engine light staying on also lpg filling cap

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Hi i have just picked up a hijet 16v efl lpg
van today. I have just drove the van up from Bristol to leeds the van is quite a cumfy ride. And quite nippy its on a o3 plate.
the only thing is well two things really. One the engine light came on just after the lpg gas ran out so I switched it to petrol, is this common for the engine light to come on and stay on.
also two I dont have a key for the LPG gas cap to open it so I can fill it up and the guy I got the van off as stopped answering the phone.is there a way I can get a replacement key for this. Hope anybody can help thanks mel.