The photocell is good and I have tried a replacement. The resistance of the solenoid is 2100 ohns or so and is thus within spec.
The boiler has a facility to monitor the oil pressure just before it goes into the burner jet. When the boiler is running normally the oil pressure is static at slightly over 100 psi. The jet is new.
The oil level in the tank maybe a few inches below the level of the burner pump, but was 4 inches lower last year before I filled it up.
The Control box is a Honeywell Satronic DKO 972-N
What do you think the fault is? Which component would you replace.