Daikin U5-00 Fault Code: Causes, Fixes & Repair Costs
What does the Daikin U5-00 fault code mean?
The U5-00 fault code indicates that the remote controller has lost its communication link with the indoor unit's control board. Daikin uses the U-series of codes for system-level communication faults, and U5 specifically points to the signal path between the controller and the indoor unit. The '00' suffix confirms this is the currently active fault being displayed. While the system classifies this as a 'service soon' level alert rather than an outright emergency shutdown, the unit will not respond reliably to commands until the communication link is restored — so it should be addressed promptly rather than ignored.
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
- Loose or corroded P1/P2 controller wiring Common
The two-wire P1/P2 connection between the wired remote controller and the indoor unit is the most frequently identified culprit. A terminal that has worked loose, a conductor affected by condensation or corrosion, or a cable that has been lightly damaged can all interrupt the low-voltage signal that keeps the two devices talking. The fault may appear intermittently at first and then become permanent as the connection deteriorates further.
- Two wired controllers both set as 'main' Common
Some installations include a secondary wired remote controller — for example one in a hallway and one in a utility room. If both controllers are configured as the primary (main) controller rather than one being set to sub-controller mode, each device attempts to take ownership of the communication bus simultaneously. This clash causes a transmission error that triggers U5-00. The fix is purely a settings change rather than any physical repair.
- Flat or failed wireless remote controller batteries Common
On systems using a wireless (non-wired) remote controller, depleted batteries are a straightforward cause of lost communication. The controller may appear to have enough charge to display its own screen but lack the power to transmit reliably to the indoor unit receiver.
- Transient fault caused by a power surge or outage Sometimes
A brief power interruption or voltage spike can disrupt the communication logic on either the controller or the indoor unit PCB. In many cases the fault clears after a proper power cycle, but repeated occurrences without an obvious trigger suggest underlying hardware weakness.
- Faulty indoor unit PCB Sometimes
If the indoor unit's main control board develops a fault — whether through age, moisture ingress, or component failure — it may lose the ability to decode signals from the remote controller even when wiring is intact. This typically produces a persistent U5-00 that does not respond to resets or wiring checks.
- Faulty remote controller PCB Sometimes
The remote controller itself contains a small circuit board that manages signal transmission. If this board fails — for instance following water ingress or a surge — the controller may power on but be unable to communicate correctly with the indoor unit. Swapping in a known-good controller during diagnosis quickly confirms or rules this out.
- Electrical interference on the communication cable Rare
In some installations, particularly where the P1/P2 cable runs close to mains wiring or other sources of electromagnetic noise, induced interference can corrupt the low-voltage communication signal. This tends to produce intermittent faults rather than a permanent lockout.
How to fix it
- Check for obvious damage or looseness at the P1/P2 wiring terminals DIY safe
Switch the heat pump off at the controller and at the wall isolator before touching any terminals. On a wired remote controller, the P1/P2 communication cable connects at screw terminals on both the controller backplate and the indoor unit terminal block. Gently tug each wire to confirm it is firmly seated, look for signs of corrosion or discolouration, and re-secure any that appear loose. On a wireless controller, skip this step and go straight to checking the batteries.
- Replace the batteries in a wireless remote controller DIY safe
If your system uses a wireless remote, open the battery compartment and fit a fresh set of alkaline batteries of the correct size (refer to the sticker inside the compartment). Even if the display looks fine, low batteries are a common cause of lost signal. Once replaced, allow the controller to re-pair with the indoor unit receiver — refer to your controller's quick-start guide for the pairing procedure, which typically involves holding a button for a few seconds.
- Check whether two wired controllers are installed and verify their settings DIY safe
If there are two wired remote controllers on the same indoor unit, one must be configured as the 'main' controller and the other as the 'sub' controller. Both being set to 'main' is a known trigger for U5-00. Access the remote controller's installation settings menu (usually reached by holding a combination of buttons — consult your model's installation manual) and confirm that only one device is set as primary. If you are unsure how to navigate this menu, an engineer can make this change quickly.
