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NTI Error Code FILL – 108: Low water pressure lockout (Boiler)

Updated · by Aurora Star Tech Inc.

NTI error code FILL - 108 on a boiler means: Low water pressure lockout. It is also called NTI diagnostic code FILL - 108.

High severityBoilerTRX

Homeowner? What NTI code FILL - 108 means for you

Call a technician Your boiler is reporting: Low water pressure lockout. The boiler has locked out or cannot run properly. One reset is fine; if the code comes back, it needs a licensed technician to find the cause.

Safe things you can check yourself

  • Check the thermostat: right mode (Heat or Cool), set a few degrees past room temperature, and fresh batteries if it uses them.
  • Make sure the unit has power: its on/off switch (often looks like a light switch near the unit) is ON and the breaker in the electrical panel is not tripped.
  • Look at the pressure gauge on the boiler. If it reads much lower than usual or near zero, or water is leaking, call a technician.
  • Outside, make sure the intake and exhaust pipes or vent cap are not blocked by snow, ice, leaves or a nest.
  • Check that the boiler gas valve is open (handle in line with the pipe).
  • Try one reset: turn the power off for 30 seconds, then back on. If the code comes back, stop resetting and call a technician.

Call (647) 657-2977Book a service call

Licensed technicians serving Vaughan, Toronto and the GTA.

What does NTI error code FILL - 108 mean?

If water pressure drops below 7 PSI, the control shows this blocking error and stops burner operation.

Applies to: TRX150, TRX199, TRX199C

Common causes of NTI code FILL - 108

  • Water pressure in the system drops below 7 PSI
Safety: Gas, electrical and refrigerant work must be done by a licensed technician. Turn off power and gas before servicing, never bypass a safety control, and always follow the manufacturer's manual for your exact model.

NTI models that use code FILL - 108

TRX085TRX110CTRX150TRX199TRX199C

Other NTI Boiler codes

CodeMeaning
Alert 496Internal error: Unknown safety error occurred
Alert 497Internal error: Safety timer was corrupt
Alert 498Internal error: Safety timer was expired
Alert 499Internal error: Safety timings
Alert 500Internal error: Safety shutdown
EMERGIncorrect wiring to BUS connection (emergency mode)
Hold / Lockout 9Flame bias out of range
Hold / Lockout 13Internal fault: PII shorted to ILK
Hold / Lockout 14Internal fault: HFS shorted to LCI
Hold / Lockout 67ILK off - interlock string open
Hold / Lockout 79Outlet high limit
Hold / Lockout 81Delta T limit

All NTI Boiler error codes →

NTI code FILL - 108: homeowner questions

What does NTI code FILL - 108 mean?

Low water pressure lockout. If water pressure drops below 7 PSI, the control shows this blocking error and stops burner operation.

Is it safe to keep using my boiler with code FILL - 108?

It depends on the cause. The boiler has locked out or cannot run properly. One reset is fine; if the code comes back, it needs a licensed technician to find the cause.

Can I reset NTI code FILL - 108 myself?

You can try one reset (power off for 30 seconds, then on). If NTI code FILL - 108 comes back, the cause is still there and a licensed technician should check it.

Who can fix NTI code FILL - 108 near me?

Aurora Star Tech Inc. repairs NTI boilers in Vaughan, Toronto and across the GTA. Call (647) 657-2977 or book a service call online.

Homeowner help: Boiler Not Heating: What to Check

Troubleshooting guides

Source: NTI TRX150 manual, p. 31. Written in our own words as a summary; always check the original manual.

General guidance for qualified technicians. The manufacturer's installation and service manual and local codes always take priority. Aurora Star Tech is not affiliated with, endorsed by or sponsored by any equipment manufacturer; brand and model names are used only to identify equipment and belong to their owners.

© 2026 Aurora Star Tech Inc. All rights reserved. This page may not be copied, republished or used to train other systems without written permission.

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