I have an audi a4 avant the temp guage is normal

common problem, heatercore needs cleaning, remove both hoses going to heatercore and run water through one hose at a time, for about 30 seconds,use water hose with a nozel spray with hose fully open , many places would just sell you a heatercore which would cost about $1000.00 when all it needs is a backflush,( cleaning) good luck, hope this helps.