I am having low heat problems. I checked the blend air door, it is operating correctly. I let my truck heat up, and it looks like the gauge is reading a little more than 1/4 the way up. I then put the temp gun on my heater hoses going into the heater core. One of the lines was at about 165 degrees, and the other was about 120-135 degrees. The blower was on low speed. Does this sound like the heater core could be plugged? I have also read on some of the posts that the thermostat could be bad and not heating up enough. Any suggestions?