Error codes such as E4 or E6 are brand- and model-specific. The same letters can mean different sensors, communication faults or protection trips across manufacturers, so a generic internet meaning is not a diagnosis. Power-cycle only if the manual allows it, photograph the code, note indoor and outdoor behaviour, and arrange competent inspection if the code returns, cooling fails, ice forms or breakers trip. Do not keep clearing protective lockouts to force the compressor to run.
Treat the code as a clue, not a parts list
Manufacturers publish code tables in the service or user manual for that exact indoor and outdoor pair. An E4 on one brand may be a sensor or discharge issue, while another brand uses E4 for something else entirely. The same applies to E6 and similar alphanumeric displays.
Write down the full code string, whether it flashes, and whether the indoor fan, outdoor fan or compressor still try to start. That record saves engineer time and stops you ordering the wrong board from a forum post.
Safe user steps
Check mode, set temperature, filters and outdoor clearances first. If the manual describes a reset sequence after a temporary trip, follow it once. Repeated resets that bring the code straight back usually mean the protection is doing its job.
Do not open electrical panels, jump sensors or bypass high-pressure, low-pressure or freeze protections. Those steps can destroy the compressor and create a safety risk.
- Photograph the display and outdoor data plate.
- Note weather, time of day and whether all rooms are affected.
- Stop if you smell burning or hear repeated contactor chatter.
- Ask the engineer to confirm the code against the OEM table on site.
What engineers usually check next
Depending on the verified code meaning, diagnosis may cover coil and ambient sensors, communication wiring between indoor and outdoor PCBs, fan motors, dirty coils, refrigerant pressures after other causes are ruled out, or control-board faults.
Ask for a written finding before authorising parts. A guessed 'E6 board' replacement without proving the cause is a common way to pay twice.
Frequently asked questions
Does E4 always mean low refrigerant?
No. Never assume refrigerant from the code letters alone. Confirm the manufacturer table for your model and let a qualified engineer diagnose.
Should I keep resetting an E6 code?
One manual-approved reset can be reasonable. Repeated clears without diagnosis risk compressor damage and usually waste call-out fees later.
Core buying guides
Start with these high-intent guides, then dig into the rest of the library.
Related guides
Sources and further reading
How this guide was prepared
This guide was written by the Local AC Installers Editorial Team. We compare official guidance, legislation and established consumer information, separate general information from project-specific advice, and show publication dates and sources. We do not accept installer accreditation claims at face value.