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Error code reference

LG Hvac Error Code 42

Low pressure switch opened, under 14 PSI.

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Low pressure switch opened, under 14 PSI.

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What 42 means on a LG hvac

Error code 42 on an LG HVAC system indicates that the low pressure switch has opened, meaning refrigerant pressure on the low side of the system has dropped below 14 PSI. The low pressure switch is a safety device that monitors suction-side pressure and cuts system operation when pressure falls to an unsafe level. When the switch opens, the system shuts down the compressor to prevent damage from running under low refrigerant charge or restricted flow conditions.

The most common reason for suction pressure to fall below 14 PSI is an insufficient refrigerant charge, often caused by a leak somewhere in the refrigerant circuit. Low outdoor ambient temperatures can also cause low suction pressure, as can a blocked or dirty filter that restricts airflow across the indoor coil. A failing expansion valve or a restriction in the refrigerant line can similarly starve the low side of the system and trigger this threshold.

Diagnosing this fault requires connecting manifold gauges to the service ports and measuring actual system pressures. If static pressure confirms a low charge, the system must be leak-checked before adding refrigerant. A technician should inspect the TXV or expansion device, check filter and coil conditions, and confirm ambient temperatures are within the system's rated operating range. Resolving the root cause is essential before resetting the fault and returning the unit to operation.

Source: manufacturer service documentation.

Common causes of 42

  1. 01

    Low refrigerant charge due to a leak

    Part replacement

    A refrigerant leak reduces suction-side pressure below the 14 PSI threshold that triggers the low pressure switch. Leak detection using UV dye or an electronic leak detector is required to locate the source before recharging.

  2. 02

    Dirty or clogged air filter

    Part replacement

    A severely restricted air filter reduces airflow across the evaporator coil, causing the coil to over-cool and refrigerant pressure to drop. Inspect and replace the filter as a first physical check.

  3. 03

    Faulty or stuck expansion valve (TXV)

    Part replacement

    A malfunctioning thermostatic expansion valve that is closed or stuck will restrict refrigerant flow into the evaporator, causing low-side pressure to fall sharply. Pressure readings at the manifold gauges will help confirm abnormal superheat values pointing to this component.

  4. 04

    Low outdoor ambient temperature

    Wiring / connection

    Operating the system in outdoor temperatures below its rated minimum can cause refrigerant to condense more than expected, reducing suction pressure and tripping the low pressure switch. Check whether the fault occurs only during cold weather conditions.

  5. 05

    Faulty low pressure switch

    Part replacement

    The low pressure switch itself may be defective and opening prematurely even when system pressure is within the acceptable range. This can be confirmed by measuring actual system pressures with gauges while the switch has opened.

See the test procedure for each cause

Frequently asked questions about 42

What does 42 mean on a LG hvac?
Error code 42 on an LG HVAC system means the low pressure switch has opened because the refrigerant pressure on the suction side of the system dropped below 14 PSI. This is a protective shutdown designed to prevent compressor damage. The system will not operate until the underlying pressure issue is resolved and the fault is cleared.
Can I fix error code 42 on my LG HVAC myself?
Some basic checks are accessible to a knowledgeable homeowner, such as inspecting and replacing a clogged air filter or verifying that outdoor temperatures are within the system's operating range. However, if the cause involves refrigerant loss or a faulty expansion valve, a licensed HVAC technician is required to handle refrigerants and perform pressure testing. Attempting to add refrigerant without identifying and repairing a leak will not solve the problem.
Is error code 42 on an LG HVAC dangerous?
The error itself is a safety feature, not a hazard indicator. The system is protecting the compressor from running under dangerously low pressure. However, if the fault is caused by a refrigerant leak, prolonged exposure to certain refrigerants in an enclosed space can be a health concern. Address the fault promptly and ensure adequate ventilation if you suspect a refrigerant leak indoors.
Will resetting my LG HVAC clear error code 42?
Resetting the unit may temporarily clear the fault code and allow the system to restart, but the error will return if the root cause has not been corrected. If suction pressure remains below 14 PSI, the low pressure switch will open again and shut the system down. A reset should only be performed after diagnosing and addressing the underlying issue.
How much does it cost to repair error code 42 on an LG HVAC?
Repair costs depend heavily on the cause. Replacing an air filter is a low-cost fix typically in the range of $10 to $30 for parts. If the system requires refrigerant leak detection, repair, and recharging, costs can range from $200 to $600 or more depending on the leak location and refrigerant type. A faulty expansion valve replacement can range from $150 to $400 including labor. A service diagnostic visit alone typically costs $75 to $150.

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