Coolant temperature sensor

Hello I've had my temperature light in my car come in for a while after reading a few threads about changing the sensor I have done, light went of until I turned my car off and on again now it won't go away.

I didn't buy my sensor of eBay for around £8, not sure if it's because I bought a cheap sensor or not

Can anyone please advise me on what do do

Note: I have also scraped the prongs in the expansion tank as a few people have said to do this

Thanks