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[Angie]

The Central heating boiler fires up and the pump runs, but the flame cuts out before the last radiator in the loop has had time to get warm. The pump keeps going. The boiler fires up again after a while and then cuts out again a few minutes later - this sequence repeats. It is not the room thermostat control turning it off. I have bled the radiators but this made no diference. What else can I try.

[sparx]

seems the boiler is cutting out &in again on it's overheat thermostat, if your pump has 3speeds move lever to pos. 3 ie move more water through boiler, or try turning boiler control down a little,
the setting limits the ultimate rad temp so will seem cooler but room will still get up to temp. even if a little bit slower.
if an old system may need flushing & new rust inhibiter adding,
Sparx

[peter the plumber]

It’s a radiator-balancing problem

The lock shields need to be look at to ensure an equal distribution of hot water.

It’s a slow job, but you can do it with a small spanner.

You just open the last radiator lock shield to full, then work back to the first on one the heating system turning each one down a ¼ turn.

[Aarovic]

Could be just an air-lock. Turn off all radiators that are working and see what happens.