Skip to main contentStart diagnosing 51 on your LG heat pump→
What this code means
Indoor connection over capacity — total connected indoor unit capacity exceeds outdoor unit rated capacity (130% for 10HP, 115% for 4.5HP, 120% for 6HP).
First step from the service manual
Verify if total indoor unit capacity is within limits; if yes, confirm indoor unit capacity code and replace PCB.
Next steps: diagnose the root cause
1
Verify the error code is active
Power cycle the unit and see if the code returns. If it does, the fault is persistent and requires diagnosis.
2
Get the full diagnostic procedure
Get step-by-step guidance, test values, and wiring diagrams from the OEM service manual.
$29/month · Cancel anytime · Or $9.95 3-day pass
What’s available for LG heat pump models
Access LG heat pump service manuals, wiring schematics, exploded parts views, and error code references on MyPros+.
Related LG heat pump error codes
01Indoor unit air sensor (room temperature sensor) open or short circuit.02Indoor unit inlet pipe sensor open or short circuit detected.03Wired remote controller to indoor unit transmission error04Drain pump malfunction05Outdoor unit to indoor unit transmission error06Indoor unit outlet pipe sensor open or short circuit detected.07Indoor unit is operating in a different mode than other connected indoor units…09Serial number error21DC peak22Max CT23DC link24High pressure25Low/over voltage32Inverter compressor discharge temperature33Constant speed compressor discharge temperature34High pressure35Low pressure40CT sensor open or short circuit.41Inverter compressor discharge pipe sensor (discharge temperature) open or short.42Low pressure sensor open or short circuit.43High pressure sensor open or short circuit.44Outdoor unit air sensor open or short circuit.45Outdoor unit pipe sensor (left) open or short circuit.46Suction pipe sensor open or short circuit.47Normal (constant speed) compressor discharge pipe sensor open or short circuit.48Outdoor unit pipe sensor (right) open or short circuit.49Voltage sensor open or short circuit.52Transmission error from inverter PCB to main PCB in outdoor unit53Transmission error from indoor unit to main PCB in outdoor unit54Reverse connection or disconnection of R,S,T power line (CRUN1008T1 only).55Transmission error from central controller to main PCB in outdoor unit56Transmission error from main PCB in outdoor unit to central controller57Transmission error from main PCB to inverter PCB in outdoor unitAxIndoor unit EVAP IN sensor breakaway error (x = indoor unit address).BxElectronic expansion valve open error (x = indoor unit address)ChCompressor preheating in progress via crank case heater (CCH)CH01Problem in Remote Room Air sensor (open/short/disconnected).CH02Problem in Refrigerant (Inlet side) sensor.CH03Bad communication between control panel and indoor unit.CH05Bad communication between Indoor unit and Outdoor unit.CH06Problem in Refrigerant (Outlet side) sensor.CH08Problem in Water Tank sensor.CH09PCB Program (EEPROM) Fault due to electrical or mechanical damage.CH13Problem in Solar-thermal sensor.CH14Problem in Flow Switch (open while water pump running, or closed while pump not…CH15Water pipe overheated (leaving water temperature above 72°C).CH16Problems in sensors (general sensor fault).CH17Problem in Water-inlet sensor.CH18Problem in Water-outlet sensor.CH19Problem in Water-interlim sensor.CH21DC Peak (IPM Fault)CH22Max CT (over current detected by current transformer).CH23DC Link voltage is low (1phCH24High Pressure / Low Pressure switch activated.CH25Input voltage is too low or too high.CH26Problem in DC Compressor positioning (position detect error).CH27PSC/PFC FaultCH28DC Link voltage is high.CH29High current at compressor