The answer to 'my heat exchanger keeps shutting off' is that there are several common causes, including a cracked heat exchanger, issues with the thermostat or control board, or a problem with the airflow. Symptoms may include the furnace shutting off unexpectedly, uneven heating, or strange noises coming from the unit.
If the heat exchanger is cracked, it will need to be replaced by a professional HVAC technician. Issues with the thermostat or control board may be fixable through troubleshooting and repair, but should also be handled by a pro. Airflow problems can sometimes be addressed through DIY maintenance like replacing air filters, but more serious airflow issues will require professional diagnosis and repair.