What can I do to fix heart rate monitor of my GPS Forerunner 305?

Wear the Forerunner 305 heart-rate monitor with the printed logo right side up, just beneath the breastplate, and directly on the skin. It is very necessary that it establish direct contact to the skin and moisture is necessary for electrical connection between the heart-rate monitor and your body. It may be necessary to wet the contacts on the monitor or you can even use electrode gel so that you can get a reliable connection for your workout. Let’s say, if the reading on the heart-rate monitor is erratic, like blinking, you can also put it on the back instead of the breastplate. This will allow better contact to skin.

You can also check if the heart-rate monitor has been reset at some point or not. You can select restart scan by going to the accessories section of the setup menu.

Try cleaning the residue, in case you use electrode gel regularly as connection problem may occur due to built up on the contacts of the heart-rate monitor.

Also check for electrical interference like computer monitors, AM, FM Tuner or MP3 Players. These things can create intermittent reception.

Try to bent the gold contact that touches the top of the battery (+ve)

Still if the Forerunner 305 heart-rate monitor is not receiving signals, you may need to replace the battery.