The usual causes are:
LPG Prime solenoid
Testing:
If the Solenoid relay fail fors any reason, LPG mode will not be available.
1. Remove the connector to the PRIME solenoid on the LPG converter.
2. Measure the resistance across the PRIME Relay Test solenoid using a suitable multimeter. Prime Solenoid Resistance Specification 10-14 Ω.
3. Check the harness for open circuits and short terminals using a suitable multimeter.
Gas Cut-out Solenoid GCO
3 solenoids control LPG flow from the tank to the LPG Stepper Motor A-D converter unit. The three solenoids are controlled by the Gas Cut-Out (GCO) relay. The GCO relay is energised by the
PCM, energising the solenoid control relay, which then energises to activate the Tank Lock-off, Engine Bay Lock-off and Idle Circuit solenoids on the LPG fuel line.
Testing:
Relay Test
1. Remove the GCO Relay.
2. Measure the resistance across the GCO relay coil
3. Check the harness for open circuits and short terminals using a suitable multimeter.
GCO Relay Coil Resistance Specification 60-120 Ω
3. Check the harness for open circuits and short
LPG Stepper Motor A-D
Can only be tested with a diagnostic tool.
It will also likely be throwing DTC codes so it might be worth scanning it.