If your LG Multi V system is showing error 42, the low pressure sensor on your outdoor unit has been detected as disconnected or short circuited.
The important distinction: this is a fault with the sensor, not with your refrigerant.
What this means for your building
The low pressure sensor measures pressure on the suction side of the refrigerant circuit. The system uses that reading for compressor protection and for calculating superheat.
Error 42 reports that the sensor circuit itself has failed — an open circuit or a short. The system is no longer receiving a trustworthy reading.
This is not a low pressure condition. That distinction matters because the instinct on anything mentioning “low pressure” is to suspect a refrigerant leak and add charge. Doing that here would be adding refrigerant to a system that never needed it, based on a sensor that is not reporting.
The sub-codes
| Sub-code | Unit |
|---|---|
| 421 | Master outdoor unit |
| 422 | Slave outdoor unit 1 |
| 423 | Slave outdoor unit 2 |
On a multi-module system this identifies which unit to attend immediately.
How urgent is it?
Moderately urgent — more than a comfort sensor, less than a compressor fault.
The system has lost part of its protective visibility. Low pressure sensing is how it detects genuinely low-side conditions and shuts down before the compressor is harmed. Without it, that protection cannot act.
There is also a knock-on: superheat calculations depend on pressure readings. A faulty sensor can trigger other codes — including flood-back warnings — that reflect bad data rather than a real condition. Chasing those wastes a visit.
What it typically costs
Low to medium.
The sensor is not an expensive component. The cost is access to the outdoor unit — rooftop or plant room, with the appropriate safe working arrangements — plus diagnostic time to confirm the sensor is genuinely faulty rather than accurately reporting a real problem.
That confirmation step matters. Occasionally the sensor is fine and the circuit around it is not.
What causes it
- Connector working loose — vibration over years of operation
- Cable damage — chafing, rodent damage, or disturbance during other work
- Moisture ingress at the connector, common in rooftop units
- Sensor failure — they do age out
- Wiring damage between the sensor and the control board
- Damage during previous service — a lead pinched on reassembly
What happens if you leave it
Two problems compound.
Protection is reduced. The system cannot see genuinely low pressure, so a real refrigerant problem developing later would not trigger the shutdown that protects the compressor.
Other diagnostics become unreliable. Superheat calculations feed several protective functions. Bad pressure data produces misleading codes elsewhere, and a technician working from those is chasing a problem that does not exist.
What we check
- Use the sub-code to identify which outdoor unit is affected.
- Test the sensor against its specified output.
- Inspect the connector and cable run for damage, moisture and looseness — frequently the actual fault.
- Verify the reading is genuinely wrong rather than reporting a real pressure condition.
- Check wiring between the sensor and the control board.
- Review other active codes that may have been triggered by bad pressure data.
- Assess nearby sensors while access is open, since paying for rooftop access twice is avoidable.
What you can tell us before we arrive
- The sub-code — 421, 422 or 423 identifies the module.
- Any other codes appearing? Some may be consequences of the bad reading rather than separate faults.
- Has the unit been serviced recently? Sensor leads are commonly disturbed during other work.
- Any change in performance? Helps establish whether there is a real pressure issue alongside.
Related codes
- 150 — Liquid flood-back warning. Depends on superheat calculations, which depend on pressure readings.
- 50 — Loss of phase. Unrelated cause, but also identified by master and slave sub-codes.
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Related codes
Error 35 means low side pressure dropped far enough to shut the system down. Usually a refrigerant leak — but a failed expansion valve reads the same.
Error 150 warns that liquid refrigerant is at risk of flooding back to the compressor. It is a protective warning before damage, not after it.
Error 50 means the outdoor unit has lost a phase of its power supply. Usually an electrical fault rather than an HVAC one — and dangerous to force past.
Frequently asked questions
What does error code 42 mean on an LG Multi V system?
The outdoor unit low pressure sensor has been detected as disconnected or short circuited. It is a fault with the sensor or its wiring rather than an indication that system pressure is genuinely wrong. The system has simply lost a reliable reading from that sensor.
Is error 42 a refrigerant problem?
Not in itself. This code reports a fault in the sensor circuit, not a low pressure condition in the system. That distinction matters, because assuming a refrigerant problem and adding charge would be exactly the wrong response. That would overcharge a system that never needed it.
What do sub-codes 421, 422 and 423 mean?
They identify which outdoor unit reported the fault. 421 is the master, 422 is slave one, and 423 is slave two. On a multi-module system that tells us which unit to attend rather than checking all of them.
Why does a faulty pressure sensor matter?
Low pressure sensing feeds compressor protection and superheat calculations. Without a reliable reading the system cannot detect genuinely low pressure, and functions that would normally shut it down before damage occurs are no longer able to. Superheat calculations also become unreliable.
How much does an error 42 cost to fix?
Usually low to medium. The sensor itself is not an expensive component. Most of the cost is access to the outdoor unit and the diagnostic time to confirm the sensor is genuinely faulty rather than reporting a real pressure condition.
Can a bad pressure sensor cause other error codes?
Yes, and this is why it is worth resolving promptly. Superheat calculations depend on pressure readings, so a faulty sensor can trigger codes such as 150 for liquid flood-back risk that reflect bad data rather than a real refrigerant condition.