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

Plenty of working equipment predates fault codes entirely: standing-pilot furnaces, straight-cool condensers with nothing but a contactor and a capacitor, and boards that have one LED that either lights or does not. There is no code to read. You go back to the sequence of operation and the meter, which is the skill the codes were built on top of anyway.

Work the sequence, one step at a time. Write the steps down for the appliance in front of you: call for heat, transformer powers control circuit, inducer starts, pressure switch closes, igniter energizes, gas valve opens, flame proves, blower delay, blower starts. Then find the first step that does not happen. That step, and the input that should have caused it, is your fault.

Meter across the load, not just at it. Voltage present at a component does not mean the component works, and voltage absent tells you the problem is upstream. On a 24 V control circuit, walk it: measure at the transformer secondary, then at each safety in series, and find where the voltage disappears. That is the open device.

Use the wiring diagram in the cabinet. On older equipment it is usually pasted inside the door and it is accurate. Photograph it before you start, because it will tear or fall out.

What replaces the code, by symptom:

• Nothing happens at all: transformer, fuse, low-voltage short, door switch, thermostat, or the disconnect
• Inducer runs, nothing follows: pressure switch, tubing, or draft
• Igniter or pilot lights, burner does not: gas supply, gas valve, or manifold pressure
• Burner lights and drops out: flame proving, whether that is a thermocouple, a flame rod, or a pilot generator
• Runs then trips on limit: airflow, filter, blower, or duct
• Compressor hums and does not start: capacitor, then start components, then the compressor windings
• Outdoor unit dead with call present: contactor, control voltage from indoor, low-pressure switch, or the disconnect

Older safeties are the diagnostic. A limit that opened, a rollout that tripped, a manual-reset button that has been pushed before - each of those is the equipment telling you what happened, in hardware instead of in a code. Look for evidence of a past trip: heat discoloration, a scorched wire, a soot pattern, a reset button that has clearly been pressed.

Look for the visible. Old equipment fails in ways you can see and smell. Cracked heat exchangers, rust-through pans, rodent damage, dry bearings, burned contacts, oil at a refrigerant joint. Ten minutes with a flashlight beats an hour of meter work on a thirty-year-old unit.

Then have the age conversation. Equipment with no diagnostics is usually equipment with limited remaining life and shrinking parts availability. Repair it if the part is available and the machine is otherwise sound, but tell the customer plainly where it sits in its life so the next failure is not a surprise.

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