The “No Heartbeat” status in your polling report indicates that there is a problem in the communication between your sensor and your data controller device (or MIU).
For wireless systems such as those using the MIU-1000, this indicates that the sensor connected to your wireless transmitter is not sending data to your wireless transmitter properly. Sensors that work with the wireless transmitter will send heartbeat information to the wireless transmitter to indicate that the sensor is still operational even when it has no count data to send. You should check all connections between the sensor(s) and your wireless transmitter and make sure your sensors are still operational.
For wired systems such as those using the MIU-4, MIU-8, or MIU-40, this indicates that the sensor(s) attached to the port indicated have not sent heartbeat information for the time period indicated. Heartbeat information is sent by the sensor to your data controller as a way of letting the data controller know that the sensor is still connected even when it has no count data to send. You should check all wiring between your sensor(s) and data controller and the make sure your sensors are still operational.