1999 Suburban LT 5.7L

 

If the rear heater works, your engine temp is OK. Using a temperature gun, measure the temp on the hoses going in and out of the front heater. With the dial set to full heat and the engine up to operating temp see what the temps are of the two hoses. They should both be hot. If not, you might have a plugged heater core. Can’t recall if there is a valve in the heater core hose but check a repair manual for that. If both heater hoses are hot, the heater blend door may be to blame. Use the search function on the forum to check out the remedy for a malfunctioning blend door and/or blend door actuator.

Let us know the results. Good luck.

SD