Really need a bit more info on model of fridge but I'll try to start you off.
Is it automatic changeover or do you have to switch the energy supply manually? If you select the 12v system (you'll need the engine running and the alternator charging) do you get a fault code on the display panel and if so what is it? Your fridge manual should have a list of fault codes and thus should point you to the problem.
First thought was a failure of the 12v element, but a bit surprising if it is given the age of the fridge. You won't get a replacement in Spain - I've been trying to get a 240v element and they have to be ordered from the UK.
Thanks for the reply's I have checked all fuses I have tried to set the 12 volt manually and it still wont work I plug into 240 volt changes to 240 volt I unplug and start the engine it tries to work on 12volt but gives up and goes on to gas I plug the mains in and it goes back to 240 volt.
The Fridge is Thetford model number N3142 automatic change over.
The on off switch on the fridge shows a red light when trying to start 12 volt.
Still in Spain I think I will have to get this sorted when I get home in a couple of weeks. Its still under warrantee
There will be 12V at the lower of these terminals if the fridge works at all, because this is the feed for the circuit board (fridge intelligence).
The fridge will not run on 12V until the ignition is switched on, when 12V should appear at the terminal above (D+). If there is 12V here and the fridge does not stay cool, the (12V) element should be checked.
Maybe you overestimate the capability of the 12V mode? It is only sufficient to keep the fridge cool (ticking over) whilst on the move (the ignition is on).
I'd check that the fridge waits around 20min before working on gas after running the engine. It should do this to avoid lighting the gas when you stop, in case you're at a petrol station. If it does, then you know that the D+ feed is working without the need to find it (or a multimeter). I suspect that, given the age of your fridge, everything is working as it should but you are expecting too much from the 12V side.