Narrow it down

Did anything else lose power at the same time?

This is the most useful thing you can tell us. Think about lights, sockets and other appliances — not just the air conditioning.

Yes — other appliances or the whole property were affected Other loads or the whole property affected
Only the air conditioning — several AC units are affected Several AC units affected, other loads unaffected
No — only this one AC unit Only this unit affected
I'm not sure Scope of the power loss not established

Is the unit completely dead, or does it power up and then stop?

Look at the indoor display and whether the remote gets any response — from outside the unit only.

Completely dead — no display, no response at all Unit completely without power
It powers up and shows the code, but will not run Powers up and displays the code without running
It starts, runs briefly, then stops with the code Starts then stops with the code
It runs, but the code appears now and then Runs with the code appearing intermittently
I'm not sure Unit state not established

Does the breaker or isolator for the air conditioning look tripped?

Look only, and do not keep switching it. If a breaker has tripped, one attempt to reset it is enough — a breaker that trips again is reporting a real fault, and repeatedly resetting it is dangerous.

Yes — and it trips again when reset once Protective device trips again after one reset
Yes — it was tripped and has stayed on since Protective device had tripped, holding since one reset
No — nothing looks tripped No protective device appears tripped
I cannot see or identify it Protective device state not established

Does the outdoor unit start at all when the system is asked to cool?

Watch and listen from a safe distance only. Never open the unit, remove a cover, or touch anything inside it.

No — it does nothing Outdoor unit does not start
It hums or clicks but does not run Outdoor unit attempts to start without running
Yes — it starts, then stops Outdoor unit starts then stops
I could not see or hear it safely Outdoor unit operation not established

Did this start after a power cut, a generator changeover or a voltage dip?

A supply event immediately before the code is strong evidence, and it is something only you can report.

Yes — right after a power cut or generator changeover Followed a power interruption or generator changeover
Yes — during a period of dimming lights or low voltage Followed observed low voltage or dimming
No — nothing unusual happened with the power No supply event reported
I'm not sure Supply event history unknown

Did this start after any electrical or installation work?

Work on the distribution board, a new isolator, a unit installation or replacement, or work by other trades in the property all count.

After electrical work in the property or building Followed electrical work in the property or building
After the unit was installed, moved or replaced Followed installation, relocation or replacement
After a part was replaced or a repair was made Followed a part replacement or repair
No work at all — it appeared on its own No recent work reported
I'm not sure Recent work history unknown

Has building management or a neighbour reported an electrical problem?

In apartments and compounds this is often the fastest way to establish that the problem is not your unit at all.

Yes — a building electrical issue has been reported Building electrical issue reported by management or neighbours
No — nobody else has reported anything No building electrical issue reported
I have not asked Building electrical status not established

Is the code still showing after the system has been switched off and on once?

Switch off at the wall switch or the isolator once, wait ten minutes, then switch back on. Do not keep resetting it, and do not touch the breaker repeatedly.

Yes — it came straight back Code returns immediately after one restart
It cleared, then returned after running a while Code clears then returns during running
It has run normally since Normal operation since one restart
It has not been switched off and on Restart not attempted

Where this points on your system

Area the diagnosis points to Electrical supply arriving at the unit Shown on: Split system Also kept in view: Communication path between the indoor and outdoor unit; Control board area inside the outdoor unit

The electrical supply arriving at the unit, including its isolator and protection.

About Lennox PC 01

The unit stopped on a voltage protection covering both a high and a low supply condition, so the code establishes that the supply was outside range without saying in which direction.

How to read this diagnosis

Everything here is a possible cause and a likely area to inspect, based on your answers — it is not a confirmed diagnosis.
One code may indicate more than one fault, so a technician would typically verify the suspected area by measurement before any part is replaced.
Manufacturer documentation should be checked for model-specific values: we do not publish voltages, pressures or resistance figures we cannot source for your exact model.

What you can check, and what needs a technician

Safe to check yourself

Confirm the exact code as it is displayed, and note which unit or controller is showing it.
Note whether anything else lost power at the same time — other appliances, lights, sockets or other AC units.
Note whether the unit is completely dead, powers up but will not run, starts and stops, or runs with the code appearing now and then.
Look at the breaker or isolator for the air conditioning and note whether it appears tripped. One reset attempt is enough — if it trips again, leave it off and report that.
Watch and listen from a safe distance to establish whether the outdoor unit starts, hums without running, or does nothing at all.
Note whether the fault began after a power cut, a generator changeover, or a period when lights were dimming.
Note whether it began after electrical work, an installation or a repair, and who carried that work out.
Ask building management or a neighbour whether an electrical problem has been reported in the building.
Where several indoor units are served by one system, note how many of them are affected.
Record the indoor and outdoor model numbers and the supply arrangement stated on the label before contacting anyone — electrical thresholds are model-specific.
Note whether lights dimmed, other appliances misbehaved, or a power cut or generator changeover happened around the time the code appeared — that observation is often what separates a supply condition from a unit fault.

