H4-00 on a Daikin Altherma: Malfunction of low pressure switch
Short answer
On a Daikin heat pump, H4-00 means: Malfunction of low pressure switch. 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: Malfunction of low pressure switch.
Part involved: thermistor / pressure sensor.
When it shows: Low pressure switch is defective or NOT well connected.
Likely causes:
- Faulty low pressure switch.; Faulty main PCB.
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: no.
Manual reset: Manual reset via user interface.
What you can check
- Read the full error code and sub-code shown on the user interface (Malfunctioning screen; press the help button for Daikin's long description) and note it, with the date/time, for your installer.
- Do NOT bleed radiators while an error/malfunction symbol is displayed: Daikin operation manuals warn refrigerant may have leaked into the water circuit - ventilate and call your installer.
- If heating/hot water has stopped, the operation manual lets the user confirm on the Malfunctioning screen whether the backup/booster heater may take over as emergency heating (much higher running cost) until the installer attends.
What the engineer will check
- Perform a check of the low pressure switch. ; Perform a check of the main PCB.
Who to ask for: Heat pump engineer (Daikin-trained).
Certification: Electrical competence to BS 7671 for sensor/PCB wiring; F-gas if a refrigerant-side sensor must be replaced.
What you might notice
What owners and engineers report
9 owner and engineer reports mention this.
- No heating on the Altherma: 7 reports
- Intermittent fault on the Altherma: 2 reports
Sources
- Service manual Daikin Altherma 3 M EBLA/EDLA09~16DA (ESIE20-06A) ↗ · ESIE20-06A · page 42 · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026