HVAC Common Error Codes + Indicator Lights
Why this matters
Modern HVAC equipment has diagnostic boards with error codes that pinpoint problems. Customer says "my AC isn't working" - board flashes 3 times: that means flame sensor failure (specific). Without knowing codes, tech replaces parts at random; with codes, fixes correctly in minutes. Manufacturers each have own code system but patterns are consistent. This is the field card.
How error codes are displayed
LED blink codes: control board LED blinks specific count (e.g., 4 blinks = pressure switch failure).
LCD displays: alphanumeric display (newer equipment).
Smartphone apps: manufacturer apps display via WiFi/Bluetooth.
Diagnostic ports: some equipment has port for handheld code reader.
Reading blink codes
Most furnaces + AC indoor + outdoor units:
- Solid red OR amber LED = power on
- Slow blink = normal operation
- Fast blink series = error code
- Count pauses between groups
Example: 3 blinks, pause, 3 blinks, pause = code 3 (look up "3" in manufacturer's chart).
Universal HVAC errors (common patterns)
Most furnaces will display one of these:
| Issue | Common code |
|---|---|
| Pressure switch failure | 3 blinks (many brands) |
| Limit switch open | 5 blinks |
| Flame sensor failure | 4 blinks |
| Ignition lockout | 3 OR 4 blinks |
| Low gas pressure | 5 OR 6 blinks |
| Faulty igniter | 6 OR 7 blinks |
Each manufacturer different - always check the equipment's specific reference.
Manufacturer-specific codes
Carrier/Bryant 58STA / Performance 80/96 (two-digit): 11 normal; 12 prior error cleared; 13 limit/rollout; 14 flame loss; 21 gas heating lockout; 22 limit fault; 31 pressure switch; 32 HSI; 33 limit/rollout; 34 ignition fault; 41 - 44 blower issues.
Trane XL/XR (blink count): off no power; solid normal; 1 hard lockout; 2 pressure switch closed when expected open; 3 pressure switch open; 4 high-limit overheat; 5 rollout; 6 igniter; 7 flame proving; 8 low flame; 9 AFUE proving; 10 gas valve.
Goodman/Amana (blink count): 1 lockout; 2 pressure switch open; 3 stuck closed; 4 limit open; 5 flame sensed no call; 6 rollout; 7 low-voltage cooling; 8 diagnostics.
Rheem/Ruud (blink count): 1 lockout; 2 pressure switch stuck closed at startup; 3 open; 4 limit open; 5 rollout open; 6 igniter fault; 7 flame loss; 8 cooling miswire.
Honeywell control codes
Honeywell smart thermostat + boards:
- E1: temperature sensor failure
- E2: humidity sensor failure
- E3: communication lost
- E4: WiFi disconnect
- E5: software fault
- Various error codes in app
Common HVAC error diagnostic steps
Pressure switch error: causes - clogged condensate drain, blocked vent terminal (bird nest, ice), disconnected hose, wrong-rated switch, failed inducer fan motor. Diagnostic: visual inspect drain + vent, test inducer amperage, verify hose continuity, check switch with manometer.
Limit/overheat error: causes - dirty blower wheel, dirty filter, closed registers, faulty blower motor/capacitor, failed limit switch, heat exchanger restriction. Diagnostic: check + change filter, verify blower clean + free spinning, check blower amperage, verify limit switch continuity.
Flame sensor error: causes - dirty flame sensor rod (oxidation/buildup)
- Cracked porcelain on flame rod
- Wrong gas pressure (flame too small)
- Misalignment with flame
Diagnostic:
- Visually inspect flame rod
- Sand lightly with fine sandpaper (steel wool OK)
- Test microamp reading (should be 4 - 10 µA when flame present)
Ignition lockout
Causes:
- HSI (hot surface igniter) failure
- No gas / low gas pressure
- Defective gas valve
- Flame sensor not detecting (loose connection)
- Wrong polarity wiring
Diagnostic:
- Verify gas valve opening
- Test HSI continuity (60 - 150 ohms typical)
- Verify gas pressure at manifold (3.5" WC NG; 10" WC LP)
- Check polarity
Compressor lockout (AC outdoor unit)
Causes:
- High pressure trip
- Low pressure trip
- Internal overload
- Capacitor failure
- Bad contactor
- Low refrigerant
Diagnostic:
- Verify capacitor (MFD rating, leak free)
- Test contactor coil + contacts
- Refrigerant pressures (high + low side)
- Compressor amperage at startup + run
Outdoor unit not running
Causes:
- No 240V power
- Failed disconnect / fuses
- Contactor not energizing (no 24V from indoor)
- Capacitor failure
- Compressor failed
Diagnostic:
- Check disconnect + breaker
- Verify 24V at contactor coil
- Test contactor (energize manually to check)
- Test capacitor (MFD)
- Compressor windings to ground
Smart thermostat diagnostic
Common issues:
- No power: C-wire connection (most common)
- Wrong setup: heat pump vs traditional (O/B wiring)
- WiFi disconnect: router OR firmware
- Customer hasn't paired to app: walkthrough required
Common smart-thermostat brands codes:
- Nest: "no power" indication
- Ecobee: various E-codes
- Honeywell Lyric: app-based
- Tado: app-based
Reading customer info correctly
At customer house:
- Photograph nameplate (model + serial)
- Note refrigerant type
- Check error code BEFORE calling support
- Reference manufacturer's service manual
Most controls reset on:
- Power cycle (60 seconds off then on)
- Manual reset button (some boards)
- Replace battery on standalone alarms
Common manufacturer service manuals
Online + in truck:
- Carrier IM (Installation Manual) + SM (Service Manual)
- Trane SM
- Goodman / Amana SM
- Rheem / Ruud SM
- Lennox SM
- York / Coleman SM
App-based (modern):
- Carrier Connect
- Trane TruComfort
- Goodman Service Library
Common error code mistakes
Skipping diagnostic: replacing parts at random.
Misreading blink codes: counting wrong, missing pauses.
Wrong manufacturer's chart: codes vary by brand.
Not power-cycling first: easier diagnostic with fresh reset.
Ignoring previous error history: board may store prior failures.
Not testing components individually: assuming error = part needs replacement (sometimes wiring OR sensor).
When to escalate
Beyond field tech:
- Multiple repeated errors
- Compressor failure
- Heat exchanger damage
- Refrigerant leak in coil
- Customer's safety concern
Escalate to:
- Senior tech
- Manufacturer warranty (if applicable)
- Customer's home insurance
- Replacement vs repair decision
Building diagnostic skills
Annual training: manufacturer-specific service training.
Code book / app: NATE app, ContractorTech, or manufacturer apps.
Reading service manuals: thoroughly + with diagrams.
Build experience: pattern recognition with each call.
Ask "what changed": customer's input often reveals root cause.
The single most-impactful diagnostic habit is the SYSTEMATIC POWER CYCLE FIRST. Before reading codes, cycle power: turn unit off for 60 seconds, then back on. 30 - 50% of "error code" complaints clear with a power cycle (intermittent fault, communication glitch, etc.). If it clears, watch for recurrence; if it returns, you have an actual fault to diagnose. Skipping this step costs hours diagnosing transient issues that resolve themselves.
References
- Manufacturer service manuals (Carrier, Trane, Goodman, Rheem, etc.)
- ACCA HVACR-specific training
- AC Service Tech (online training)
- HVAC School (online education)
- Manuall internal: Refrigeration Cycle Theory, Refrigerant Pressure-Temperature Quick