phases (inverter compressor input current over 30A).CH32Temperature at discharge pipe is too high (105°C or above).CH39Transmission error between PFC Micom and INV Micom.CH40Problem in CT (current transformer) sensorCH41Problem in discharge-pipe temperature sensor (inverter side).CH44Problem in ambient temperature sensor.CH45Problem in condenser-pipe temperature sensor.CH46Problem in suction-pipe temperature sensor.CH51Over capacity combinationCH52Bad communication between internal PCBs (Inverter PCB to Main PCB).CH53Bad communication between Indoor unit and Outdoor unit (outdoor-side code for…CH60EEPROM Checksum mismatched.CH61Temperature at condenser-pipe is too high (65°C).CH62Temperature at heatsink is too high (85°C).CH63Temperature at condenser-pipe is too low.CH65Problem in heatsink temperature sensor (short if ≥130°C, open if < -30°C).CH67BLDC motor fan lock (outdoor)CH73AC input instant over current errorCxElectronic expansion valve open error for xth indoor unitDxOPEN/SHORT error of EVAP IN sensor in indoor unit (x = indoor unit address).E1System down due to communication error after completion of trackingE101Communication error between the indoor unit and the wired remote panel (PAC).E102Indoor unit room temperature sensor open/short error.E103Indoor unit EVAP IN sensor open/short error.E104Indoor unit EVAP OUT sensor open/short error.E105Indoor unit EVAP IN sensor breakaway.E106Indoor unit EVAP OUT sensor breakaway.E107Simultaneous breakaway of indoor unit EVAP IN/OUT sensor.E108Auxiliary (electric) heater sensor 1 error.E109Auxiliary (electric) heater sensor 2 error.E110Auxiliary (electric) heater sensor 3 error.E111Shutter sensor error.E112Perfect fan sensor error (PAC).E113MCU temperature sensor OPEN/SHORT error.E121MCU temperature sensor open/short error.E122Indoor unit EEV close failure.E123MCU temperature sensor open/short error.E151Indoor unit EEV open failure.E152Indoor unit EEV close failure.E153Indoor unit float switch detectionE154Indoor fan error.E155Mixed operation errorE156EEPROM error.E157Option setting error for indoor unit.E158Electric heater discharge temperature protection error.E159No fan error related to the electric heater.E160Software setup error in peripheral control device using option.E161The 1st MCU SOL V/V cooling/heating opening at the same time.E162The 2nd MCU SOL V/V cooling/heating opening at the same time.E163The 2nd MCU SOL V/V error.E164No change of EVAP IN temperature during the piping check.E165No change of EVAP OUT temperature during the piping check.E166Change of EVAP IN temperature detected on other indoor unit during piping check.E167Change of EVAP OUT temperature detected on other indoor unit during piping…E170There is no change of EVAP IN temperature during the piping check.E171There is no change of EVAP OUT temperature during the piping check.E172There is a change of EVAP IN temperature of other indoor unit during the piping…E173There is a change of EVAP OUT temperature of other indoor unit during the…E181No change of EVAP IN temperature during the piping check.E182No change of EVAP OUT temperature during the piping check.E183Change of EVAP IN temperature of other indoor unit during the piping check.E184Change of EVAP OUT temperature of other indoor unit during the piping check.E2Tracking errorE201Installed indoor unit number setting error on outdoor unit.E202Communication error between indoor and outdoor unit.E203Outdoor PCB MICOM communication error between main and sub.E204Communication error between the outdoor unit and MCU.E205Indoor unit connected with confluence kit without continuity.E206Indoor unit connected with confluence kit and setting address was overlapped…E207MCU address does not match with indoor unit