Can one service water heaters with a sealed combustion chamber?

The main burner won't stay lit, so I called the gas company (I am on their "guardian plan").

But the tech said I need to replace the water heater and he did not do any service.

The manufacturer says that (i) the thermocouple can be replaced, (ii) air intake screen can be cleaned (iii) flame arrestor opening can be unblocked (iv) defective gas control can be replaced etc.

What are my options?

I don't like taking cold showers. Certainly not for this reason.