Prefer model charts when the cabinet plate is readable.
57 brand-level codes
01
Communication failure
Carrier / Bryant
Indoor and outdoor communicating controls are not talking.
- AB / data wires polarity and continuity (not 24V)
- Outdoor control board LED / status
- Transformer and C connection
- Damaged harness or water in outdoor control
31
High pressure switch
Carrier / Bryant
Discharge / liquid pressure opened the HPS.
- Dirty outdoor coil, fan not running, bent fins
- Overcharge or non-condensables
- Restriction in liquid line or drier
- Recirc of condenser air
32
Low pressure switch
Carrier / Bryant
Suction pressure dropped below LPS cutout.
- Low charge / leak
- Indoor airflow (filter, blower, coil freeze)
- Restriction / TXV stuck almost closed
- Outdoor coil iced in heat mode
33
High discharge temperature
Carrier / Bryant
Compressor discharge is too hot.
- Low charge, high superheat
- Failed outdoor fan or dirty coil
- Reversing valve leaking by
- Bad discharge thermistor
41
Compressor / inverter fault
Carrier / Bryant
Drive detected a compressor or inverter problem.
- Winding ohms and ground
- Inverter heat sink and cooling fan
- DC bus / input voltage
- OEM status flash count before reset
53
Outdoor coil thermistor
Carrier / Bryant
Coil temperature sensor open, shorted, or out of range.
- Sensor ohms vs OEM chart at known temp
- Connector corrosion
- Sensor not clipped to the right circuit
55
Outdoor air thermistor
Carrier / Bryant
Ambient sensor failed or unplugged.
- Ohms vs chart
- Sun load / sensor placement
- Board input if sensor is good
E1
Communication error
Trane / American Standard
ComfortLink / communicating pair lost the data bus.
- D / R / B wiring (not thermostat 24V assumptions)
- Outdoor TAM / control LEDs
- Voltage on communicating pair per OEM
E2
Outdoor temperature sensor
Trane / American Standard
ODT sensor open or shorted.
- Sensor ohms
- Harness pinch at corner panels
- Replace sensor before board
E5
Compressor fault
Trane / American Standard
Compressor failed to start or dropped on protection.
- Hard start / capacitor (if non-inverter)
- Voltage at contactor under load
- Winding resistance and ground
- Lockout history on ComfortLink
E6
High pressure
Trane / American Standard
High pressure switch or transducer trip.
- Condenser airflow and fan motor
- Charge and subcooling
- Liquid line restriction
E7
Low pressure
Trane / American Standard
Low pressure switch or transducer trip.
- Charge / leak
- Indoor airflow and freeze
- TXV / piston restriction
iComfort / communicating indoor-outdoor link down.
- R / I+ / I− or OEM data pair
- Outdoor control heartbeat LED
- Replace one device at a time after wiring checks out
High pressure switch open.
- Outdoor fan and coil
- Overcharge
- Restriction
Low pressure switch open.
- Charge
- Indoor coil freeze
- Restriction / TXV
E 180
Outdoor coil sensor
Lennox
Coil thermistor out of range.
- Ohms vs chart
- Defrost impact if HP
- Connector
Heat pump defrost did not complete or sensor disagrees.
- Defrost sensor placement and ohms
- Reversing valve shift (pressure swap)
- Outdoor fan during defrost
- Board defrost timing vs OEM
E1
High pressure
Goodman / Amana
HPS open — condenser side problem or overcharge.
- Fan capacitor / motor
- Dirty coil
- Charge
- Recirc
E2
Low pressure / freeze
Goodman / Amana
LPS or freeze stat opened.
- Filter and indoor blower
- Low charge
- Metering device
- Reset after coil thaws
E3
Indoor coil freeze
Goodman / Amana
Freeze protection opened.
- Airflow first
- Low charge second
- Blower speed / ECM settings
E4
High discharge
Goodman / Amana
Discharge temperature protection.
- Charge and SH
- Outdoor fan
- Sensor
E5
Communication
Goodman / Amana
Communicating board lost the indoor unit.
- Data pair
- 24V common
- Board LEDs
11
Communication
Rheem / Ruud
EcoNet / communicating failure.
- E1 / E2 polarity
- Outdoor control
- Dip switches / addressing
13
High pressure
Rheem / Ruud
High pressure lockout.
