The answer to 'my heat exchanger won't turn on' is that there are several common causes, including a faulty thermostat, a problem with the furnace's control board, or a blockage in the heat exchanger itself. Symptoms may include the furnace not starting, the blower fan not running, or a lack of warm air coming from the vents.
To troubleshoot, first check the thermostat to ensure it is set to 'heat' and the temperature is set higher than the current room temperature. If that's not the issue, you may need to inspect the furnace's control board or blower fan for any signs of damage or malfunction. A blocked or cracked heat exchanger is a more serious problem that will require professional repair or replacement.