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Almost always an earthing or screening issue rather than a failed sensor. Check the thermocouple screen is earthed at one end only, that the loom is not routed alongside a drive cable, and that the vessel itself has a sound earth bond. Swapping the sensor first is the common mistake.

Work the permissive chain in order: door fully closed and locked, interlock proved, cooling water available, air or nitrogen pressure present, no standing alarm, recipe selected and approved. CPC lists the blocking permissive on the start screen.

An open circuit, in nearly every case: a broken conductor, a loose terminal, or a disconnected plug in the charge. The reading is the instrument reporting an open input, not a real temperature. Check the joint at the vessel wall first.

Usually a failed level control or a heat exchanger leak putting process water into the tank. Isolate, establish which, and do not simply drain and restart — a leaking exchanger will do it again mid-cycle.

Check gas supply and isolation valve, flame detector cleanliness, purge completion and the ignition transformer. Repeated lockouts should be investigated rather than reset — a burner that needs three attempts is telling you something.

Check cooling water flow and inlet temperature first, then exchanger fouling, then the modulating valve actually stroking. On hot days an undersized cooling circuit shows up here before anywhere else.

Typically the pre-run vacuum integrity check finding a marginal port and retrying. That is the system doing its job — check the port report rather than bypassing the check.