there is a HPK document out on this code
DTC E35

Low or no 12v power supply detected

A detection of an abnormal electric current in the LPG injector driving circuit. Will set a trouble code and turns the MIL lamp on.

In other words the injector circuit is malfunctioning, this could pertain to many components in that circuit and not specifically the injector. One of the most common issues cat has had with these trucks is the relays in the relay box. I would start there.

they list some possible causes:
- broken wires (at the injector)
- damaged or loose pins in the fuel injector connector or ecm connector
- failed EGI 2 relay or Main C relay
- loose connection at the base of the relay box 2 (the blue box)
- open or short internally in the relay box 2
- fuel pressure too high at the injector
- failed injector
- failed ecm

in the relay box there is a fuel pump relay that is in there but is not used, you can swap them around and see if that cures the problem. If not then you will have to do more troubleshooting but start with the relays and connections first.

Also check fuel pressure and make sure its not overpressuring the system. Sometimes the LPG converters go bad and cause this issue.

you can try what edward says and it will probably clear the code but these codes are usually intermittent but consistent