Most Common Meaning
On popular UK brands like Ideal and Glow-Worm, the F9 code means the boiler has detected a problem with the water pressure sensor or that system pressure is outside the safe operating range. The boiler locks out to prevent damage.
Common Causes
- Low system pressure — often from leaks, recently bled radiators, or natural pressure loss over time
- Faulty water pressure sensor — the sensor itself may be worn, blocked by debris, or giving false readings
- Worn expansion vessel — fails to absorb pressure fluctuations, causing sensor trips
- Wiring/connection faults — loose or damaged wires between the sensor and control board (PCB)
- System blockages or airlocks — restricted water flow can trigger related pressure/flow errors on some models.