Armstrong Air Furnace Error Codes & What They Mean
Compiled & reviewed by Nikolai Tsyrenov · Updated 2026-06-25
How do I read Armstrong Air furnace error codes?
Armstrong Air furnaces signal faults by flashing a single red LED on the integrated control a set number of times to indicate a numbered fault code.
How to read Armstrong Air furnace codes
Armstrong Air furnaces signal faults by flashing a single red LED on the integrated control a set number of times to indicate a numbered fault code. Armstrong Air and its Allied Air sister brands (Ducane, AirEase, Concord) share one control. A slow one-second 'heartbeat' means powered with no errors; a fault is a rapid flash repeated the code's number of times. Note: some two-stage/variable-speed and communicating models use a DIFFERENT scheme (including 'E###' display codes) — always match the chart on your unit's blower-door label.
Reverse line-voltage polarity — the incoming hot (L1) and neutral are reversed; the furnace won't fire until correct polarity is restored. This is licensed electrical work, not a homeowner fix.
2 flashes
Improper earth ground — the control detects a missing or inadequate ground, which it needs as a reference for safe operation and flame sensing. Have a technician or electrician verify and repair the ground.
12 flashes
Flame sensed without the gas valve energized — the control detects flame when there should be none, which can indicate a gas valve leaking through. Turn the furnace off and call a technician promptly; never tamper with the gas valve.
13 flashes
Low line voltage — the incoming line voltage is below the level the control needs for safe operation (a brownout, overloaded circuit, or loose connections). If it persists, call an electrician or HVAC technician.
⚠️ Safety first. Gas furnaces burn fuel and produce combustion gases. If you ever smell gas (a rotten-egg or sulfur odor), hear hissing, or your carbon-monoxide alarm sounds, do not touch light switches, thermostats, or the furnace — leave the home immediately and call your gas utility's emergency line or 911 from outside. Homeowners should limit themselves to simple, safe steps: setting the thermostat, checking and replacing the air filter, confirming the furnace switch and breaker are on, keeping supply registers and return grilles open, and clearing snow, ice, or debris from outdoor vent and intake terminations. Never bypass, jumper, or reset safety switches (limit, flame-rollout, or pressure switches), never open the gas valve or burner compartment, and never attempt combustion, electrical, or control-board repairs yourself. Error codes point toward a problem but are not a substitute for proper diagnosis — when in doubt, turn the system off and call a licensed HVAC technician. Keep working carbon-monoxide detectors on every level of your home.
Armstrong Air furnace code FAQ
How do I read the error code on a Armstrong Air furnace?
Armstrong Air furnaces signal faults by flashing a single red LED on the integrated control a set number of times to indicate a numbered fault code.
Where is the diagnostic light on a Armstrong Air furnace?
It's on the control board, usually visible through a small sight glass on the lower blower-compartment door. You may need to remove the door panel to read it clearly.
What is the most common Armstrong Air furnace code?
3 flashes — Burner failed to light, or lost flame during a heat demand.