01 GTI check engine and low idle
 

i bought an 01 gti 1.8t with 101,000 and after less than a day the check engine came on and also noticed the idle low. went to advanced auto and had the code checked. it was for coolant temp sensor. so i bought it and had it replaced. approx 50 miles and like came back on again. the mechanic said his computer said it was running to lean. what can cause it to run too lean? and what would give me the same reading after changing the cts? thanx for any advise or help

i would try a higher octane fuel. and changing the oil to a differnet synthetic.