From the manual (First Google result when I searched) there are 2 LEDs on the receiver.
Top centre is the Signal LED.
Centre right is the Relay display LED.
Relay display LED is either off, yellow or flashing yellow. I assume that's not the one you are on about, though it would have been nice for some definite description.
Signal Display (RF) LED is red in the following conditions:
1) Indicates Power up (After 5 seconds. Flashes RED + GREEN (Amber)first)
2) No RF (signal from the remote unit) within the past 25 minutes
3) Software reset
Based on 2) above, no RF from the remote unit within the past 25 minutes could be failing batteries or, possibly interference (unlikely though I have had issues with some baby monitors. With on e particular one the receiver (yes, that's right, the receiver) caused a lot of interference and swamped my WiFi signal.
You could also try a reset and re-learn. See the manuals at:
http://w3.siemens.co.uk/buildingtechnologies/uk/en/building-automation/residential-controls/documentation/Documents/RDJ10RF%20Data%20Sheet.pdf
and
http://w3.siemens.co.uk/buildingtechnologies/uk/en/building-automation/residential-controls/documentation/Documents/RDJ10_1.pdf
Gladys, February 2015
