E4 on a Worcester Bosch Bosch Compress 2000 AWF: Alarm table: Storage tank water temperature sensor T5 failure / Troubleshooting table: T5 DHW temperature sensor error
Short answer
On a Worcester Bosch heat pump, E4 means: Alarm table: Storage tank water temperature sensor T5 failure / Troubleshooting table: T5 DHW temperature sensor error. The heat pump keeps running with some functions stopped. An engineer is needed to fix the cause.
Reviewed 23 September 2026 · 1 source
What it means
The manual says: Alarm table: Storage tank water temperature sensor T5 failure / Troubleshooting table: T5 DHW temperature sensor error.
Part involved: T5 DHW cylinder sensor.
When it shows: Storage tank water temperature sensor T5 failure.
Likely causes:
- Sensor connector loose, wet/water ingress in connector, or sensor faulty (manual: reconnect, dry and seal connector with waterproof adhesive, or replace sensor).
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.
Manual reset: Manual 13.2: switch the unit off and on again to reset the safety device; if reset fails contact dealer. Manual 11.4 warns repeated resets can cause irreversible damage.
What you can check
- Read and note the full code (and sub-code) shown on the controller and pass it to your installer.
- Manual 13.2: one off/on restart may be tried; if the code returns, stop and call an engineer - repeated resets can damage the unit.
What the engineer will check
- Check sensor connector seating and moisture; measure sensor resistance; replace sensor if faulty (inside unit casing - isolate power, wait 10 min for DC bus discharge).
Who to ask for: heat pump installer / electrician.
Certification: Electrical work to BS 7671 by competent person.
What you might notice
Sources
- Installation and operating instructions Compress 2000 AWF CS2000AWF 4-30 R-S/T ↗ · 6721859133 (2025/10) · page 128; 135 · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026