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Allied Error Code 4 flashes: High limit switch open (Furnace)

Updated · by Aurora Star Tech Inc.

Allied error code 4 flashes on a furnace means: High limit switch open. It is also called Allied diagnostic code 4 flashes.

High severityFurnaceA95/A93/95G1/92G1UH · A95/95G1UH · A93/92G1UH

Homeowner? What Allied code 4 flashes means for you

Call a technician Your furnace is reporting: High limit switch open. The furnace 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.
  • Make sure the furnace front door or panel is closed properly; many furnaces will not run with the door switch open.
  • Look at the air filter. If it is grey and clogged, replace it; a blocked filter causes many shutdowns.
  • Outside, make sure the intake and exhaust pipes or vent cap are not blocked by snow, ice, leaves or a nest.
  • Check that the gas valve next to the furnace is open (handle in line with the pipe). Do not force it.
  • 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 Allied error code 4 flashes mean?

The primary limit circuit is open. Code listed only in the manual; no causes or steps given.

Applies to: A95UH, A93UH, 95G1UH, 92G1UH (sizes 045-08 to 135-20)

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.

Allied models that use code 4 flashes

92G1UH92G1UH0450892G1UH0451292G1UH0700892G1UH0701292G1UH0901292G1UH0901692G1UH1101692G1UH1102092G1UH1352095G1UHA93UHA93UH1DA95UHA95UH1D

Other Allied Furnace codes

CodeMeaning
1 flashFlame sensed with gas valve off
2 flashesPressure switch closed with inducer off
3 flashesPressure switch open with inducer on
5 flashesRollout switch open
6 flashesPressure switch cycle lockout
7 flashesLockout - no ignition
8 flashesLockout - too many flame dropouts
9 flashesIncorrect line voltage phasing
LED offNo power to control or control hardware fault
LED onNormal operation

All Allied Furnace error codes →

Allied code 4 flashes: homeowner questions

What does Allied code 4 flashes mean?

High limit switch open. The primary limit circuit is open. Code listed only in the manual; no causes or steps given.

Is it safe to keep using my furnace with code 4 flashes?

It depends on the cause. The furnace 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 Allied code 4 flashes myself?

You can try one reset (power off for 30 seconds, then on). If Allied code 4 flashes comes back, the cause is still there and a licensed technician should check it.

Who can fix Allied code 4 flashes near me?

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

Homeowner help: Furnace Not Turning On: What to Check · Furnace Blinking Light: What the Flash Codes Mean

Troubleshooting guides

Source: Allied 506724-01 (Issue 1108) A95UH/A93UH/95G1UH/92G1UH Installation Instructions, Diagnostic Codes p.53. 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 Furnace Repair

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