I based my suggestion on the fact that this code can get set and become a historical code when someone tries 'troubleshooting by disconnection'.
A better idea might be to know if this code is showing up while the engine is running (no hour meter reading visible, and is an active error code), or if it is showing up only when the key is first turned on and goes away after the engine starts (a code in history, but not an active error).
Since what sets this code usually would prevent the engine from running normally (or at all), and no mention was made in the original post of performance, only of the code, I assume it is a historical code, either caused by an intermittent circuit break or 'troubleshooting by disconnection' while the key was in the on position. [and I can obviously spell assume with the help of me speel checker ;-) ]

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

I would clear the code and see if it comes back before I spent much time checking that code, and if it comes back soon, then go through what Kevin_t said.

E35 is open circuit to the LPG fuel injector....in the pigtail harness at the injector is a fuse check this first....then check your relays and fuses in the black/blue/red rectangular boxes.....one might be under the left or right foot step the others will be close to the battery.......I would also ohm the injector coil and see if it is open should read LOW ohms but not be open or have HIGH resistance