Guided diagnosis
Lennox 400: guided diagnosis
Technician recommended
Narrow it down
What is the outdoor unit doing when the system is switched on?
Listen and look from a safe distance outside — a balcony, a window or the yard. Do not open anything or go closer than you normally would.
- It runs — the fan turns and I can hear it working Outdoor unit runs with the fan turning
- The fan turns, but I never hear it settle into running Outdoor fan turns but the unit does not settle into running
- It hums or clicks briefly, then goes quiet Outdoor unit hums or clicks briefly then stops
- Nothing at all — it is completely silent Outdoor unit completely silent
- I'm not sure, or I cannot see or hear it safely Outdoor unit behaviour not established
When does the error appear?
Whether a code arrives at the start or after some running separates a starting condition from a load or heat condition.
- Straight away, within a minute of switching on Code appears immediately on starting
- After several minutes of running Code appears after several minutes of running
- At random, with no pattern Code appears intermittently with no pattern
- I'm not sure Timing pattern not established
What happened just before the code first appeared?
This is something only you can report, and on a compressor code it changes the answer more than anything else you can tell us.
- A power cut, generator changeover or voltage dip Followed a power interruption or supply event
- AC servicing, cleaning or a repair Followed servicing, cleaning or a repair
- A part was replaced — a compressor, a board or something else Followed a compressor, board or part replacement
- Nothing at all — it appeared on its own No preceding event reported
- I'm not sure History before the code unknown
Is there any cooling at all from the indoor unit?
Stand in front of the indoor unit with the system running and judge it as you normally would.
- None — the air is not cold at all No cooling reported
- Some, but much weaker than it used to be Weak cooling reported
- It cools normally, but the code still shows Cooling apparently normal with the code displayed
- I'm not sure Cooling performance not established
Is more than one air conditioner or appliance affected?
If other units or other appliances in the property behave oddly too, the answer is more likely to sit upstream of this AC.
- Yes — other units or appliances are affected too Other units or appliances affected as well
- No — only this air conditioner Only this air conditioner affected
- I'm not sure Whether other units are affected is unknown
Is the code still showing after the system has been switched off and on once?
Switch off at the wall switch or isolator once, wait ten minutes, then switch back on. Do not keep resetting it, and do not open anything.
- 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
From where you normally stand, does the outdoor unit seem unusually hot or the air around it unusually still?
Judge this from a distance only — never touch the outdoor unit, its pipework or its casing.
- Yes — it seems much hotter than usual, or the air around it is trapped Outdoor unit appears unusually hot or poorly ventilated
- No — it seems as it always does No unusual heat or ventilation problem observed
- I'm not sure Outdoor unit heat and ventilation not established
Where this points on your system
The compressor in the outdoor unit. A code pointing here still needs measurement before any conclusion is drawn.
About Lennox 400
The control reported that the compressor's internal overload had tripped — a protection built into the compressor itself operated, which is the compressor protecting itself rather than the compressor failing.
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, including any letters and separators, and note whether it is shown indoors, on the outdoor unit or on a wall controller.
- Note what the indoor unit is doing: running, blowing air that is not cold, or not starting at all.
- From a safe distance outside, note what the outdoor unit does when the system is switched on — runs, fan only, a brief hum or click, or nothing at all.
- Note whether the code appears immediately or only after several minutes of running.
- Note what happened immediately before the code first appeared — a power cut, a generator changeover, servicing, or a part being replaced.
- Note whether other air conditioners or other appliances in the property are affected as well.
- Switch the system off at its wall switch or isolator once, wait ten minutes, switch it back on, and note whether the code returns immediately, returns later, or stays away.
- Record the indoor and outdoor model numbers from their labels, where they can be read without opening anything — compressor protection behaviour is model-specific and a technician needs them.
- From a safe distance outside, note whether the outdoor unit's grille looks dusty or blocked, whether the fan turns, and whether anything is trapping hot air around it — never touch it or clean it while it may start.
