Fireye NX6300 (Nexus) fault F11: False Flame
F11 means the controller saw a flame signal when there should be no flame, for example before purge, during purge or after shutdown, or while a shuttered scanner’s shutter was closed. The controller cannot tell a real flame from a faulty signal, so it locks out.
Diagnose this with AI
Try Free Boiler Diagnostic Upload a keypad photo, the fault history or a parameter export from your Fireye NX6300 (Nexus). InduQT compares the evidence and suggests the next useful checks.
When it happens
- Status 01–02: burner off or waiting for purge
- Status 05–09: proving positions and pre-purge
- Status 10: pre-ignition
- Status 17–18: post-purge
Reading the subset
| Subset | What it tells you |
|---|---|
| 1xx | Flame 1 (the first flame input); the last two digits are the burner status number |
| 2xx | Flame 2 (the second flame input) |
| x01, x02 | Burner off or waiting to purge: flame seen at rest |
| x05 to x09 | During closed/air/purge position proving or pre-purge |
| x10 | Pre-ignition, with the igniter on but no fuel |
| x17, x18 | Post-purge, after the fuel valves closed |
Where it sits in the burner
Supervised function: Flame supervision.
| Component | Signals to look at | How it fails |
|---|---|---|
| Flame scanner (UV / IR) | Flame signal | Scanner no longer sighted on the flame root; Detector or amplifier ageing; Electrical noise on signal wiring; Dirty or fogged scanner sight path |
| Flame amplifier / flame input | Flame signal | Detector or amplifier ageing; Electrical noise on signal wiring; Wrong detector, amplifier or configuration |
| Main safety shutoff valves | Main fuel valve output, Valve proof of closure | Valve seat leakage or slow closing; Fuel valve not opening fully; Welded, jumpered or miswired contact |
Likely causes and checks
1Fuel still burning after shutdown (subset x17–x18 or x01)
- Points to it
- Visible flame after the cycle; oil after-drip or leaking safety valve.
- Check
- Look into the furnace. If flame persists, close the manual fuel valves and have the safety shutoff valves tightness-tested; the valve leak test (if fitted) helps here.
2Detector seeing ignition spark (subset x10)
- Points to it
- Only at pre-ignition when the spark starts.
- Check
- Re-sight the scanner so it cannot view the spark; check scanner and ignition cable routing.
3Failing UV tube or self-check detector
- Points to it
- Signal in a dark, cold furnace; often accompanied by F66 events.
- Check
- Cover the scanner with the burner off; any remaining signal means a detector or input fault.
4Noise pickup on the flame input
- Points to it
- Intermittent, coinciding with motor or ignition switching.
- Check
- Check screen termination and separation of the scanner cable from power and ignition wiring.
Telling it apart
- F11 is flame when none is expected; F66 is the controller’s own dark test failing, which can precede F11 as a UV tube fails.
Evidence to collect
- Fault prefix and subset
- Event log
- Scanner model (UV, self-check UV, IR)
- Whether flame is visible in the furnace at the time
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.