U3 on a Worcester Bosch Greenstar Plus Hybrid: Open / short circuit on hot gas temperature sensor TH4 or compressor temperature sensor TH32
Short answer
On a Worcester Bosch heat pump, U3 means: Open / short circuit on hot gas temperature sensor TH4 or compressor temperature sensor TH32. The heat pump stops until the fault clears or is reset. An engineer is needed to fix the cause.
Reviewed 23 September 2026 · 1 source
What it means
The manual says: Open / short circuit on hot gas temperature sensor TH4 or compressor temperature sensor TH32.
Part involved: external unit PCB / refrigerant circuit.
When it shows: Open / short circuit on hot gas temperature sensor TH4 or compressor temperature sensor TH32.
Likely causes:
- Loose plug/contacts; faulty sensor; faulty PCB.
Is it safe?
| Does the heat pump stop? | Yes |
|---|---|
| Severity | Fault — clears by itself when the condition goes |
| Engineer needed? | Yes |
The heat pump stops until the fault clears or is reset.
Resetting itself: unknown.
Manual reset: Codes read via outdoor unit LEDs or service tool 7-716-161-051; SW1-2 DIP switch cancels fault displays (engineer).
What you can check
- Read and note the full FW200/hybrid manager fault code shown on the controller and pass it to your installer.
- Check the heat pump isolator and consumer-unit breaker(s) are switched on; if a breaker has tripped more than once, leave it off and call an engineer.
- Check the outdoor unit is not blocked by leaves, snow, ice or other debris (do not open the casing).
What the engineer will check
- Loose plug/contacts; faulty sensor; faulty PCB.
Who to ask for: F-gas refrigeration engineer / heat pump installer.
Certification: F-gas certification (Category I) required for refrigerant circuit work; qualified electrician for electrical work.
What you might notice
Sources
- Installation and Maintenance Instructions Greenstar Plus Hybrid ↗ · 6 720 803 687 (2012/11) · page 61; 65 · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026