E4 on a Baxi heat pump: Domestic hot water temperature sensor (T5) error
Short answer
On a Baxi heat pump, E4 means: Domestic hot water temperature sensor (T5) error. The manual does not say whether the heat pump stops. An engineer is needed to fix the cause. The wording differs by model range — each is shown below.
Reviewed 23 September 2026 · 3 sources
What it means
On the Assure Monobloc ASHP (Assure Mono 5/7/9/12/16)
The manual says: Domestic hot water temperature sensor (T5) error.
Part involved: temperature sensor.
Likely causes:
- T5 connector loose or wet; T5 sensor failed - replace.
On the HP40 / Baxi Assure HP50 Monobloc ASHP
The manual says: Water tank temp. sensor (T5) fault.
Part involved: temperature sensor.
Likely causes:
- Check sensor resistance; T5 connector loose; connector wet; T5 sensor failed - replace. If no T5 fitted, DHW mode must be disabled (10.5.1 DHW MODE SETTING).
On the HP60 / Baxi Assure HP55 High Temperature Monobloc ASHP (R290)
The manual says: T5 water cylinder temperature sensor error.
Part involved: temperature sensor.
Is it safe?
On the Assure Monobloc ASHP (Assure Mono 5/7/9/12/16)
| Does the heat pump stop? | Not stated |
|---|---|
| Engineer needed? | Yes |
The manual does not say whether the heat pump stops.
Resetting itself: unknown.
Manual reset: Power OFF/ON after correcting cause (ISM 13.4).
On the HP40 / Baxi Assure HP50 Monobloc ASHP
| Does the heat pump stop? | Not stated |
|---|---|
| Engineer needed? | Yes |
The manual does not say whether the heat pump stops.
Resetting itself: unknown.
Manual reset: Installer: correct cause then power OFF/ON (ISM 13.4).
On the HP60 / Baxi Assure HP55 High Temperature Monobloc ASHP (R290)
| Does the heat pump stop? | Not stated |
|---|---|
| Engineer needed? | Yes |
The manual does not say whether the heat pump stops.
Resetting itself: unknown.
Manual reset: unknown.
What you can check
- Note the exact code shown on the controller and when it appeared.
- If the code persists or returns, contact a heat pump engineer.
What the engineer will check
On the Assure Monobloc ASHP (Assure Mono 5/7/9/12/16)
- T5 connector loose or wet; T5 sensor failed - replace.
Who to ask for: heat pump engineer (controls / sensors).
Certification: Manufacturer-trained heat pump engineer.
On the HP40 / Baxi Assure HP50 Monobloc ASHP
- Check sensor resistance; T5 connector loose; connector wet; T5 sensor failed - replace. If no T5 fitted, DHW mode must be disabled (10.5.1 DHW MODE SETTING).
Who to ask for: heat pump engineer (controls / sensors).
Certification: Manufacturer-trained heat pump engineer.
On the HP60 / Baxi Assure HP55 High Temperature Monobloc ASHP (R290)
- Diagnose per manufacturer service procedure (manual gives description only).
Who to ask for: heat pump engineer (controls / sensors).
Certification: Manufacturer-trained heat pump engineer.
What you might notice
Sources
- Installation and owner's manual - Heat Pump air/water 'inverter' Assure Mono 5, 7, 9, 12, 16 ↗ · Back cover shows '7762758 - v05 - 19082020' (same number as the Assure Split manual - probably shared back-cover artwork) · page 71 · Reviewed 23 September 2026
- Installation and Service Manual - Heat Pump air/water 'inverter' HP40/50-4-1PHMB to HP40/50-13-3PHMB (United Kingdom) ↗ · 784570-01 (v1) · page 72 · Reviewed 23 September 2026
- Installation, user and service manual - Reversible monobloc air-to-water heat pump, outdoor unit HP60-4-1-PHMB to HP60-13-3-PHMB ↗ · 7888668 - v03 - 17072025 · page 58 · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026