P1246 code

Hi I recently bought a mk4 golf gti 1,8t AUQ

last week I was driving around and the temp gauge slowly dropped to nearly zero then engine management light came on !

I took it to my friend who put is machine on and it said cooling temp sensor fault !

I bought a sensor from euro car parts and it fixed problem, however, today I was driving car and the same thing happened again, so I got someone else to check problem and code P1246 came up on diagnostics !

I've been informed possibly a faulty thermostat !

Can anybody help pls !