address.E208Installed MCU quantity set-up error in the outdoor PCB or MCU address setting…E209Rotary switch for indoor unit address in MCU was overlapped among MCUs.E210DIP switch for indoor unit setting was ON position even though indoor unit was…E211DIP switch for indoor unit setting was ON position even though indoor unit was…E212DIP switch for indoor unit setting was OFF position even though indoor unit was…E221Outdoor temperature sensor OPEN/SHORT error.E222Outdoor temperature sensor breakaway.E223COND-OUT MAIN temperature sensor error OPEN/SHORT.E224COND-OUT SUB 1 temperature sensor error OPEN/SHORT.E225COND-MID sensor breakaway of outdoor unit.E226COND-OUT 1 breakaway.E231PWM discharge temperature sensor error OPEN/SHORTE232Fixed capacity compressor 1 discharge sensor error OPEN/SHORTE233Fixed capacity compressor 2 discharge sensor error OPEN/SHORTE234Fixed capacity compressor 3 discharge sensor error OPEN/SHORTE235COND-MID sensor breakaway of outdoor unit.E236COND-OUT 1 sensor breakaway.E241PWM DISCHARGE TEMP sensor error OPEN/SHORTE251Fixed capacity compressor 1 discharge sensor error OPEN/SHORTE252Fixed capacity compressor 2 discharge sensor error OPEN/SHORTE253Fixed capacity compressor 3 discharge sensor error OPEN/SHORTE261Digital discharge sensor breakaway.E262Discharge temperature sensor breakawayE263Discharge temperature sensor breakawayE264Discharge temperature sensor breakawayE265Fixed 3 discharge sensor breakaway.E266Sump Sub 1 breakaway error.E267Sump Sub 1 breakaway error.E268Sump Sub 2 breakaway error.E269Sump Sub 3 breakaway error.E270Suction breakaway error.E271SUMP TEMP DIGITAL sensor error OPEN/SHORTE272SUMP TEMP FIXED 1 sensor error OPEN/SHORTE273SUMP TEMP FIXED 2 sensor error OPEN/SHORTE274SUMP TEMP FIXED 3 sensor error OPEN/SHORTE275High pressure sensor error OPEN/SHORTE276Low pressure sensor error OPEN/SHORTE277High pressure sensor breakaway.E278Low pressure sensor breakaway.E279Suction temperature sensor error OPEN/SHORT.E280Main cooling solenoid valve open error.E281SUMP TEMP DIGITAL sensor error OPEN/SHORTE282SUMP TEMP FIXED 1 sensor error (OPEN/SHORT)E283SUMP TEMP FIXED 2 sensor error (OPEN/SHORT)E284SUMP TEMP FIXED 3 sensor error (OPEN/SHORT)E291High pressure sensor error OPEN/SHORTE292Low pressure sensor error OPEN/SHORTE293High pressure sensor breakaway.E294Low pressure sensor breakaway.E295Suction temperature sensor error (OPEN/SHORT).E296Main cooling solenoid valve open error.E2974-Way valve operation error.E3Indoor unit float switch errorE301Outdoor temperature (OUT-TEMP) sensor OPEN/SHORT error.E302OUT-TEMP temperature sensor breakaway.E303COND-OUT MAIN temperature sensor error OPEN/SHORT.E304COND-OUT SUB 1 temperature sensor OPEN/SHORT error.E305COND-MID sensor breakaway of outdoor unit.E306COND-OUT 1 sensor breakaway.E307PWM DISCHARGE TEMP sensor error OPEN/SHORT. Detected when outdoor temperature…E308Fixed capacity compressor 1 discharge sensor error OPEN/SHORTE309Fixed capacity compressor 2 discharge sensor error OPEN/SHORT. Detected when…E310Fixed capacity compressor 3 discharge sensor error OPEN/SHORT. Detected when…E311PWM discharge temperature sensor error OPEN/SHORTE312SUMP TEMP FIXED 1 sensor error OPEN/SHORT. Detected when outdoor temperature is…E313Fixed capacity compressor 2 discharge sensor error OPEN/SHORTE314Fixed capacity compressor 3 discharge sensor error OPEN/SHORTE315High pressure sensor error OPEN/SHORTE316Low pressure sensor error OPEN/SHORTE320Main cooling solenoid valve open error.E321SUMP TEMP DIGITAL sensor error OPEN/SHORT (detected when outdoor temp over…E322SUMP TEMP FIXED 1 sensor error OPEN/SHORT (detected when outdoor temp over…E323SUMP TEMP FIXED 2 sensor error OPEN/SHORT (detected when outdoor temp