Just as info I had this
problem for ages, replaced the sender for a green on from ebay for £6-7 and it made no difference.
Had a fault code which the garage logged as a faulty sender. But I thought it was the thermostat since I'd replaced the green thing. They replaced the thermostat as part of a whole range of work they did for me the other day and said the fault didn't seem to resolve, so replaced the sender too.
It's now resolved which is great but for some reason they charged me £21 for the temp sender unit, seems odd if it's the same thing I paid £6-7 as I'd be surprised if they took the piss seeing as they charged me £9 for the 'stat and generally are the most trustworthy bunch I know.
Is is likely the unit of ebay was crap? or are we talking two different things here? the receipt shows 4 pin temp sender unit, which I can only assume is the same thing! Maybe I fitted it wrong :s...or it was the 'stat.
In addition, I was getting 120-30 from half a tank, I've done 170 and not reached half way yet, I know things aren't linear but it feels like I might make 50-100 extra per tank....as Lee said a while back, how can you afford NOT to do it!