this "fault", since it sets no codes, means the engine computer thinks it is a "normal" situation, such as when the key switch is turned off, naturally you get no code because of that. (or the LPG fuel running out before the pressure sender).
Here is the thing about "swapping the relays around", the engine computer gets it's power through 2 different (and not next to each other) relays that both have to work, and if either 1 does not work, it is possible the computer sees it as "switched off". In my opinion, it is far better to try a "known good" relay, one socket at a time, or swap out all the relays together.
You may have a bad relay box even, it would not be the first one that ever proved bad.
Someone should also tell you not to use a test light on a 'pulsed signal' like the injectors or fuel trim valves, you can let the smoke out of the black boxes.