5203 on a Worcester Bosch Bosch Compress 5800i AW: Alarm: outside temperature sensor T1 fault
Short answer
On a Worcester Bosch heat pump, 5203 means: Alarm: outside temperature sensor T1 fault. 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: outside temperature sensor T1 fault.
Part involved: outside temperature sensor T1.
When it shows: Alarm: outside temperature sensor T1 fault.
Likely causes:
- Open circuit in lead; corroded terminals; faulty 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: 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.
What the engineer will check
- Check lead continuity and terminals; test sensor against table; replace.
Who to ask for: heat pump installer / controls engineer.
Certification: None specific; electrical isolation required before wiring checks.
What you might notice
Sources
- Installationsanleitung Bedienfeld UI 800 (Luft-Wasser-Wärmepumpe) 6721833375 ↗ · 6721833375 (2023/09) DE, AT, CH, LU · page 21 · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026