- Note whether a power cut, a generator changeover or a voltage dip happened around the time the code first appeared — the compressor is the largest load in the system and the first thing to object to a supply event.
Do not
- Do not open the outdoor unit, and do not remove its casing, its electrical cover or the compressor terminal cover — the compressor circuit carries mains voltage and high current, and inverter equipment stores energy that remains after the power is switched off.
- Do not touch the compressor, its terminals or its wiring, and do not touch the pipework: the discharge line reaches temperatures that burn on contact.
- Do not measure or test anything on the compressor — no resistance, no insulation or megger test, no continuity, no voltage, and no starting or running current.
- Do not use a multimeter, clamp meter or any test instrument on this system: on a compressor circuit that is qualified work, not a homeowner check.
- Do not discharge, drain or touch a capacitor, and do not probe an inverter or drive board.
- Do not bypass, bridge or defeat an overload, a protection device, a pressure switch or a thermal protector, and do not force or jump a contactor to make the compressor run.
- Do not energise the compressor directly or attempt to start it by any means other than the system's own controls.
- Do not connect gauges or a manifold, do not measure refrigerant pressure, and do not add, top up, recover or release refrigerant.
- Do not open or close a service valve, and do not braze, cut or pressure-test any part of the sealed circuit.
- Do not replace the compressor, its capacitor or its wiring, and do not have a compressor fitted on the strength of an error code alone.
- Do not keep resetting the system or the breaker to get it to run: repeated start attempts and repeated protection trips can cause damage that was not there before.
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 compressor operation, a protection, a start failure, a thermal state, a drive condition or a mechanical condition.
- Establish which of those the code actually reports before anything is changed — the families of cause behind them do not overlap.
- Review the history: power interruptions, recent servicing and any part replacement, since each one changes which cause is likely.
- Separate a building or supply-side condition from a unit-side one before any work inside the unit.
- Inspect the compressor circuit's wiring, terminals, switching device and connections for heat damage, corrosion and partial seating, following correct isolation and stored-energy practice.
- Verify the relevant protection and control inputs the code depends on, to the manufacturer's documented method for this model.
- Assess heat rejection, airflow and the operating load where the code or the evidence implicates them, including condenser condition and outdoor fan operation.
- Evaluate refrigerant-side and load conditions only where the code supports it, by measurement rather than by assumption.
- Evaluate the inverter drive and its power stage where the equipment and the code involve them, observing stored-energy precautions.
- Establish whether the compressor is electrically or mechanically failed only once the supply, connections, starting path, protection, load and drive have been excluded — and only by measurement to the manufacturer's method.
- Decide between repair and compressor replacement only after that model-specific diagnosis, and establish what caused the failure before any replacement is fitted.
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 genuinely useful evidence: a compressor code that clears and stays away behaves differently from one that returns immediately.
When not to reset: Do not keep resetting it. Each reset is another start attempt into whatever condition caused the trip, and repeated start attempts and repeated protection trips are themselves capable of causing damage — as well as removing the pattern a technician would use.
What this code can point to
- Wiring, terminals or connections in the compressor circuit Most likely
- The compressor circuit runs through terminals, a switching device and connections that carry high current, and heat, vibration and servicing all affect them. A connection is a likely area to inspect before the compressor is suspected, because it is common, cheap to correct and frequently the actual cause.
- A protection device or limit operated Most likely
- A protection trip records that a limit was reached and the system acted on it. That is the protection working, not a broken part: what pushed the system to the limit is the diagnosis, and it is usually somewhere other than the component the protection sits on.
- The supply arriving at the unit Likely
- An internal overload responds to current and temperature together, so a low or unbalanced supply is one of the conditions that trips it with the compressor undamaged.
