I've said it before and I'll say it again... Don't use Profess Autogas
You need to get it to someone who knows what they're doing but I agree it sounds like temp sensor. If they fitted the LPG I'm pretty confident the coolant feed to the vap will be wrong
My car switches over to gas within 200yds of leaving home. I've always thought this to be too quick, although I get warm air within about a mile.
Just for giggles, turn the heater on full hot next time you go out for a drive and see if it switches to LPG ok
I took a look at the 2 vapouriser coolant connections yesterday. One of the pipes disappears to the rear of the engine and is presumably tapped into the HBV. The other pipe comes up and is tapped into the pipe coming from the coolant header tank. Would that be correct connections?
I emailed Profess LPG yesterday complaining that the switch back to petrol would not be caused by the faulty injectors, if indeed they are?, but that the fault was more likely to be one of the ones suggested in this thread. They just replied that they couldn't comment until the car had been seen by their factory trained technician....It just has been! Have cancelled the replacement of the injectors with them.
Regarding your suggestion of driving away with the heater on full. Presumably we would expect the car to not switch to LPG as there would not be enough heat to the vap?