I agree with Bob. A fairly common problem, usually due to a dirty/faulty thermal probe. Not a DIY job. Get it fixed when you put the 'van in for its annual service.
AcmeUK
July 2010
Sounds like it needs a thermocoupler get it checked by gas eningeer who is registered for working on LPG appliances. I'm telling you this I may be wrong that's why it should be checked by profesional.
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Try fixfault.com to fix caravans/motorhomes.