A03 on a Toshiba heat pump: Temperature increase failure (hot water supply)
Short answer
On a Toshiba heat pump, A03 means: Temperature increase failure (hot water supply). The heat pump keeps running with some functions stopped. An engineer is needed to fix the cause. The wording differs by model range — each is shown below.
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: Temperature increase failure (hot water supply).
Part involved: backup heater, overheat thermostat, cylinder immersion and thermal cut-out.
When it shows: TTW cylinder sensor exceeds 85C.
Likely causes:
- Faulty cylinder sensor TTW.
- Hot water cylinder thermal cut-out.
On the Estia (Series 4) Standard & Powerful
The manual says: Temperature increase error (hot water supply).
Part involved: backup heater, overheat thermostat, cylinder immersion and thermal cut-out.
When it shows: TTW exceeds 85C.
Likely causes:
- TTW cylinder sensor.
- Cylinder thermal cut-out.
On the Estia 5 Series (Standard)
The manual says: Temperature increase error (hot water supply).
Part involved: backup heater, overheat thermostat, cylinder immersion and thermal cut-out.
When it shows: TTW exceeds 85C.
Likely causes:
- TTW cylinder sensor.
- Cylinder thermal cut-out.
On the Estia 5 Series Powerful (Hi Power / Extreme)
The manual says: Temperature increase error (hot water supply).
Part involved: backup heater, overheat thermostat, cylinder immersion and thermal cut-out.
When it shows: Trigger thresholds not printed in the IM; see SM.
Likely causes:
- TTW cylinder sensor.
- Cylinder thermal cut-out.
Is it safe?
| Does the heat pump stop? | Partly |
|---|---|
| Severity | Fault — clears by itself when the condition goes |
| Engineer needed? | Yes |
The heat pump keeps running with some functions stopped.
Resetting itself: yes (per manual).
Manual reset: Clear the code from the remote controller once the cause is fixed (service procedure).
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 consumer-unit breakers/isolators for the outdoor unit, hydro unit and backup heater are switched on.
What the engineer will check
On the R32 Estia 1-Series (split, Wall-Mounted and All-in-One)
- Follow manufacturer's check-code flowchart.
- Faulty cylinder sensor TTW.
- Hot water cylinder thermal cut-out.
Who to ask for: Heat pump engineer with electrical competence.
Certification: Electrical competence (Part P / 18th Edition).
On the Estia (Series 4) Standard & Powerful
- Follow manufacturer's check-code flowchart.
- TTW cylinder sensor.
- Cylinder thermal cut-out.
Who to ask for: Heat pump engineer with electrical competence.
Certification: Electrical competence (Part P / 18th Edition).
On the Estia 5 Series (Standard)
- Follow manufacturer's check-code flowchart.
- TTW cylinder sensor.
- Cylinder thermal cut-out.
Who to ask for: Heat pump engineer with electrical competence.
Certification: Electrical competence (Part P / 18th Edition).
On the Estia 5 Series Powerful (Hi Power / Extreme)
- Follow manufacturer's check-code flowchart.
- TTW cylinder sensor.
- Cylinder thermal cut-out.
Who to ask for: Heat pump engineer with electrical competence.
Certification: Electrical competence (Part P / 18th Edition).
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