Fireye NX6300 (Nexus) fault F41: Boiler sensor fault or safety limit exceeded
F41 covers two very different situations: the measured boiler pressure or temperature went above the controller’s safety limit, or the sensor stopped responding correctly. The subset separates them, and only subset 4 is a real limit exceedance.
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
- Any status; most often while firing at high load or as load drops suddenly
Reading the subset
| Subset | What it tells you |
|---|---|
| 1 | Sensor feedback below 1 V |
| 2 | Sensor feedback above 5 V |
| 3 | Sensor failed during its test |
| 4 | Safety limit exceeded |
| 255 | CANbus communication fault to the sensor |
Where it sits in the burner
Supervised function: Limit and interlock supervision.
| Component | Signals to look at | How it fails |
|---|---|---|
| Steam pressure / temperature sensor and control loop | Steam pressure, Boiler water temperature, Firing rate | Transmitter or sensor failure; Communication wiring fault; Control loop tuning or strategy |
| Operating and safety limits | Running interlock / limit string, Steam pressure, Boiler water temperature, Drum / water level | Operating or safety limit open; Pressure or position switch out of adjustment; Welded, jumpered or miswired contact |
| Controller communication network | Burner sequence state | Communication wiring fault; Electrical noise on signal wiring |
Likely causes and checks
1Real over-pressure or over-temperature (subset 4)
- Points to it
- Steam demand dropped suddenly or the burner stayed at high fire; pressure trend rises to the limit.
- Check
- Trend steam pressure or water temperature and firing rate before the trip; check the modulation setpoint and PID against the safety limit.
2Sensor or wiring fault (subsets 1–3)
- Points to it
- Pressure reading jumps or reads full scale or zero.
- Check
- Check sensor wiring and supply, and that no analog input exceeds 5 V.
3CANbus fault (subset 255)
- Points to it
- Other CAN devices also show errors.
- Check
- Check CANbus wiring and termination to the sensor.
Telling it apart
- Subset 4 is a process event; subsets 1–3 and 255 are instrument faults. Treat a subset 4 trip as a real over-pressure until proven otherwise.
Evidence to collect
- Subset
- Steam pressure or water temperature trend before the trip
- Firing rate trend
- Sensor model and wiring
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.