Daikin U7-00 Fault Code: Causes, Fixes & Repair Costs
What does the Daikin U7-00 fault code mean?
The U7-00 code indicates that the main control PCB and the inverter PCB inside the outdoor unit have lost the ability to communicate with each other. Daikin groups all communication-related faults under the 'U' family of codes, and U7-00 specifically points to a breakdown in the internal data link between these two boards. When this fault is active, the heat pump will typically shut down and stop providing heating or hot water until the underlying issue is resolved.
General guidance only — not a substitute for professional advice, and not certified by an heat pump engineer. Always have your specific appliance assessed by a qualified professional. Any heat pump work must be carried out by an MCS-certified heat pump engineer. If you notice a refrigerant leak, a burning smell or water leaking from the unit, switch it off at the isolator and call an MCS-certified heat pump engineer. Refrigerant work must be carried out by an F-Gas registered engineer.
Common causes
- Faulty inverter PCB Common
The inverter PCB inside the outdoor unit is the most frequently implicated component when U7-00 appears. If the board has developed an internal fault or its communication circuitry has failed, the main PCB will be unable to establish or maintain the data link, triggering this code. Board failures can result from age, voltage spikes or moisture ingress.
- Loose, damaged or corroded wiring and connectors Common
The ribbon cables and plug-in connectors that carry signals between the main PCB and the inverter PCB can work loose over time due to vibration from the compressor and fan, or can corrode in the damp environment inside an outdoor unit. Even a partially seated connector can break the communication link and produce this fault.
- Defective main control PCB Sometimes
If the main PCB has sustained damage — from a power surge, a lightning-related transient or component aging — it may no longer be capable of initiating or sustaining the communication handshake with the inverter board. This is less common than an inverter PCB fault but is still a well-documented cause of U7-00.
- Power surge or transient following an outage Sometimes
Technicians report that U7-00 can appear after mains power is restored following a grid outage or a nearby lightning strike. The communication handshake between the two boards fails to re-establish cleanly. In some cases a controlled power cycle clears the fault entirely; in others it indicates that a surge has permanently damaged a board.
- Electrical noise or interference Rare
Strong electromagnetic interference from nearby industrial equipment, variable-speed drives or poorly screened cabling can corrupt the internal communication signal. Daikin's own service documentation lists external electrical noise as a recognised trigger for communication-family fault codes.
How to fix it
- Switch the heat pump off at the controller and isolate it at the outdoor unit isolator. Wait at least three minutes, then restore power and allow the unit to restart normally. DIY safe
A transient glitch — for example, after a brief mains outage — can sometimes prevent the two boards from completing their startup handshake. A full, controlled power cycle gives both microcomputers a clean restart. If the fault clears and does not return within a few heating cycles, the event was likely a one-off. If U7-00 reappears, move to the next steps and do not continue running the unit.
- Check the MCB or circuit breaker for the heat pump in your consumer unit has not tripped. DIY safe
A tripped breaker can cause an uncontrolled power loss that leaves the PCBs in an inconsistent state. Reset the breaker if tripped, then attempt the power cycle above. If the breaker trips again immediately, do not reset it a second time — this points to an electrical fault requiring an engineer.
- Inspect the area around the outdoor unit and ensure there is no obvious physical damage to the unit casing or visible wiring entering it. DIY safe
Look for signs of pest damage, physical impact or water pooling around cable entry points. Do not open the outdoor unit casing or touch any internal components. If you can see damaged external wiring, report this to the engineer when you call.
- Retrieve the fault history from the indoor controller if your model supports it. DIY safe
Many Daikin Altherma controllers allow you to scroll through the last ten stored fault codes. Note down the codes and any timestamps shown. This information will help an engineer understand whether U7-00 is recurring, whether it appeared alongside another code, and what the unit was doing when the fault triggered.
- Contact an MCS-certified heat pump engineer to carry out a full diagnostic. Refrigerant-circuit work legally requires an F-Gas registered engineer. heat pump engineer
An engineer will use Daikin's service software to interrogate the outdoor unit, check communication signals between the main and inverter PCBs, inspect and re-terminate all internal connectors and wiring harnesses, and test each PCB in turn. If a board has failed, it will need to be replaced — this is not homeowner-serviceable work. Check whether your unit is still within Daikin's three-year parts warranty (five years for commercial installations) and ensure your annual service records are available, as Daikin may reject warranty claims without them.
Parts you may need
- Daikin outdoor unit inverter PCB · from £380
- Daikin outdoor unit main control PCB · from £290
- Internal wiring harness / ribbon connector set · from £45
The exact spare depends on your boiler's GC number (on the data badge). Check this against the part before buying.
Typical repair cost
Expect to pay roughly £200–£600, depending on the underlying cause.
Frequently asked questions
Can I keep using my Daikin heat pump while the U7-00 code is showing?
No — it is best to switch the unit off until the fault has been diagnosed. A communication breakdown between the main and inverter PCBs means the compressor could be operating without proper supervisory control, which risks further damage to the unit. If the fault clears after a single power cycle and does not return, the event was likely a transient glitch, but any recurring U7-00 should be investigated promptly by an engineer.
Could U7-00 be caused by a power cut or surge?
Yes, this is a recognised trigger. When mains power is restored abruptly after an outage, the two PCBs inside the outdoor unit sometimes fail to re-synchronise during startup, producing a U7-00. A controlled power cycle — switching off at the isolator, waiting three minutes, then restarting — often clears a surge-related instance. If the fault returns, a transient may have permanently damaged one of the boards, and an engineer's inspection is needed.
How much does it cost to fix a U7-00 fault in the UK?
For most households, the repair falls between £200 and £600. A wiring inspection and re-termination at the lower end of that range; a single PCB replacement (inverter or main board, including parts and labour) typically sits between £400 and £700 once a callout fee is factored in. In the less common scenario where both boards need replacing simultaneously, costs can exceed £900. It is worth checking whether your unit is within Daikin's standard warranty period and whether you have annual service records, as a valid warranty claim could eliminate parts costs entirely.
Does fixing a U7-00 fault require an F-Gas engineer?
The communication fault itself does not involve the refrigerant circuit, so a straightforward PCB swap or wiring repair does not legally require F-Gas registration. However, any Daikin Altherma work on the outdoor unit is best carried out by an MCS-certified heat pump engineer who will also hold F-Gas registration, ensuring they can safely work around the refrigerant circuit if needed and that your Daikin warranty remains valid. Always confirm the engineer's credentials before booking.