- Condenser airflow
- Charge
- Restriction
14
Low pressure
Rheem / Ruud
Low pressure lockout.
- Charge / leak
- Indoor airflow
- TXV
21
Ambient sensor
Rheem / Ruud
Outdoor air sensor fault.
22
Coil sensor
Rheem / Ruud
Coil temperature sensor fault.
- Ohms
- Clip location
- Defrost behavior
01
High pressure
York / Coleman
High pressure switch trip.
02
Low pressure
York / Coleman
Low pressure switch trip.
03
Outdoor sensor
York / Coleman
ODT or coil sensor out of range.
- Identify which sensor from flash / display
- Ohms vs chart
05
Communication
York / Coleman
Communicating thermostat or indoor lost outdoor.
- Data wiring
- Common
- Control firmware pair
U4
Indoor / outdoor communication
Daikin
Transmission error between indoor and outdoor.
- Interconnecting cable (no splices, correct gauge)
- Polarity on communication pair
- Outdoor PCB LED
U0
Low pressure / shortage
Daikin
Suction pressure or sensor indicates shortage or restriction.
- Weigh-in vs nameplate
- Leak search
- EEV operation
Compressor failed to start or locked.
- Winding / insulation
- Inverter output
- Mechanical lock
L5
Inverter overcurrent
Daikin
Output overcurrent on compressor drive.
- Compressor to ground
- Inverter cooling
- Supply voltage
L4
Inverter overheat
Daikin
Heat sink temperature too high.
- Inverter fan
- Fins packed with lint
- Ambient recirc
A5
Freeze / overheat indoor
Daikin
Indoor coil protection.
- Filter and fan
- Charge
- Room load
Condensate level or pump fault.
- Pump lift and check valve
- Float switch
- Trap / pitch
F3
Discharge temperature
Daikin
Discharge thermistor high or failed.
- Charge / SH
- Sensor ohms
- EEV
U4
Communication
Mitsubishi
Indoor-outdoor transmission error (M-NET / S series pair).
- S1/S2/S3 or A/B wiring
- No high-voltage on comm
- Outdoor board
U2
High pressure / overcurrent
Mitsubishi
Compressor protection from high load or pressure.
- Outdoor coil and fan
- Charge
- Indoor filter
P6
Freeze / overheat protection
Mitsubishi
Coil temperature outside allowed band.
Float switch or drain pump.
P1
Indoor intake thermistor
Mitsubishi
Return air sensor failed.
- Ohms
- Connector on indoor board
P2
Pipe thermistor
Mitsubishi
Indoor pipe sensor failed.
F3
Discharge thermistor
Mitsubishi
Outdoor discharge sensor.
- Ohms vs chart
- High SH / low charge if reading is real
Compressor or outdoor overcurrent.
- Winding / ground
- Inverter
- Voltage
Reversing valve did not shift or sensor disagrees.
- Solenoid 24V / 12V during call
- Pressure swap heat vs cool
- Stuck piston — tap test only after power-down policy
E1
Thermostat internal
Honeywell / Resideo
Control electronics fault.
- Power cycle
- C-wire voltage
- Replace thermostat if repeat
E2
Sensor
Honeywell / Resideo
Onboard or remote temperature sensor failed.
- Remote sensor wiring
- Replace stat if onboard
E5
Memory / config
Honeywell / Resideo
Installer setup corrupted.
- Re-enter ISU
- Replace if it will not hold
01 / lockout
Module lockout
Copeland
CoreSense / Comfort Alert locked the compressor after repeats.
- Flash code on module (count yellow)
- Run capacitor and voltage
- Charge and airflow that caused the trips
- Reset only after the root cause is fixed
2 flash
System pressure trip
Copeland
High or low pressure related trip (module-specific).
4 flash
Locked rotor
Copeland
Compressor failed to start.
- Hard start
- Capacitor µF
- Voltage sag
- Mechanical lock
5 flash
Open circuit
Copeland
No compressor current when it should be running.
- Contactor
- Wiring
- Internal overload / windings
6 flash
Open start circuit
Copeland
Start winding / start components open.
- Start capacitor
- Potential relay
- Start winding ohms
7 flash
Open run circuit
Copeland
Run winding or run capacitor path open.
- Run capacitor
- Run winding
- Connections