U9 on a Worcester Bosch Greenstar Plus Hybrid: Inverter over or undercurrent and fault in the serial PCB communication
Short answer
On a Worcester Bosch heat pump, U9 means: Inverter over or undercurrent and fault in the serial PCB communication. 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: Inverter over or undercurrent and fault in the serial PCB communication.
Part involved: external unit PCB / refrigerant circuit.
When it shows: Inverter over or undercurrent and fault in the serial PCB communication.
Likely causes:
- Bus voltage <200 V or >420 V, or current drop: supply voltage; compressor wiring detached; PFC module; ACT module; CNAF/CN5/CN2 plugs; 52C circuit.
Is it safe?
| Does the heat pump stop? | Yes |
|---|---|
| Severity | Fault — needs a manual reset once the cause is fixed |
| 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
- Bus voltage <200 V or >420 V, or current drop: supply voltage; compressor wiring detached; PFC module; ACT module; CNAF/CN5/CN2 plugs; 52C circuit.
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; 66 · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026