is caused by the lack of a catalytic converter in the downpipe. The Electronic and Mechanical CEL fix assume that you have the stock rear O2 sesnor connected.