- Heat rejection, airflow or thermal load Likely
- Compressor and discharge temperatures are set by how well the system rejects heat: a blocked or dirty condenser, an outdoor fan not running, restricted airflow, recirculating hot air or extreme ambient conditions raise them long before anything inside the compressor is at fault. This is a likely area on any code that appears after some minutes of running.
- The starting path — command, switching device, capacitor or drive output Possible
- Between the control's decision to start and the compressor turning sit a command, a switching device, a starting or run capacitor on some equipment, and the drive output on inverter equipment. Any of them can prevent a perfectly healthy compressor from starting, which is why the path is proven before the compressor is judged.
- Refrigerant-side or operating-load condition Possible
- The load the compressor works against is a refrigerant-circuit condition: flow control, a restriction, the charge and the indoor load all change it. A code produced by that condition is a system finding — and it is not by itself an indication that refrigerant should be added, which is a claim only a technician's measurements can support or refuse.
- Temporary protection state that may clear Possible
- Systems latch a protection and hold it, and some hold it until a proper restart. Where the code followed a supply event and cleared cleanly on one controlled restart, that is a possible cause worth establishing first because it costs nothing to rule in or out.
- The compressor did not achieve a start Possible
- The system commanded a start and did not get it. That is a finding about the outcome, not about the reason: the supply, the starting components, the drive and the compressor all decide whether a start succeeds, and this stays a symptom until one of them is proven.
- Inverter drive or power-stage condition Possible
- On inverter equipment the drive decides how the compressor is fed and monitors what it does. A drive fault, its own supply, its cooling and the connections to the compressor can all produce a compressor-labelled code with the compressor undamaged — which is why a drive condition is separated from a compressor condition rather than merged with it.
- Model-specific diagnosis required Possible
- Compressor codes carry model-specific protection behaviour, drive logic and documented test procedures, and several brands reuse the same characters across generations with different meanings. Where that applies, documentation for the exact model decides what the code reports and what may safely be tested.
- Compressor operation feedback or detection Less common
- Some codes report that the system could not confirm the compressor was doing what it was told, rather than that it failed. The sensing or feedback path is then part of the picture alongside the compressor, and a detection fault with a running compressor is a recognised outcome.
- Mechanical condition inside the compressor Less common
- A compressor can be mechanically restricted, stalled or unable to start, and these codes exist because that happens. It is established by measurement after the supply, the starting path, the connections, the drive and the load have been excluded — because each of those imitates it exactly.
Which compressor condition this code refers to
Compressor overload or load-protection condition. At the compressor's own overload protection and the operating load acting on it, in the outdoor unit.
A protection operated because the compressor was working outside its limits. The important word is protection: it did its job, and what pushed the compressor there — heat rejection, airflow, the operating load, the supply, or a genuine mechanical condition — is the diagnosis. A protection trip records a condition; it does not name a broken part.
Common questions
What does Lennox 400 mean?
400 on a Lennox system is a compressor-side report: it points at Compressor overload or load-protection condition. The control reported that the compressor's internal overload had tripped — a protection built into the compressor itself operated, which is the compressor protecting itself rather than the compressor failing. At the compressor's own overload protection and the operating load acting on it, in the outdoor unit. What the system has told you is that something about the compressor's starting, protection, load, temperature or operation was outside what it accepts, and it stopped rather than continuing. The code identifies the area involved, not the component that failed.
Does Lennox 400 mean the compressor needs replacing?
No. A compressor-related error code does not by itself prove a compressor failure or a need for replacement. The same display can be produced by a temporary protection state, the supply arriving at the unit, a connection or terminal in the compressor circuit, the starting path, heat rejection and airflow, the operating or refrigerant load, the inverter drive, or a mechanical condition inside the compressor — and only the last of those needs a compressor. A technician establishes the upstream causes first, because a compressor is the costliest part of the system and fitting one on the strength of a code frequently leaves the fault exactly where it was.
Can a power cut cause Lennox 400?
