It's possible but unlikely the thermocouple is just dirty  - they have a habit of splitting open after a few years - this is more likely.

The solution is to change it (I'm assuming you are competant at such things :-)). Clibegas don't carry spares but you will probably be able to get one at Blascos (near the police station in Tortosa). There are various sizes and shapes so take the old one with you. They can test it (or you can yourself if you know what you are doing). If it isn't this then it is most likely the gas valve.

You didn't say what make the boiler is - I'm assuming it's a wall hung gas water heater.

Nigel