Why does one of the door safety sensors have a flashing light instead of solid?

HVAC & Fireplace Bucket EXPERT August 07, 2008

A dim or intermittent LED is an indication of a misalignment of the sensor "eyes." The dim LED or blinking LED indicates that the signal being received from the other sensor is weak or intermittent due to alignment or interference.

If you are certain the alignment is correct and not blocked or thrown off by another light source, then there is a possibility that one of the sensors is failing. It could be the one flashing but it could also be that the sending sensor is not putting out a strong enough signals to be picked-up properly. There is also a chance the logic board, which controls the sensors, is failing.

Unfortunately, the only way to determine if it is a sensor or logic board is through a process of elimination, which can be quite costly. So it is worth the extra time to keep playing with the alignment or even have a professional adjust it to be sure that is not the problem. Sometimes a loose mount or other problem can throw off the adjustments and complicate the process.

What a technician would normally do in this situation is use a working pair of sensors they had for testing purposes and hook them directly to the overhead unit with the sensor "eyes" held together. If the problems are still present it is likely a logic board issue since this test bypasses the sensor alignment and wiring possibilities.

If the problem went away, the tech would then take your sensors and attach them direct to the overhead unit to repeat the test and see if it is a problem in the wiring, if not then he would test each sensor against his testing ones to see which one might be failing and so on. It is the only certain way to pinpoint the cause and, for most people, not very cost effective as you might end up buying a lot of parts you don't need.

There are some specifications for voltages going to the sensors from the Logic, or control, board in the overhead opener. If you measure across the terminals the sensor wires go on (typically number 2 and 3) on the Logic Board you should read around 5 VDC. If you do not have that voltage there then the Logic Board may have failed but, unfortunately, that does little to help with diagnosing the sensor or sensor wiring in this case. Since the sensor LEDs are already lit, we know some voltage is reaching them and any drop that was enough to alter the sensor performance will most times cause the LEDs to go out as well.

One exception could be an intermittent connection that breaks only momentarily due to a pinched or partly broken wire going from the overhead unit to a sensor. But, again, if you could duplicate the loss of connection well enough to pick it up on a meter it would also be visible on the LEDs.

You could start by testing your sensors eye-to-eye at the overhead unit and at least get an idea if you have a wiring or logic board problem or what else you might be dealing with. That might require cutting the wiring in some cases and, if so, be sure to leave enough wire to reattach the sensor if it is still good or to attach any replacement sensor you might need.