Temp outside has been 20 degrees at night I WARM.

Temp outside has been 20 degrees at night I WARM CAR UP IN MORNING FOR 15 MINUTES TEMP GAUGE USED TO READ BETWEEN COLD AND HALF AFTER WARM UP NOW AFTER 15 MINUTES IT READS JUST ABOVE COLD HAS A FAINT WARMTH COMING FROM VENTS WOULD THIS BE A THERMOSTAT ISSUE OR POSSIBLY SOMETHING LARGER (2) TOOK CAR TO DEALER FOR OIL CHANGE I WAS TOLD THERE MAY BE A SMALL CRACK IN HEAD GASKET COST TO FIX $1,500.00 IS THERE ANY WAY TO CHECK THIS OUT ON MY OWN BEFORE PAYING THAT KIND OF MONEY IF IT IS NOT NEEDED THANK YOU!