I have 1999 Seville STS with 140,000 miles. Very well maintained. Zero codes. I have to add a gallon of coolant about every 6 to 8 months. No visual signs of leakage so I assume it's going onto the street. It doesn't ever run hot. I get message "check coolant level" about every 6 to 8 months. The heater doesn't get very hot which makes me think they're related. Any suggestions to fix my problem would be appreciated.