Fireye NX6300 F66 Flame Test Failed, what to check first?

Question

I'm getting an F66 fault on my Fireye NX6300 burner control system. It happens during firing, and I've seen some F11 events before this. What should I check first?

Top answer · Written by InduQT from the fault guide and manufacturer manuals cited below · Oct 28, 2026

The F66 fault on a Fireye NX6300 indicates that the controller's internal flame input test failed, meaning it can no longer trust the flame signal. This test ensures the system reads 'no flame' when it should, often using a scanner shutter or the detector's self-check mechanism. A common cause is a failing UV tube or a stuck shutter, which can lead to a false flame signal. This fault is distinct from an F11, which signifies an actual flame detected at an inappropriate time.

Likely causes and checks

1UV tube failing or shutter stuck

Points to it: A failing UV tube can falsely indicate the presence of a flame, or a stuck shutter can prevent the 'no flame' condition from being properly tested. This is often indicated by Subset 1 or +7, and may be preceded by F11 events.
Check: Test the scanner outside the burner as directed by the manual. If the dark check still fails, replace the scanner.

2Flame input wiring short

Points to it: A short in the flame input wiring can cause an incorrect flame signal, leading to the F66 fault. This is typically indicated by Subset 3 and may occur after recent wiring work or if the cable has been damaged.
Check: Inspect the scanner cable and its terminals for any shorts.

3Self-check detector fault

Points to it: If the self-check detector itself has an internal fault or memory issue, it can fail its own diagnostic test, resulting in an F66 fault. This is indicated by Subsets x02–x06.
Check: The manual directs replacing the detector for memory or internal faults.

4CANbus to the detector

Points to it: Issues with the CANbus communication to the flame detector can prevent the controller from properly performing the flame test. This is indicated by Subset x01.
Check: Check the CANbus wiring connecting to the detector.

Next check: Given the prior F11 events and the F66 fault, the most likely cause is a failing UV tube or a stuck shutter. You should start by testing the scanner outside the burner as per the manual's instructions.

Work on fuel trains, burners, pressure vessels and electrical equipment must be done by qualified personnel following the manufacturer’s documentation and local codes. Never bypass a safety device or interlock.