Heat pumps · Error codes · Navien
F12Navien · PEM750
F12 on a Navien PEM750: Communication failure between the PCB and driver board
Short answer
On a Navien heat pump, F12 means: Communication failure between the PCB and driver board. The heat pump stops until the fault clears or is reset. An engineer is needed to fix the cause.
Reviewed 24 September 2026 · 1 source
What it means
The manual says: Communication failure between the PCB and driver board.
Part involved: compressor inverter drive board.
Likely causes:
- Check main board-driver board connection (12V, GND, A, B); check 12 V supply; replace compressor driver board.
Is it safe?
| Does the heat pump stop? | Yes |
|---|---|
| Engineer needed? | Yes |
The heat pump stops until the fault clears or is reset.
Resetting itself: unknown.
What you can check
- Note the code shown on the controller and the model number on the unit's rating label before contacting your installer.
- Check that the heat pump's isolator and consumer-unit breaker are on; do not open any casing.
What the engineer will check
- Check supply voltage and phase at the unit terminals.
- Check compressor U/V/W connections and compressor winding resistance.
- Check drive board heatsink cooling and fan.
- Replace the inverter/driver board if faults persist.
Who to ask for: Heat pump engineer.
Certification: Heat pump installer competence; F-gas and flammable-refrigerant (R32/R290) handling competence for any refrigerant-circuit work.
What you might notice
Sources
- Navien Air to Water Heat Pump UK Service Manual (EN, 250408) ↗ · 250408 · page printed p.71 (PDF p.72) · Reviewed 24 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 24 September 2026