Fireye NX6100 (Nexus) fault F43: Gas pressure limit exceeded, or sensor fault
F43 means the controller’s own gas pressure sensor reads outside the operating limits set for the burner, or the sensor stopped responding. The subset separates low pressure, high pressure and sensor faults.
Diagnose this with AI
Try Free Boiler Diagnostic Upload a keypad photo, the fault history or a parameter export from your Fireye NX6100 (Nexus). InduQT compares the evidence and suggests the next useful checks.
When it happens
- Any status where gas pressure is supervised, typically from pilot ignition through modulation
Reading the subset
| Subset | What it tells you |
|---|---|
| 1 | Pressure too low |
| 2 | Pressure too high |
| 10 | Gas pressure sensor not responding correctly |
| 255 | CANbus connection fault |
Where it sits in the burner
Supervised function: Main fuel delivery.
| Component | Signals to look at | How it fails |
|---|---|---|
| Fuel supply and regulator | Main gas pressure, Oil pressure | Fuel supply pressure droop; Regulator out of adjustment or sticking |
| Controller communication network | Burner sequence state | Communication wiring fault; Electrical noise on signal wiring |
Likely causes and checks
1Supply pressure low (subset 1)
- Points to it
- At high fire or when other users start; utility or plant supply issue.
- Check
- Check supply pressure upstream of the burner regulator; the manual’s first instruction is to check the gas supply pressure.
2Regulator lock-up or creep (subset 2)
- Points to it
- High pressure at light-off or after shutdown.
- Check
- Have the regulator inspected for lock-up and creep.
3Sensor or CANbus (subset 10 or 255)
- Points to it
- Reading jumps or is missing.
- Check
- Check the sensor and its CANbus wiring.
Telling it apart
- F43 is the controller’s analog gas pressure limit; a discrete low or high gas pressure switch usually shows as an external alarm (F01–F09, F12, F13) depending on wiring.
Evidence to collect
- Subset
- Gas pressure trend around the trip
- Supply pressure upstream of the regulator
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.