HVAC Manufacturer Error Codes Reference

Why this matters

Modern HVAC equipment reports faults via LED blink codes, display codes, or app notifications. The tech who knows the common code patterns by manufacturer narrows diagnosis in seconds. Without that knowledge, you're starting from scratch every call. This is the field card - common patterns, where to look on each major manufacturer.

Where to find codes

Furnace control board: LED visible through inspection window OR door interlock; blink pattern indicates fault.

AC / heat pump outdoor unit: some have LED on control board; many don't show codes (must read from indoor stat OR app).

Mini-split indoor head: digital display OR LED on indoor unit; some require button-combination to view fault.

Smart / communicating thermostat (Carrier Infinity, Trane ComfortLink, Lennox iComfort): displays codes from any system component.

Mobile app (Bryant Hybrid Heat, Trane Home, Lennox iComfort, Carrier Cor): real-time fault reporting + history.

The legend for codes lives in the installation manual (often inside the cabinet). Photograph it the first time you encounter a brand + you have it forever.

Common fault categories across brands

Ignition/flame (gas furnaces): pilot won't light (gas valve, igniter, pilot orifice); flame sensed but lost (flame sensor dirty, gas pressure); pilot lights but main burner doesn't.

Airflow: pressure switch open (inducer, blocked flue); high-limit tripped (filter, blower); rollout switch tripped (flue blocked, flame escape).

Combustion air (sealed/condensing): inducer stalled; vent restriction; condensate drain plugged.

Refrigerant/pressure (AC, heat pump): low-pressure cutout (low charge, low airflow); high-pressure cutout (high charge, dirty condenser, fan failure); compressor lockout from multiple consecutive trips.

Control/communication: communicating-system loss-of-comm; thermostat-board mismatch; outdoor unit can't talk to indoor.

Sensor: open sensor (thermistor failed); out-of-range; disconnected.

Carrier / Bryant (often share platforms)

Furnace control board: LED blinks for fault codes. Steady = normal. Common codes:

• 13 = limit switch open (airflow)
• 14 = flame sensor lockout
• 33 = limit switch open (during prior cycle)
• 34 = flame failure
• 41 = blower motor fault
• Yellow LED indicators on communicating boards (Infinity)

Infinity / Evolution communicating systems: display fault codes on the thermostat (Code-XX with description) + log history.

Heat pump outdoor: defrost board displays codes via LED OR via the indoor stat.

Trane / American Standard

XV/XL/XR series: 3-digit codes via LED on outdoor + indoor. ComfortLink II / Symbio communicating: codes on thermostat OR app. XV series common: 90/91 = communication; 79 = high pressure (cooling); 80 = low pressure; 50s = sensor faults. Furnace board blink codes similar to Carrier; check cabinet manual.

Lennox

iComfort / S30 thermostat: displays alerts with multi-digit code (e.g., E105) + plain-text description. Easiest to read of major brands.

Pulse / G71 furnaces (older models still in field): LED blink codes for ignition + limit faults.

Outdoor units (XC / XP series): status codes via LED on the control board; coordinated with the indoor.

Goodman / Amana / Daikin

Furnace control board (Janitrol PCBKF / White-Rodgers): LED blinks. 1 = normal; 3 = pressure switch open; 4 = limit switch open; 5 = igniter failure; 6 = flame sensor; 7 = gas valve. Outdoor units: dip-switch board OR LED per model.

Rheem / Ruud

Furnace IFC board: LED blink + status indicator. Codes vary by series; check manual.

Outdoor units: limited self-diagnostic; rely on system measurements (pressures, voltages).

Heat pumps specifically

Defrost board: separate fault codes vs the heating-mode codes. Common: defrost cycle stuck, outdoor sensor failed, reversing valve fault.

Aux heat strip lockout: heat pump locks out aux heat when outdoor temp above setpoint, OR aux heat fault detected. Customer complaint "expensive auxiliary running all the time" = check lockout settings.

Mini-split specifics (ductless)

Mitsubishi Mr. Slim: indoor unit blinks operation LED + checks LED in pattern. Service mode via remote → diagnostic display.

Daikin: indoor display shows "E + 2 chars" code (E0, U4, etc.).

LG: similar 2-char code.

Fujitsu: dip-switch test mode + LED codes.

The codes on mini-splits are denser + more cryptic than central HVAC. The manual is essential; without it, you're guessing.

Communicating system "loss of communication"

Common on Carrier Infinity, Trane ComfortLink, Lennox iComfort:

• Indoor board + outdoor board + thermostat must all be on the same firmware/protocol
• One bad component drops the whole system into "loss of comm"
• Diagnosis: physical inspection of communication wiring (4-wire), terminal connections, firmware versions, replacement of suspect board

These systems are amazing when working + frustrating when they fail. Customer pays a premium for them; expect higher service-call expectations.

Smart-stat error codes

Nest, Ecobee, Honeywell Wi-Fi:

• Power loss (C-wire issue): smart stat fault
• Wi-Fi disconnection
• System-specific codes passed through from the equipment

Don't let smart stats confuse: the underlying HVAC fault is usually upstream. The stat is reporting what it sees, not diagnosing root cause.

How to use codes effectively

Read the code FIRST: before opening any panel. The code tells you which area to investigate.

Look up the meaning (manual, manufacturer website, manufacturer support chat). Don't assume across brands.

Cross-check with measurements: a "low pressure" code on AC could be low charge OR low airflow OR sensor fault. Measure to confirm.

Clear the code after repair: most boards retain fault history. Clear after the fix so the next call can see fresh history.

Photograph + log the code: in service ticket. Useful for warranty + history.

What codes don't tell you

Codes report symptoms, not causes. "Pressure switch open" tells you the switch is open - not WHY. Could be:

• Inducer motor failed (no draft → switch doesn't close)
• Flue blocked
• Pressure switch hose disconnected
• Pressure switch failed itself

Always investigate the cause behind the code. Replacing the pressure switch when the inducer was the actual problem = repeat callback.

Manual access

OEM manuals: free online from manufacturers (most). Some require dealer account.

Aftermarket databases (Building Intelligence Center, Aaon Service Bench): cross-brand quick reference.

Bookmark + offline access: when you're at the customer's location with no Wi-Fi, having the manuals downloaded saves you.

Older equipment without diagnostics

References

• Manufacturer service literature (Carrier, Trane, Lennox, Goodman, Rheem, Mitsubishi, Daikin, LG, Fujitsu)
• ACCA technical resources
• AHRI directory + technical bulletins
• Mike Holt / Refrigeration School curricula
• Manuall internal: HVAC Short-Cycling Diagnosis, Diagnostic Test Procedures, Reading Wiring Schematics