There is a valve that controls coolant flow to the heater core on the firewall. MAXIMA 97'..Where IS this??
That valve is most likely sticky or clogged. When that happens, very little coolant will flow through the heater core resulting in luke warm if any heat. You can manually open it and get all the heat you'll ever need until you replace or fix it. ANYBODY
That valve shuts off the hot water going to the heater core it is on the firewall and it has 2 cables going to it. I had one freeze up on me on my Toyota. I used PB Blaster on it. I sprayed it and let it sit for a bit, then I sprayed it again and lightly