Reducing 12v DC to 0.5V DC no more than 1A draw

[birchy]

To be honest, I think you'd be better off using a small AA battery or similar for the 0.5V signal. It will certainly be a lot safer as you don't want to be shorting out a battery with 300A+ peak current!

[birchy]

On further thinking...I don't quite understand what you are trying to do here? If you have a suspect O2 sensor (i.e. the output signal voltage is not within tolerance), then it's faulty. If it's not faulty, then you can use the (0.5V?) signal from it to test the ECU or whatever.

Might be worth making yourself a little variable power supply. Basically all you'd need is a 9V PP3 battery and a potentiometer.