6243 on a Worcester Bosch Bosch Compress 5800i AW: Warning: high temperature difference between heat pump flow and return sensors (TC1-TC0)
Short answer
On a Worcester Bosch heat pump, 6243 means: Warning: high temperature difference between heat pump flow and return sensors (TC1-TC0). The heat pump keeps running. An engineer is needed to fix the cause.
Reviewed 23 September 2026 · 1 source
What it means
The manual says: Warning: high temperature difference between heat pump flow and return sensors (TC1-TC0).
Part involved: TC1/TC0 delta T.
When it shows: Warning: high temperature difference between heat pump flow and return sensors (TC1-TC0).
Likely causes:
- Primary circulation too low.
Is it safe?
| Does the heat pump stop? | No |
|---|---|
| Severity | Warning |
| Engineer needed? | Yes |
The heat pump keeps running.
Resetting itself: Clears when cause removed.
Manual reset: Reset/acknowledge via UI 800 fault menu after cause removed.
What you can check
- Read and note the full code (and sub-code) shown on the controller and pass it to your installer.
- Check the heating system pressure gauge reading (Bosch guidance is typically 1-2 bar when cold); report low pressure to your installer.
What the engineer will check
- Check/clean particle filter; ensure all valves open.
Who to ask for: heat pump installer / heating engineer.
Certification: None specific for hydraulic checks; MCS-certified installer recommended for system changes.
What you might notice
Sources
- Installationsanleitung Bedienfeld UI 800 (Luft-Wasser-Wärmepumpe) 6721833375 ↗ · 6721833375 (2023/09) DE, AT, CH, LU · page 22 · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026