I think disconnecting wont tell you anything. The error will different, i.e. "Open Circuit", not "Invalid Signal". What the ECU does it the specific instances may be different, which may mask the the diagnostic process. I.e. it runs different when its disconnected anyway, regardless of the sensor being faulty. I agree with Paul, its not like the sensors have a good reputation, so its a good shout. A genuine Bosch part is going to cost somewhere in the region of £70 - £90 (more for Ford branding). There are cheaper options out there. But my method would be to have the ECU tell you the problem, then replace the sensor as needed, I know this adds £100ish to the eventual cost of repair, but this is to get the tools for the Job. If you purchase the oxygen sensor, and its not that, then you would have spent the money anyway, and I bet you can sell the lead a few years down the line if you want some of your money back.