We were using a Fujitsu Mini Split Heating and Air Conditioner with a remote controller. First, they didn’t seem to be working because the IR controllers just beep when a signal is received, then we got a strange beep with the three lights just flashing. Well, fortunately, we could query the internet. The manual was completely useless, but the troubleshooting said that this meant an invalid code or a simpler the remote was low on power.
Fortunately, we have two identical remotes, so discovered that was the problem (the remotes are not coded to specific units which are nice). And it worked. The biggest issue is that Fujitsu in its infinite wisdom put a screw on the back so you need to have a small screw driver to change it which is crazy, so just beware.