I think I have the answer.
On Yahoo Answers Blue Rizla Girl put up the idea that the sensor is some form of hydrometer. (i.e. floats in salty water sinks if not salty enough)
Based on this thought I postulated that the sensor is some form of hydrometer float in a tube with perhaps an optical sensor.
If this is correct why is it not working?
Is it because the float is stuck at the bottom of the tube, perhaps by some food debris? If so how can we fix it?
This is what I did on my Hotpoint FWD60:-
I took the salt filler cap off. There was no salt in the reservoir as it has been a problem for months, and I was only refilling it when dishes were not coming clean.
I poured 2 litres of hot water into the salt reservoir.
I then put the empty machine on the intensive 65 degree wash, half load and with the reservoir cap still OFF.
When finished I filled up with salt and HEY PRESTO the salt light is OFF!!!!
Just hope it stays that way until the reservoir really needs refilling.
While I was down on my hands and knees I took the opportunity to clean the filters in the sump. They do get full of bits!
I also flushed the sump by dashing a couple of cups of hot water into it and then bailing it out with a small yoghurt pot. I repeated this until the water in the yoghurt pot and the sump no longer had any signs of bits in them. (Dashing = vigorously pour)
I was amazed at how many bits I got out, especially considering I had given the machine a good clean in the past 6 weeks.
I even got part of a paper label!
So there we have it, try it out and report back.
AcmeUK, December 2009