A04 on a Toshiba heat pump: Antifreeze operation (1)
Short answer
On a Toshiba heat pump, A04 means: Antifreeze operation (1). The heat pump keeps running with some functions stopped. An engineer is needed to fix the cause.
Reviewed 23 September 2026 · 4 sources
What it means
On the R32 Estia 1-Series (split, Wall-Mounted and All-in-One)
The manual says: Antifreeze operation (1).
Part involved: circulation pump, flow switch/sensor, strainer, water pipework.
When it shows: Freeze-risk condition from TC/TWO, or TWI<=15C during defrost.
Likely causes:
- Little or no water flow (dirt, piping too long/short).
- Heater power circuit (supply, breaker, connection).
- Backup heater presence setting.
- TWI/TWO/TC sensors or flow sensor.
On the Estia (Series 4) Standard & Powerful
The manual says: Antifreeze operation (1).
Part involved: circulation pump, flow switch/sensor, strainer, water pipework.
When it shows: Freeze-risk condition from TC/TWO, or TWI<=10C during defrost.
Likely causes:
- Little or no water flow.
- Heater power circuit.
- Backup heater presence setting.
- TWI/TWO/TC sensors and flow switch.
On the Estia 5 Series (Standard)
The manual says: Antifreeze operation (1).
Part involved: circulation pump, flow switch/sensor, strainer, water pipework.
When it shows: Freeze-risk condition from TC/TWO, or TWI<=10C during defrost.
Likely causes:
- Little or no water flow.
- Heater power circuit.
- Backup heater presence setting.
- TWI/TWO/TC sensors and flow switch.
On the Estia 5 Series Powerful (Hi Power / Extreme)
The manual says: Antifreeze operation (1).
Part involved: circulation pump, flow switch/sensor, strainer, water pipework.
When it shows: Trigger thresholds not printed in the IM; see SM.
Likely causes:
- Little or no water flow.
- Heater power circuit.
- Backup heater presence setting.
- TWI/TWO/TC sensors and flow switch.
Is it safe?
| Does the heat pump stop? | Partly |
|---|---|
| Severity | Fault — needs a manual reset once the cause is fixed |
| Engineer needed? | Yes |
The heat pump keeps running with some functions stopped.
Resetting itself: no.
Manual reset: Cause must be fixed by an engineer; hardware faults reappear until repaired.
What you can check
- Note the check code shown on the remote controller and the outdoor/hydro model numbers before contacting an engineer.
- Check the system pressure gauge on the hydro unit / heating system shows the normal fill pressure the installer set (the manuals ask for at least about 1 bar); if it is low, report it to the engineer. Check the filling loop is closed.
- Check radiator/underfloor zone valves and thermostats are calling for heat and have not all been turned off.
What the engineer will check
On the R32 Estia 1-Series (split, Wall-Mounted and All-in-One)
- Follow manufacturer's check-code flowchart.
- Little or no water flow (dirt, piping too long/short).
- Heater power circuit (supply, breaker, connection).
- Backup heater presence setting.
- TWI/TWO/TC sensors or flow sensor.
Who to ask for: Heat pump / heating engineer (hydronic).
Certification: Heat pump installer competence (e.g. MCS-installer staff); no F-gas needed unless refrigerant work follows.
On the Estia (Series 4) Standard & Powerful
- Follow manufacturer's check-code flowchart.
- Little or no water flow.
- Heater power circuit.
- Backup heater presence setting.
- TWI/TWO/TC sensors and flow switch.
Who to ask for: Heat pump / heating engineer (hydronic).
Certification: Heat pump installer competence (e.g. MCS-installer staff); no F-gas needed unless refrigerant work follows.
On the Estia 5 Series (Standard)
- Follow manufacturer's check-code flowchart.
- Little or no water flow.
- Heater power circuit.
- Backup heater presence setting.
- TWI/TWO/TC sensors and flow switch.
Who to ask for: Heat pump / heating engineer (hydronic).
Certification: Heat pump installer competence (e.g. MCS-installer staff); no F-gas needed unless refrigerant work follows.
On the Estia 5 Series Powerful (Hi Power / Extreme)
- Follow manufacturer's check-code flowchart.
- Little or no water flow.
- Heater power circuit.
- Backup heater presence setting.
- TWI/TWO/TC sensors and flow switch.
Who to ask for: Heat pump / heating engineer (hydronic).
Certification: Heat pump installer competence (e.g. MCS-installer staff); no F-gas needed unless refrigerant work follows.
What you might notice
Sources
- Air to Water Heat Pump Service Manual - Wall Mounted Type (File No. AE0-2101W) ↗ · AE0-2101W · page p.111 (printed page; PDF page +1) · Reviewed 23 September 2026
- Quick Reference Air to Water Heat Pump Series 4 Standard & Powerful Types ↗ · Quick Reference Series 4 Standard & Powerful (undated) · page p.60 (printed page; PDF page +1) · Reviewed 23 September 2026
- Air to Water Heat Pump Service Manual (File No. A10-1704) - Estia 5 Series Indoor/Outdoor ↗ · A10-1704 · page p.86 (printed page; PDF page +1) · Reviewed 23 September 2026
- Hydro Unit Installation Manual EF99812301-4 (Powerful) ↗ · EF99812301-4 · page p.57 (printed page; PDF page +1) · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026