Primary controls: read the expected state before making a diagnosis
The primary safety control supervises a defined sequence, not merely an on-off motor command. Establish the actual model and its documentation. Identify what demand must exist, which limits and interlocks must be satisfied, whether a pre-purge or delayed oil valve is used, when ignition is energized, and how flame proof is evaluated. A motor that runs establishes only one fact. It does not prove correct fuel delivery, spark location, air movement or the control’s complete operation.
A common oil-burner flame detector is a cadmium-sulfide cell. It is light-sensitive: resistance is high in darkness and lower when the intended flame is visible to it. That optical method is different from a gas burner’s flame-rectification circuit. The detector must have its specified view, condition and wiring. An early flame-present signal may mean stray light, a sensing fault or an unexpected actual flame. A technician seeing flame through an inspection opening does not prove that the control receives a valid electrical signal. Use the exact control’s specified test and threshold rather than import a value from another model.
Distinguish sequence timing functions. Purge establishes the intended airflow condition. Trial for ignition limits an unsuccessful attempt to establish required proof. Interrupted ignition stops the ignition source after the specified establishment interval, while another design may maintain it through firing. A post-purge continues designed airflow after fuel cutoff. Recycle and lockout states are not universal permission for unlimited resetting; the control’s actual state and the chamber condition determine the safe next action.
Use circuit measurements deliberately. A schematic’s normal contact state is a reference, not a statement that every powered contact is closed. A series path is interrupted by an open required contact. In a simple energized path with a return, an open point can have supply voltage across it, while a good closed contact has little drop. More complex circuits need the full diagram. Verify voltage at the relevant point, not an unrelated energized branch.
Protection must retain its intended role. A manual-reset limit is not changed into a normal operating control merely because it trips inconveniently. Confirm replacement voltage, capacity, contact duty, timing and approval. Motor starting components and mechanical load can explain overload trips. After a repair, verify the complete affected start, run and stop sequence rather than accept one successful ignition.