Heater

There's two things that can cause no heat to come from the heater core; (1) no coolant flow or blocked heater core, (2) the blend door is stuck in the AC position. Touch or squeeze the heater hoses under the hood, do they feel hot? Are the hoses pressurized? If yes then it's a safe bet you have flow to the core. Next, try moving the temp selector between cold and hot. Can you hear or detect the blend door moving? If not, the actuator may have went bad. Getting to the actuator and blend door is a challenge, as it's inside the dash, sitting on top of the heater core box. You *might* be able to reach it by removing the air bag and/or glove box, and manually putting it in the heat position as a temporary fix. Actuators run about $35. If your heater core itself is bad, then you have no choice to pony up and pay for a new one and the labor to install it (which involves removing the dash). Check on a Tahoe forum for the exact procedure, some vehicles make it easier by going through the firewall rather than the dash. You may also want to verify the electronic controls for the blend door are working first before tearing anything apart, as the fault could lie with the selector switch.