It can, and it is one of the first things worth establishing. A compressor is the largest load in the system, so an interruption, a generator changeover or a voltage dip is exactly the kind of event it objects to — and the protection that follows can latch until the system is properly restarted. That is why what happened immediately before the code, and what one controlled restart changes, are both real evidence rather than background detail.
The outdoor unit hums or clicks but never starts — does that mean the compressor is seized?
It does not establish that. A brief hum or click and no start is the sound of a start attempt that did not succeed, and the reason may be the supply, a starting or run component, the switching device, the drive output, a connection, or a mechanical condition in the compressor. Several of those are inexpensive to correct and all of them imitate each other from outside the unit. What separates them is measurement on the starting path, in that order — and repeatedly trying to start it while the condition remains can add damage that was not there before.
Is Lennox 400 a protection or a failure?
That is exactly what the diagnosis has to separate, and the code alone does not answer it. A protection trip records that a limit was reached and that the system acted on it — which means the protection worked. What pushed the system to the limit is the diagnosis, and it is often heat rejection, airflow, the supply or the load rather than the component the protection sits on.
Can I reset Lennox 400?
Switching the system off at its wall switch or isolator, waiting ten minutes and switching it back on is reasonable once, and how it behaves afterwards is informative. Repeatedly resetting it is not advisable: every reset is another start attempt into whatever caused the trip, and repeated attempts and repeated protection trips can cause damage as well as removing the history a technician would use.
What can I safely check myself?
Observations and history only, from where you normally stand: the exact code, what the indoor unit is doing, what the outdoor unit does when the system is switched on, whether the code appears immediately or after some minutes, what happened before it appeared, whether other units or appliances are affected, and what one controlled restart changes. Do not open the unit, do not touch the compressor, its terminals, its wiring or the pipework, do not use any test instrument, and do not go near the refrigerant circuit.
Why can this not be checked with a meter at home?
Because the compressor circuit combines mains voltage, high current, stored energy in inverter equipment, a line hot enough to burn and a sealed circuit under pressure — and a compressor terminal failure under load can be violent. The tests that actually separate these causes are done with the system isolated, following the manufacturer's method for that model, by someone equipped to do it. We do not publish resistance, current, pressure or temperature figures here for the same reason: they are model-specific, and applying a generic value is how healthy compressors get condemned.
Can I keep using the AC with Lennox 400?
Usually it will not let you: the system stops rather than running the compressor outside the limits it protects. Where it does keep running or keeps retrying, leaving it that way is not advisable — repeated protection trips and repeated start attempts are capable of turning a condition that was correctable into a failure that is not.
What will a technician check for Lennox 400?
In order: the code against documentation for your exact model, which condition it actually reports, the history of power events and recent work, whether the condition is supply-side or unit-side, the compressor circuit's wiring, terminals and switching device, the relevant protection and control inputs, heat rejection and airflow, the refrigerant-side load where the code supports it, and the inverter drive where the equipment has one. Only once those are excluded is the compressor itself assessed by measurement — and repair or replacement is decided after that, never before it.
The internal overload tripped — has the compressor already been damaged?
Not by the trip itself: the internal overload exists precisely to stop the compressor before damage occurs, so a trip is evidence that the protection worked. What matters is the condition that caused it — heat rejection, airflow, the supply, the load, or a starting problem — because leaving that in place and repeatedly restarting is what turns a protected compressor into a failed one.
Does Lennox 400 on its own justify a compressor quotation?
No. What the code establishes is a condition on the compressor side; it does not establish a compressor failure. The supply, the connections and terminals, the starting path, heat rejection and airflow, the operating load and — on inverter equipment — the drive all produce this code with the compressor undamaged. Any quotation for the costliest part of the system should follow measurement that excluded those, not a code read off the display.
Did this help you find the fault?
Thank you — recorded. This helps us improve the guidance, and is never published.
Need this looked at properly?
Call or send a photo on WhatsApp and we will tell you whether it needs an inspection, and what the work involves, before anyone travels.
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)