over…E324SUMP TEMP FIXED 3 sensor error OPEN/SHORT (detected when outdoor temp over…E331High pressure sensor OPEN/SHORT error (SHORT below 0.4V, OPEN over 4.2V…E332Low pressure sensor OPEN/SHORT error (SHORT below 0.4V, OPEN over 4.7V…E333High pressure sensor breakaway.E334Low pressure sensor breakaway.E335Suction temperature sensor OPEN/SHORT error.E336Main cooling solenoid valve open error.E3374-Way valve operation error.E341SUMP TEMP DIGITAL sensor error OPEN/SHORTE342SUMP TEMP FIXED 1 sensor error OPEN/SHORTE343SUMP TEMP FIXED 2 sensor error OPEN/SHORTE344SUMP TEMP FIXED 3 sensor error OPEN/SHORTE351High pressure sensor error OPEN/SHORTE352Low pressure sensor error OPEN/SHORTE353High pressure sensor breakaway.E354Low pressure sensor breakaway.E355Suction temperature sensor ERROR (OPEN/SHORT).E356Main cooling solenoid valve open error.E3574-Way valve operation error.E4Setting error of option switch in indoor unitE401Outdoor temperature sensor OPEN/SHORT error.E402Outdoor temperature sensor breakaway.E403Outdoor unit freezing detection 3E404Outdoor unit overload 1 protection control error.E405Outdoor unit overload 2 protection control error.E406Outdoor unit overload 3 protection control error.E407PWM DISCHARGE TEMP sensor error OPEN/SHORTE408Fixed capacity compressor 1 DCHRG sensor error OPEN/SHORTE409Fixed capacity compressor 2 DCHRG sensor error OPEN/SHORTE410Fixed capacity compressor 3 DCHRG sensor error OPEN/SHORTE411Compressor down by low pressure sensor protection control 2.E412Compressor down by low pressure sensor protection control 3.E413Outdoor unit overload 3 protection control error.E414Outdoor unit SUMP DOWN 2 protection control.E415Outdoor unit SUMP DOWN 3 protection control.E416Outdoor unit discharge temperature 1 protection control.E417Outdoor unit discharge temperature 2 protection control.E418Outdoor unit discharge temperature 3 protection control.E419Outdoor unit EEV open 1 self-diagnosis error.E420SUMP TEMP DIGITAL sensor error OPEN/SHORTE421SUMP TEMP FIXED 1 sensor error OPEN/SHORTE422SUMP TEMP FIXED 2 sensor error OPEN/SHORTE423SUMP TEMP FIXED 3 sensor error OPEN/SHORTE424High pressure sensor error OPEN/SHORTE425Low pressure sensor error OPEN/SHORTE426Outdoor unit reverse phase detection 2 error.E427Outdoor unit reverse phase detection 3 error.E428Outdoor unit compression rate 1 error (compression ratio over 8.5 for more than…E429Main cooling solenoid valve open error.E430Outdoor unit compression rate 3 error.E431Outdoor unit compression rate 1 errorE432Cooling mode operation forbidden when outdoor temperature below -5°C.E433Refrigerant charging operation restriction at 15°C or above outdoor temperature…E434Instant power off error (disappears when compressor runs).E440Low pressure detection error at initial compressor startE441Outdoor unit SUMP DOWN 1 protection control (SUMP temperature maintains over…E442Refrigerant charging operation at over 15°C outdoor temperature in heating mode.E443Instant power offE451Outdoor unit discharge temperature 1 protection control.E452Outdoor unit discharge temperature 2 protection control.E453Outdoor unit discharge temperature 3 protection control.E461Outdoor unit EEV open 1 self diagnosis error.E462Outdoor unit EEV open 2 self diagnosis error.E463Outdoor unit EEV open 3 self diagnosis error.E464Outdoor unit EEV close 1 self diagnosis error.E465Outdoor unit EEV close 2 self diagnosis error.E466Outdoor unit EEV close 3 self diagnosis error.E471Outdoor unit compression rate 1 error (compression ratio over 8.5 for more than…E472Outdoor unit compression rate 2 error.E473Outdoor unit compression rate 3 error.E481Forbid heating mode operation when outdoor temperature over 30°C.E482Forbid cooling mode operation when outdoor temperature