Acson Inverter Y / APJ2 Aircon C4 Error Code: What It Means

What does Acson C4 mean?

Acson Inverter Y / APJ2 C4 is listed as indoor heat-exchanger thermistor is open or shorted and heat-exchanger temperature-sensor fault in the Acson G18 / GS02 Inverter Y display family and Acson APJ2 A5WMY10/15KR fault lists. This listing is unofficial, so do not approve parts from this code alone. Do not collapse this code across model families; use the model, series, or system type to choose the right row. Confirm the exact model, series, or signal row before accepting a parts quote.

Choose the listing that matches your unit

The same code has more than one listed meaning. Match the model family, the system type and the unit that shows the code before approving parts.

Code, what it means, and what to check first
Code	What it means	What to check first
Acson G18 / GS02 Inverter Y display family	Indoor heat-exchanger thermistor is open or shorted	Match this meaning to your installed model and the unit that shows the code, then use the checks below.
Acson APJ2 A5WMY10/15KR	Heat-exchanger temperature-sensor fault	Match this meaning to your installed model and the unit that shows the code, then use the checks below.

Code
Acson G18 / GS02 Inverter Y display family
What it means
Indoor heat-exchanger thermistor is open or shorted
What to check first
Match this meaning to your installed model and the unit that shows the code, then use the checks below.
Code
Acson APJ2 A5WMY10/15KR
What it means
Heat-exchanger temperature-sensor fault
What to check first
Match this meaning to your installed model and the unit that shows the code, then use the checks below.

What to do now

Use these steps to record evidence and match the installed model to its service manual before diagnosis or a repair quote.

Record C4 and match the service manual
Record C4 and stop there; the manufacturer gives no customer reset procedure. Arrange diagnosis with the matching service manual.
No manufacturer reset steps listed for C4
Capture the complete C4 error code and model stickers first. The manufacturer lists the meaning but no customer reset procedure; do not treat a generic power-cycle as manufacturer guidance.
Send these to us
C4 error code, a clear display photo, Acson Inverter Y / APJ2 indoor and outdoor model stickers, the controller, gateway, or unit that reported the code, operating mode and affected area, and any power or setting changes made after the code appeared

Guidance for your model

Notes that match the code to your exact model. Electrical, refrigerant, wiring, resistance and component checks are for a technician.

Guidance for your model
Detail	What it says
Singapore / model note	Unofficial public copy of a manufacturer-authored Acson International service guide. This table is scoped to the G18/GS02 Inverter Y reporting family shown in the guide; it is not a universal table for current Acson models.
Singapore / model note	Unofficial public copy of a manufacturer-authored Acson International service guide. This table is model-scoped to APJ2 with A5WMY10/15KR and is kept separate from other Acson meanings.

Detail
Singapore / model note
What it says
Unofficial public copy of a manufacturer-authored Acson International service guide. This table is scoped to the G18/GS02 Inverter Y reporting family shown in the guide; it is not a universal table for current Acson models.
Detail
Singapore / model note
What it says
Unofficial public copy of a manufacturer-authored Acson International service guide. This table is model-scoped to APJ2 with A5WMY10/15KR and is kept separate from other Acson meanings.

What to check before repair

The manufacturer's fault table gives the code's meaning, not a repair procedure. Capture evidence first; model-specific tests belong to the matching service manual.

Homeowner checks and technician confirmations
You can check	Technician should confirm
Photograph the complete C4 error code before taking any action.	Confirm C4 exactly and record which controller, gateway, or unit reported it.
Capture the Acson Inverter Y / APJ2 indoor and outdoor model stickers and identify which controller or unit reported it.	Match the indoor and outdoor model stickers, installed family, and reporting equipment to the source scope.
The manufacturer provides no customer reset procedure. Preserve the evidence and use the matching service manual for any system action.	Follow the matching service-manual procedure and keep measured test evidence before naming a failed part.

You can check
Photograph the complete C4 error code before taking any action.
Technician should confirm
Confirm C4 exactly and record which controller, gateway, or unit reported it.
You can check
Capture the Acson Inverter Y / APJ2 indoor and outdoor model stickers and identify which controller or unit reported it.
Technician should confirm
Match the indoor and outdoor model stickers, installed family, and reporting equipment to the source scope.
You can check
The manufacturer provides no customer reset procedure. Preserve the evidence and use the matching service manual for any system action.
Technician should confirm
Follow the matching service-manual procedure and keep measured test evidence before naming a failed part.

Source-bounded repair decision

Acson Inverter Y / APJ2 C4 confirms the fault label the manufacturer lists, not a failed part or repair procedure. Match the installed model to its service manual and require measured diagnostic evidence before approving parts, settings, or refrigerant work.

Whether the reporting controller or unit and exact installed model are confirmed.
What the matching service-manual procedure and measured tests show.
Whether the proposed repair names the evidence for the failed part or condition.

Acson C4 questions

What does the Acson C4 error mean?

Acson Inverter Y / APJ2 C4 is listed as indoor heat-exchanger thermistor is open or shorted and heat-exchanger temperature-sensor fault in the Acson G18 / GS02 Inverter Y display family and Acson APJ2 A5WMY10/15KR fault lists. This listing is unofficial, so do not approve parts from this code alone. Do not collapse this code across model families; use the model, series, or system type to choose the right row. Confirm the exact model, series, or signal row before accepting a parts quote.

Can I fix a Acson C4 error myself?

No. C4 needs a technician's measurement before parts are decided. Photograph the complete C4 error code before taking any action. Capture the Acson Inverter Y / APJ2 indoor and outdoor model stickers and identify which controller or unit reported it.

Is it safe to keep running the aircon with a C4 error?

Record C4 and stop there; the manufacturer gives no customer reset procedure. Arrange diagnosis with the matching service manual.

What causes a Acson C4 error?

Acson C4 has a listed meaning that depends on the exact model and on which unit shows the code. Use the decoder on this page before assuming a specific cause.

Will a Acson C4 error need a repair or a full replacement?

Acson Inverter Y / APJ2 C4 confirms the fault label the manufacturer lists, not a failed part or repair procedure. Match the installed model to its service manual and require measured diagnostic evidence before approving parts, settings, or refrigerant work.

What should I send a technician about a Acson C4 error?

Before booking, send: C4 error code, a clear display photo, Acson Inverter Y / APJ2 indoor and outdoor model stickers, the controller, gateway, or unit that reported the code, operating mode and affected area, and any power or setting changes made after the code appeared.

Confirm the fault

Share the error code, model number and symptoms so we can assess the likely cause.

Not the right code?

Browse the full Acson list or check a few codes in the same fault category.