It looks like a high temp cutout switch. Also known as a flame rollout switch.
If that is tripping you have a very unsafe condition.
The flames from your burners are rolling back out towards the gas valve and front of the burners and hitting it.
Could be the insides of the boiler ( flue passages ) are partly plugged with soot or rust or scale. Or you have a blockage in your chimney and the smoke cannot get out fast enough.
Or you have a negative pressure in your house caused by too much exhausting from a fireplace or exhaust fans etc.
Point is you should get the problem checked by a Pro and not keep resetting it.
You could also get CO, Carbon monoxide poisoning and spillage if it is not burning cleanly or fumes are spilling into your house.
It is also possible the burner is overfired but that is very rare.
If that is tripping you have a very unsafe condition.
The flames from your burners are rolling back out towards the gas valve and front of the burners and hitting it.
Could be the insides of the boiler ( flue passages ) are partly plugged with soot or rust or scale. Or you have a blockage in your chimney and the smoke cannot get out fast enough.
Or you have a negative pressure in your house caused by too much exhausting from a fireplace or exhaust fans etc.
Point is you should get the problem checked by a Pro and not keep resetting it.
You could also get CO, Carbon monoxide poisoning and spillage if it is not burning cleanly or fumes are spilling into your house.
It is also possible the burner is overfired but that is very rare.