AM-21 Off-Air Receiver shows as removed. - DirecTV DVR

For AM-21 Off-Air Receiver shows as removed., relating to DirecTV DVRs and receivers. My AM-21 recently quit working properly. When you tune to an off-air channel, the screen goes black and shows off-air tuner (either 1 or 2) removed. Also, the green power light stays on all the time, even when you turn the power off on the HR21-700 receiver. I have checked all the cable connections. I have two other AM-21's that are working fine.