After fitting a decat pipe to my focus the engine light is on; as i understand it, due to the ecu not seeing a differential in the readings between the pre and post cat o2 sensors.
I have had a look on the net and it seems that the signal given from the o2 sensor is a voltage one, so i was wondering if:

A - i could manipulate this signal, fit a resistor inline to alter the signal being given from the o2 sensor to make it appear that the cat is fitted and working ok.

B - The post cat o2 sensor is solely for monitoring the condition of the cat and that by messing with it will not adjust the way the car fuels to adjust the emissions.

Could someone that knows about this stuff tell me if this seems feasable or even just give me some more information about the system. Likewise if anyone has done anything similar with sucess.

Cheers