Do not

Do not measure voltage, current or continuity anywhere on this system, and do not use a multimeter or test instrument on mains circuits — this is qualified work, not a homeowner check.
Do not remove any electrical cover or panel, and do not open the indoor or outdoor unit, a distribution board or a terminal box.
Do not touch, test or discharge a capacitor, and do not probe an inverter board or the DC bus: these hold stored energy after the power is switched off.
Do not touch compressor terminals or any exposed terminal, busbar or conductor.
Do not test, alter or reconnect phase wiring, and never attempt to check or change phase sequence.
Do not bridge, bypass or jumper a fuse, an overload or any protective device, and do not hold a breaker closed.
Do not keep resetting the breaker: one attempt is enough, and a device that trips again is reporting a real fault rather than a nuisance.
Do not run the system on an extension lead, a temporary supply or a generator to see whether it works.
If there is a smell of burning, visible scorching, a buzzing distribution board, or the breaker trips again, leave the supply off and report that instead of the code.

Technician verification sequence

For the attending technician — these steps involve live equipment or sealed systems and are not homeowner checks.

Confirm the exact model and the manufacturer's own definition for the displayed code, including whether it describes the incoming supply, a protection operation, a sensing circuit or the unit's own power stage.
Establish scope before anything is opened: whether the condition affects this unit alone, several units, or the property — the three lead to different investigations.
Inspect the supply path and protective devices serving the unit, including the isolator, its rating and evidence of thermal distress.
Inspect wiring, terminals and connectors along the power path for looseness, corrosion, overheating or incorrect connection, and correct what is found before parts are suspected.
Evaluate the supply condition against the manufacturer's stated requirements for that model, using approved instruments and correct isolation practice.
Evaluate the phase condition on three-phase equipment, including sequence and balance, where the code or the installation history implicates it.
Assess the current sensing and supply-synchronisation circuits, separating a reporting fault from a real electrical condition.
Assess the inverter power stage and DC bus only where the code or the evidence implicates it, following the manufacturer's isolation and stored-energy procedure.
Evaluate load-side condition where a protection is being triggered, including what the load is being asked to do rather than the load alone.
Confirm the root cause before any board or compressor is replaced, and where a protective device operated, establish what it responded to.

About resetting

Switching the system off at its wall switch or isolator once, waiting ten minutes and switching it back on is reasonable, and what happens next is useful evidence: a code that returns immediately behaves differently from one that only returns while the system is working.

When not to reset: Do not keep resetting it, and in particular do not keep resetting a breaker. A protective device that operates again is reporting a real electrical fault, and forcing the supply back on is how an electrical fault becomes a fire or a destroyed compressor. Repeated resets also erase the pattern a technician would use.

What this code can point to

Building or supply-side electrical condition Most likely: The supply reaching the unit may be low, high, unstable or missing a phase because of something upstream of the air conditioner: the building circuit, a shared or loaded feed, a distribution board, a generator changeover or work elsewhere in the property. Where several loads or several units are affected, this becomes a likely area to inspect — and it is not the occupant's to touch.
Local supply interruption to this unit Most likely: Power to this unit alone may have been interrupted or restored — a tripped protective device on its circuit, an isolator left off, or a supply that came back after an outage. It is a possible cause worth establishing early because it is the cheapest thing to rule in or out.
Wiring, terminals or connectors in the power path Most likely: A loose, corroded, overheated or incorrectly made connection changes what the unit receives without anything having failed inside it. Connections are inspected before parts are suspected because they are common, cheap to correct and often the actual cause.
Supply quality and operating conditions Likely: Voltage that sags under load, generator or changeover events, long or shared feeds and sustained peak-summer demand all shift what arrives at the unit. That is an installation and supply condition rather than a broken part, and it is one of the reasons the same code can come and go.
Protective device operation — breaker, fuse or overload Possible: A breaker, fuse, thermal device or overload may have operated. That is the protection doing its job, so what needs finding is the condition behind it. A device that operates again is reporting a real fault, and it must never be bridged or held closed.
Phase condition on a three-phase supply Less common: On three-phase equipment a missing phase, a reversed or wrong-sequence connection, or an imbalance between phases will stop the unit. On a new or recently altered installation the sequence is a commissioning matter; on a running system a lost phase usually sits in the supply path or a protective device.
Inverter power stage or DC bus condition Less common: The rectifier, power-factor stage and DC bus condition incoming power before it drives the compressor. This section follows the supply, so a bus or module condition may be reporting the supply rather than itself. It holds stored energy after switch-off, which is why it is never opened outside qualified work.
Control board power input or signalling circuit Less common: The board's own power input, and the circuits that read the supply waveform and current, can report a fault while the supply is healthy. This is a likely area to inspect for signalling codes — and it is a possible cause of a fault that looks like a supply problem, not proof that the board needs replacing.
Current sensing or detection circuit Less common: Current is measured by a sensing device reporting through wiring to a controller input. A fault on that path may indicate an overcurrent that is not happening, which is a completely different repair from a load genuinely drawing too much.
Load-side current condition Less common: The compressor, a motor or another load may genuinely be drawing more current than the unit accepts. Even then the current is a symptom: a low supply voltage, restricted airflow, high heat rejection load or a mechanical condition can all raise it, so the draw is explained before any part is condemned.
Low-voltage control supply Less common: Controls run on a separate lower-voltage supply from a transformer. Where that supply is short or interrupted the system stops while the main feed is healthy — the transformer, the control wiring and whatever loads that circuit are all part of the possibility.
Model-specific electrical condition needing further diagnosis Less common: Electrical codes carry model-specific thresholds, protection sequences and lockout behaviour, and some brands reuse the same digits across ranges with different meanings. Where that applies, documentation for the exact model decides what the code means and which values are correct for it.

