My 97 gmc subrban heater quits working when its really cold out.

What is your question?

When the temp outside is moderate cold, the heater works fine. When the outside temp get to about 30°f and lower, it blows cold air. Water levels are good, hoses are good

What makes this problem better or worse?

outside temp

How long has your 2015 Chevrolet Suburban had this problem?

2 winters