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NTI Error Code Hold 21: Internal fault: Flame number of sample mismatch (Boiler)

Updated · by Aurora Star Tech Inc.

NTI error code Hold 21 on a boiler means: Internal fault: Flame number of sample mismatch. It is also called NTI diagnostic code Hold 21.

Medium severityBoilerAlso BoilerTrinity Tft

Homeowner? What NTI code Hold 21 means for you

Check, then call Your boiler is reporting: Internal fault: Flame number of sample mismatch. This is often caused by something simple. Try the safe checks below; if the code returns, book a service call before it turns into a no-heat or no-hot-water emergency.

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 Hold 21 mean?

Control board self-check reported an internal problem: Flame number of sample mismatch.

Applies to: Trinity Tft60, Tft85, Tft110, Tft155, Tft175, Tft200, Tft250, Tft300, Tft399

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 Hold 21

Tft60Tft60Tft85Tft85Tft110Tft110Tft154Tft155Tft175Tft200Tft201Tft250Tft285Tft300Tft340Tft399Tft399

Other NTI Boiler codes

CodeMeaning
Hold 8Internal fault: Zero crossing not detected
Hold 8Internal fault: Zero crossing not detected
Hold 12Internal fault: Safety relay drive cap short
Hold 12Internal fault: Safety relay drive cap short
Hold 20Internal fault: Flame ripple and overflow
Hold 20Internal fault: Flame ripple and overflow
Hold 21Internal fault: Flame number of sample mismatch
Hold 22Internal fault: Flame bias out of range
Hold 22Internal fault: Flame bias out of range
Hold 23Internal fault: Bias changed since heating cycle starts
Hold 23Internal fault: Bias changed since heating cycle starts
Hold 24Internal fault: Spark voltage stuck low or high

All NTI Boiler error codes →

NTI code Hold 21: homeowner questions

What does NTI code Hold 21 mean?

Internal fault: Flame number of sample mismatch. Control board self-check reported an internal problem: Flame number of sample mismatch.

Is it safe to keep using my boiler with code Hold 21?

It depends on the cause. This is often caused by something simple. Try the safe checks below; if the code returns, book a service call before it turns into a no-heat or no-hot-water emergency.

Can I reset NTI code Hold 21 myself?

You can try one reset (power off for 30 seconds, then on). If NTI code Hold 21 comes back, the cause is still there and a licensed technician should check it.

Who can fix NTI code Hold 21 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 Tft60-399 Installation and Operation Instructions (rev. 2012-04-09), Sec. 15.0 Troubleshooting, Table 15-3. 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.

Emergency Boiler Repair

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