Which electrical condition this code refers to

Supply voltage outside range (high or low). On the incoming supply to the unit — the code covers both directions without saying which one it saw.

This code covers both a high and a low supply condition, so the display cannot tell you which one happened. What was going on at the time — a power cut, a generator changeover, heavy load, or nothing unusual — is genuinely useful evidence here, and it is something only you can observe.

Common questions

What does Lennox PC 01 mean?

PC 01 on a Lennox system is an electrical report: it points at Supply voltage outside range (high or low). The unit stopped on a voltage protection covering both a high and a low supply condition, so the code establishes that the supply was outside range without saying in which direction. On the incoming supply to the unit — the code covers both directions without saying which one it saw. The unit judged the power it received, or the power it handled, to be outside what it accepts, and stopped rather than continuing. The code names the area involved, not the component that failed.

Does Lennox PC 01 mean the control board needs replacing?

No — an electrical code is not a diagnosis of a board, a capacitor or a compressor, and that conclusion does not follow from the code. The same display can be produced by the incoming supply, the building circuit, a connection in the power path, a protective device, the phase condition on three-phase equipment, the inverter power stage, a current sensing circuit, the low-voltage control supply or what the load is drawing. Boards, capacitors and compressors are replaced on the strength of measurement by a technician, not on the strength of an error code — and on a low or unstable supply nothing inside the unit is faulty at all.

Is Lennox PC 01 a problem with my AC or with the building supply?

That is the first thing worth narrowing, and you can help decide it without touching anything. If other appliances, lights or several AC units lost power at the same time, a building or supply-side condition becomes a likely area to inspect. If only this unit is affected, its own power path — connections, protective device, power input or load — moves up the list. Neither is a certainty, but the answer changes who should be called first.

Can a power cut cause Lennox PC 01?

It can. A power interruption, a generator changeover or a period of low voltage can leave the unit reporting an electrical condition, and some codes are latched until the system is restarted. That is why what happened with the power immediately before the code is genuinely useful evidence, and why one controlled restart tells a technician something.

Can I reset Lennox PC 01?

Switching the system off at its wall switch or isolator, waiting ten minutes and switching it back on is reasonable once, and how quickly the code returns is itself informative. Repeatedly resetting a breaker is not reasonable: a protective device that trips again is reporting a real electrical fault, and forcing it back on is dangerous.

Can I keep using the AC with Lennox PC 01?

It is better left off until it is diagnosed. The unit will usually refuse to run anyway, and where it does keep running it is operating on a supply or a power condition it has already judged unacceptable — which is how electrical faults damage compressors and boards.

What can I safely check myself?

Observations only: the exact code, whether anything else lost power, whether the unit is completely dead or powers up and stops, whether the breaker looks tripped (one reset attempt only), whether the outdoor unit starts, whether it followed a power cut or electrical work, and whether the building has reported a problem. Do not measure anything, do not open any cover, and do not touch a capacitor, a terminal or a breaker repeatedly.

Why should I not test this myself with a multimeter?

Because the circuits involved are mains or three-phase, and the inverter section stores energy after the power is off — so a measurement that looks simple carries a genuine risk of electrocution, arc flash and damage to the unit. Everything electrical in this diagnosis therefore sits on the technician side of the boundary, and nothing here asks you to measure, probe or discharge anything.

What will a technician check for Lennox PC 01?

In order: the code against the documentation for your exact model, the scope of the condition, the supply path and its protective devices, wiring and terminal condition, the supply itself against the manufacturer's stated requirements, the phase condition where three-phase applies, the current sensing and synchronisation circuits, the inverter power stage only where implicated, and finally the load. We do not publish voltage or current figures, because they depend on your model and installation.

Does Lennox PC 01 identify the compressor as the fault?

Not on the evidence of a code. Where a code reports missing current or an overcurrent, the power path, a protective device, a switching device, a connection and the sensing circuit are all in the same picture as the compressor — and a compressor drawing high current is often responding to a low supply voltage or a heat-rejection condition rather than to its own condition. A technician would typically confirm by measurement what is producing it before anything that expensive is replaced.

Did this help you find the fault?

Sources consulted

Official Lennox mini-split user guide and service manual (indoor display / outdoor LED tables)
Official Lennox residential communicating systems alert code guide
Official Lennox Climatic 2 LF20 rooftop codes-and-faults document
Cross-validated independent Lennox compilation (legacy alphanumeric table)

Lennox PC 01: guided diagnosis