below -5°C.E483Refrigerant charging operation restriction at over 15°C outdoor temperature in…E484Instant power offE5Setting error of option switch for peripheral equipment controlE501Forbid heating mode operation when outdoor temperature over 30°C.E502Low discharge pressureE503Refrigerant charging operation restriction at over 15°C outdoor temperature in…E504Instant power off (disappears when compressor runs).E505Full leakage of refrigerant of outdoor unit.E506Low pressure detection error at initial compressor start (low pressure below…E551Defrosting operation (not an error).E552Low discharge pressureE553Equability (oil leveling) operationE554Loading failure of digital compressor.E555Full leakage of refrigerant of outdoor unit.E601Communication error between wired remote controller and indoor unit.E602Communication error between master wired remote controller and slave wired…E603Communication error between 7 day scheduler and wired remote controller.FxSimultaneous breakaway of indoor unit EVAP IN and EVAP OUT sensor (x = indoor…G2Total leakage of refrigerant in outdoor unit or compressor loading failure.G3Breakaway of COND MID sensor in outdoor unit.G4Breakaway of OIL temperature sensor (detected once or more times)G5Breakaway of SUCTION sensorG6Breakaway of fixed compressor discharge sensor (detected once or more times)G7Outdoor unit freezing detection (indoor unit heat exchanger below -4°C for over…G8No heating operationG9OPEN/SHORT error of EVAP OUT sensor in indoor unit (x = indoor unit address).P0System down due to high temperature of discharge sensor (discharge temperature…P1System down due to high temperature of COND sensor (condensate temperature of…P2SUMP COMP DOWNP3System down due to low pressure switch detection (low pressure lower than 1.2…P4Reverse phase or open phase errorP431Forbid heating mode operation when outdoor temperature over 30°C.P432Forbid cooling mode operation when outdoor temperature below -5°C.P434Instant power off (disappears when compressor runs).P5System down due to frozen heat exchangerP501Outdoor unit freezing detection 1.P502Low discharge pressure (high pressure of 9 kg/cm² for 10 minutes).P503Outdoor unit freezing detection 3P504Outdoor unit overload 1 protection control error.P505Outdoor unit overload 2 protection control error.P506Outdoor unit overload 3 protection control error.P507Compressor down by high pressure sensor protection control 1.P508Compressor down by high pressure sensor protection control 2.P509Compressor down by high pressure sensor protection control 3.P510Compressor down by low pressure sensor protection control 1.P511Compressor down by low pressure sensor protection control 2.P512Compressor down by low pressure sensor protection control 3.P513Outdoor unit SUMP DOWN 1 protection control.P514Outdoor unit SUMP DOWN 2 protection control.P515Outdoor unit SUMP DOWN 3 protection control.P516Outdoor unit discharge temperature 1 protection control.P517Outdoor unit discharge temperature 2 protection control.P518Outdoor unit discharge temperature 3 protection control.P519Outdoor unit EEV open 1 self diagnosis error.P520Outdoor unit EEV open 2 self diagnosis error.P521Outdoor unit EEV open 3 self diagnosis error.P522Outdoor unit EEV close 1 self diagnosis error.P523Outdoor unit EEV close 2 self diagnosis error.P524Outdoor unit EEV close 3 self diagnosis error.P525Outdoor unit reverse phase detection 1 error.P526Outdoor unit reverse phase detection 2 error.P527Outdoor unit reverse phase detection 3 error.P528Outdoor unit compression rate 1 error.P529Outdoor unit compression rate 2 error.P530Outdoor unit compression rate 3 error.P6Error of momentary power failureP7Error of momentary power failureP701Float switch detection error (low level riskP702Compressor down by high pressure sensor protection control 2.P703Compressor down by high pressure sensor protection control 3.P704Outdoor unit overload 1 protection control error.P705Outdoor unit overload 2 protection control error.P706Outdoor unit overload 3 protection control error.P707Compressor down by high pressure sensor protection control 1.P708Compressor down by high pressure sensor protection control 2.P709Compressor down by high pressure sensor protection control 3.P710Compressor down by low pressure sensor protection control 1.P711Compressor down by high pressure sensor protection control 1.P712Compressor down by high pressure sensor protection control 2.P713Outdoor unit SUMP DOWN 1 protection control (SUMP temperature maintains over…P714Outdoor unit SUMP DOWN 2 protection control.P715Outdoor unit SUMP DOWN 3 protection control.P716Outdoor unit discharge temperature 1 protection control.P717Outdoor unit discharge temperature 2 protection control.P718Outdoor unit discharge temperature 3 protection control.P721Outdoor unit SUMP DOWN 1 protection controlP722Compressor down by low pressure sensor protection control 2.P723Compressor down by low pressure sensor protection control 3.P731Outdoor unit SUMP DOWN 1 protection controlP732Outdoor unit discharge temperature 2 protection control.P733Outdoor unit discharge temperature 3 protection control.P741Outdoor unit discharge temperature 1 protection control.P742Outdoor unit discharge temperature 2 protection control.P743Outdoor unit discharge temperature 3 protection control.P751Outdoor unit EEV open 1 self diagnosis error.P752Outdoor unit EEV open 2 self diagnosis error.P753Outdoor unit EEV open 3 self diagnosis error.P754Outdoor unit EEV close 1 self diagnosis error.P755Outdoor unit EEV close 2 self diagnosis error.P756Outdoor unit EEV close 3 self diagnosis error.P761Outdoor unit reverse phase detection 1 error.P762Outdoor unit reverse phase detection 2 error.P763Outdoor unit reverse phase detection 3 error.P771Outdoor unit compression rate 1 errorP772Outdoor unit compression rate 2 error.P773Outdoor unit compression rate 3 error.P781Forbid heating mode operation when outdoor temperature over 30°C.P782Forbid cooling mode operation when outdoor temperature below -5°C.P783Refrigerant charging operation restriction at 15°C or over of outdoor…P784Instant power offP8Compressor down due to protection control of low pressure sensor (low pressure…P801Float S/W detection error (low level riskP802Indoor unit EEV closing error (low level riskP803Indoor unit EEV opening error (low level riskP804MCU solenoid valve failure (low level riskP9Compressor down due to low pressure sensor protection control (less than 3…PACompressor down due to protection control of condensation ratio (compression…qxOPEN/SHORT error of room sensor in indoor unit (x = indoor unit address).rxCooling operation forbidden when outdoor temperature is below -5°C (flickering…sxHeating operation forbidden when outdoor temperature is over 30°C, or charging…t1Error of outdoor temperature sensor (OPEN/SHORT)t2Error of condenser temperature sensor (OPEN/SHORT)t3Error of PWM discharge temperature sensor (OPEN/SHORT)t4Error of liquid refrigerant pipe (COND OUT) temperature sensor (OPEN/SHORT)t5Error of oil temperature sensor (OPEN/SHORT)t6Error of suction temperature sensor (OPEN/SHORT)t7Error of high pressure sensor (OPEN/SHORT)t8Error of low pressure sensor (OPEN/SHORT)t9Error of fixed compressor discharge sensor (OPEN/SHORT)toOver 30°C when heatingtuBelow -5°C when coolingvxIndoor unit fan motor starting error (x = indoor unit address).
STOP GUESSING·Start verifying
Start diagnosing faster today.
$29/month · Cancel anytime · Or $9.95 3-day pass
Get Started