F91

Panasonic Air Conditioner

Severity: Critical

What it means

Panasonic air conditioner F91 is the refrigeration cycle abnormality error, documented in Panasonic's troubleshooting documentation.
Panasonic's published description: 'refrigeration cycle problem caused by low gas or blockage — protective functions for low pressure activated.'
F91 fires when the outdoor PCB's low-pressure protection trips — the refrigeration cycle isn't building expected suction-side pressure.
The most common causes are refrigerant shortage (slow leak), expansion valve blockage, or a closed service valve.

Affected Models

Panasonic split-system residential air conditioners (CS/CU series with inverter outdoor)
Panasonic Etherea and Z-Series models
Panasonic ducted and cassette indoor models with separate outdoor
F91 specifically points at the outdoor refrigeration cycle — not the indoor side
Panasonic documentation: F91 = refrigeration cycle abnormality, low pressure protective function active

Common Causes

Refrigerant shortage from a slow leak over years
Refrigerant lost suddenly from a damaged flare or pinhole in the lineset
Stop valves at the outdoor unit not fully open after install or service
Strainer / dryer in the refrigerant circuit blocked
Linear expansion valve (LEV) stuck closed

How to Fix It

Stop running the AC.

Turn the unit off at the remote.
Don't keep retrying.
Running a Panasonic compressor on low refrigerant overheats it — and compressor replacement is far more expensive than a leak fix.
Leave the unit off until a technician arrives.
Confirm outdoor stop valves are fully open.

If F91 appeared right after install or service, the most likely cause is closed stop valves.
At the outdoor unit, look for service valve caps on the two refrigerant line connections.
Remove the caps.
The valve stem should be fully back-seated (turned counter-clockwise as far as it goes).
If a valve is closed, that's the cause — open it fully.
This is a quick visual check; if you're unsure, leave it for the technician.
Look for visible refrigerant leak signs.

While waiting for service, walk the line set between indoor and outdoor.
Refrigerant oil escapes with leaking gas — look for oil staining at flare nuts, joints, or anywhere along the copper.
Note any oil-stained areas to tell the technician — it saves hours of leak hunting.
Note when F91 first appeared.

F91 right after install or service = likely closed stop valve or installer-caused leak.
F91 after years of operation = slow refrigerant loss.
F91 sudden with a hissing noise = a sudden leak from a damaged joint or pinhole.
F91 after a hard freeze or storm = sometimes lineset insulation damage led to a leak.
Telling the technician the timing helps them find the cause faster.
Schedule HVAC service with refrigerant licence.

F91 requires a technician licensed for the refrigerant your system uses (R-32 or R-410A on modern Panasonic, R-22 on much older units).
EPA Section 608 in the US, F-Gas in Europe, or local equivalent.
A Panasonic-trained technician is preferred because they have the model-specific charge weights and access to genuine service parts.
Schedule through Panasonic Service or your installer.
Expect leak repair, evacuation, and recharge by weight.

A proper F91 fix is: pressure-test the system to confirm a leak (or rule one out), find the leak with electronic detector or UV dye, repair the leak, pull a deep vacuum to evacuate the system, recharge with the correct refrigerant to the nameplate weight.
A simple 'top-up' without leak find is a band-aid — the gas will leak again and F91 will return in months.
Insist on the full leak find + repair + evacuate + recharge sequence.

When to Call a Professional

F91 needs HVAC service with refrigerant capability — Panasonic's published guidance is explicit that F91 is a refrigeration-side fault, not something the owner can fix.
Don't keep cycling the unit hoping F91 will clear — running with low pressure stresses the compressor and shortens its life.
The fix is: technician verifies pressures with manifold gauges, finds and fixes any leak, evacuates the system, recharges by weight to Panasonic's specification.

Frequently Asked Questions

Is F91 the same as a Panasonic 'low refrigerant warning'?

Closely related but not the same.
F91 is the hard fault — the low-pressure protection has tripped and the AC has shut down.
Some Panasonic models also have softer warnings (in the Comfort Cloud app or as flashing operation lamps) that fire before F91, indicating the refrigeration cycle is starting to drift.
If you saw soft warnings for weeks and then F91 appeared, the system has been leaking slowly and the gauge of how much has been lost is significant.
If F91 appeared without prior warning, the loss was sudden (often a flare crack or lineset damage).
Both go to the same fix — but the timing tells the technician what to look for.