U0 on a Daikin heat pump: Low pressure drop due to refrigerant shortage or electronic expansion valve failure
Short answer
On a Daikin heat pump, U0 means: Low pressure drop due to refrigerant shortage or electronic expansion valve failure. The manual does not say whether the heat pump stops. An engineer is needed to fix the cause. The wording differs by model range — each is shown below.
Reviewed 23 September 2026 · 2 sources
What it means
On the Altherma HT (high temperature split)
The manual says: Low pressure drop due to refrigerant shortage or electronic expansion valve failure.
Part involved: refrigerant circuit / pressure protection.
When it shows: Microcomputer judge and detect if the system is short of refrigerant. Malfunction is not decided while the unit operation is continued.
Likely causes:
- Out of gas or refrigerant system clogging (incorrect piping); Defect of pressure sensor; Defect of outdoor unit PC board (A1P); Defect of thermistor R3T ESIE09-08C.
On the Altherma LT (low temperature split)
The manual says: Refrigerant failure.
Part involved: refrigerant circuit / pressure protection.
When it shows: Gas shortage detection I : DC current ≤ (A/Hz) × Output frequency + However, when the status of running frequency > 55 (Hz) is kept on for a certain time. Note: The values are different from model to model. Gas shortage detection II : If a gas shortage error takes place 4 times successively, the system will be shut down. The error counter will reset itself if this or any other error does not occur during the following 60-minute compressor running time (total time) | Gas shortage detection I : DC current ≤ (A/Hz) × Output frequency + However, when the status of running frequency > 55 (Hz) is kept on for a certain time. Note: The values are different from model to model. 71 class 27 / 1000 2.5 Gas shortage detection II : If a gas shortage error takes place 4 times successively, the system will be shut down. The error counter will reset itself if this or any other error does not occur during the following 60-minute compressor running time (total time).
Likely causes:
- Refrigerant shortage (refrigerant leakage); Poor compression performance of compressor; Discharge pipe thermistor disconnected, or hydro-box or outdoor unit heat exchanger thermistor disconnected, room or outside air temperature thermistor disconnected; Stop valve closed; Electronic expansion valve defective 006~008 class 27 / 1000 2.5 ESIE10-02 Error Codes: System Malfunctions Part 3 –; Discharge pipe thermistor disconnected, or hydro-box or outdoor unit heat exchanger thermistor; disconnected, room or outside air temperature thermistor disconnected; Electronic expansion valve defective.
Is it safe?
On the Altherma HT (high temperature split)
| Does the heat pump stop? | Not stated |
|---|---|
| Engineer needed? | Yes |
The manual does not say whether the heat pump stops.
Resetting itself: unknown.
On the Altherma LT (low temperature split)
| 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: Reset via the remote controller (Inspection/Test button) after the cause is fixed; some codes need a manual reset of the remote-controller memory (service manual section 1.4).
What you can check
- Read the full error code shown on the user interface and note it, with the date/time, for your installer.
- Check the heating-system water pressure reading on the gauge/display: Daikin operation manuals ask users to check it is above 1 bar. If it is below that, report it to your installer (do not open the unit).
- Look at the outdoor unit from outside: check the air inlet/outlet and fan grille are not blocked by leaves, debris, snow or heavy ice build-up (do not remove panels or touch the fan).
What the engineer will check
On the Altherma HT (high temperature split)
- Follow the service-manual troubleshooting flow chart for this code: check out of gas or refrigerant system clogging (incorrect piping); check defect of pressure sensor; check defect of outdoor unit PC board (A1P); check defect of thermistor R3T ESIE09-08C.
Who to ask for: F-gas certified heat pump engineer (Daikin-trained installer / Stand By Me partner).
Certification: UK F-gas certification (Category I) for refrigerant circuit work; R-290 (Altherma 4) also needs flammable-refrigerant/hydrocarbon competence.
On the Altherma LT (low temperature split)
- Follow the service-manual troubleshooting flow chart for this code: check refrigerant shortage (refrigerant leakage); check poor compression performance of compressor; check discharge pipe thermistor disconnected, or hydro-box or outdoor unit heat exchanger thermistor disconnected, room or outside air temperature thermistor disconnected; check stop valve closed; check electronic expansion valve defective 006~008 class 27 / 1000 2.5 ESIE10-02 Error Codes: System Malfunctions Part 3 –; check discharge pipe thermistor disconnected, or hydro-box or outdoor unit heat exchanger thermistor.
Who to ask for: F-gas certified heat pump engineer (Daikin-trained installer / Stand By Me partner).
Certification: UK F-gas certification (Category I) for refrigerant circuit work; R-290 (Altherma 4) also needs flammable-refrigerant/hydrocarbon competence.
What you might notice
What owners and engineers report
1 owner and engineer report mentions this.
- No heating on the Altherma LT (low temperature split): 1 report
Sources
- Service Manual Daikin ALTHERMA HT ERSQ/ERRQ 011-016 AA V1/Y1 + EKHBRD (ESIE09-08C) ↗ · ESIE09-08C · page 208 · Reviewed 23 September 2026
- Service Manual ALTHERMA ERHQ006~008BAV3 / ERLQ006~008BAV3(9) / EKHBH(X)008BA (ESIE10-02) ↗ · ESIE10-02 · page 188 · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026