engine light problems?

i have recently purchased a 53 plate golf gti. light came on on the first day i purchased, private sale sold as seen. a mobile mechanic diagnosed a temperature sensor and showed me how to fit it.a few days later the light came on again, i cancelled it and it stayed off for a week, its now on again. my thinking is if it takes the system a week to show the fault its intermittent or not that obvious any advice appreciated.the car has done 85,000 miles and its an 1800cc turbo.