check the corresponding black box is slowly blinking red (one blink every 3 seconds) and within range of the gray box (10ft for athletic, 150 ft for equestrian)
if the red light is blinking fast, you must replace the batteries with new ones
align the black and gray boxes so the light on the gray box blinks green
bring the black box closer
change the batteries on the black box
check the lenses on the timers are clean and free of scratches. Clean only with an eyeglass wipe, never paper or your shirt. rubbing sand will scratch the lenses and render them opaque

check the corresponding black box is slowly blinking red (one blink every 3 seconds) and within range of the gray box (10ft for athletic, 150 ft for equestrian)

if the red light is blinking fast, you must replace the batteries with new ones