does anyone know how these work. Mine has stopped working properly. The warning light flashes a few times the goes off.

I've got an earth and 5volts. Is this right. I've had a look at the sensor and can't see how it works. There is a bit of metal on the flap part of the sensor. Does this rise up and something inside the sensor reckonise it?

I just wanna confirm its the sensor.

Thanks.