- Perform a full power cycle of the heat pump system DIY safe
Turn the system off using the remote controller first, then switch it off at the dedicated wall isolator or fused spur. Wait a full five minutes — this allows any residual charge in the control electronics to fully discharge and gives the PCBs a genuine reset. After five minutes, restore power at the isolator, allow the system to complete its start-up sequence (this typically takes one to two minutes), and check whether U5-00 reappears.
- Observe whether the fault returns and note any pattern DIY safe
If the power cycle clears the fault, monitor the system over the next 24–48 hours. A fault that returns only during cold or damp weather points towards a marginal wiring connection expanding and contracting with temperature. A fault that returns immediately after every reset suggests a hardware failure on either the controller or the indoor unit PCB. Note down any pattern before calling an engineer — this information significantly speeds up diagnosis.
- If the fault persists, contact an MCS-certified heat pump engineer for further diagnosis heat pump engineer
A qualified engineer will use a multimeter to measure the signal voltage across the P1/P2 terminals, check communication continuity along the full cable run, and swap in a test controller if needed to isolate whether the fault lies with the remote controller PCB or the indoor unit PCB. PCB replacement and any work inside the electrical enclosure of the indoor unit must only be carried out by a competent, qualified engineer. Although U5-00 is a communication fault rather than a refrigerant issue, if refrigerant-circuit components such as the inverter board are implicated, an F-Gas registered engineer will be required for that portion of the work.
Parts you may need
- Daikin wired remote controller (BRC series) · from £165
- Indoor unit main PCB (model-specific) · from £320
- P1/P2 communication cable (per metre, shielded) · from £3
- Remote controller backplate / terminal assembly · from £28
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 £180–£500, depending on the underlying cause.
Frequently asked questions
Is the Daikin U5-00 fault dangerous — do I need to stop using the heat pump immediately?
U5-00 is a communication fault rather than a safety-critical issue such as a refrigerant leak or electrical hazard, so it does not pose an immediate danger. Daikin classifies it as a 'service soon' alert. However, because the indoor unit and controller are no longer communicating reliably, you may lose the ability to control temperatures or modes properly. It is advisable to address the fault within a few days rather than leaving it running in an unknown state for weeks.
Can I fix the Daikin U5-00 fault myself, or do I always need an engineer?
Some of the most common causes — flat batteries in a wireless remote, a loose terminal on the P1/P2 wiring, a dual-controller settings conflict, or a transient glitch cleared by a proper power cycle — are straightforward for a competent homeowner to check. If those steps clear the fault and it does not return, no engineer visit is needed. However, if the fault persists after checking wiring, batteries, and controller settings, and after a full power cycle, the likely cause is a failed PCB on either the controller or the indoor unit, and a qualified engineer will be needed to confirm this and carry out the replacement.
How much does it typically cost to fix a U5-00 fault in the UK?
If the issue is simply a wiring reconnection or a controller settings adjustment, you would typically pay a callout fee plus an hour's labour — expect roughly £150–£250 all in. If the remote controller itself needs replacing, parts run from around £130–£200 for a standard BRC-series wired controller, plus labour, bringing the total to roughly £250–£400. An indoor unit PCB replacement is more involved: parts and labour combined typically fall in the £450–£900 range. In rare cases where the fault is traced to the inverter board or another major electronic component, costs can exceed £900 — worth factoring in when deciding whether to repair or replace an older unit.
Why does U5-00 keep coming back on my Daikin heat pump even after I reset it?
A fault that clears on reset but reappears within hours or days usually points to a connection that is marginal rather than completely broken — for example a corroded terminal that makes contact when the metal is warm but loses continuity as temperatures drop, or a controller PCB that is beginning to fail. Repeatedly resetting the system without addressing the underlying cause can mask the problem and potentially lead to further damage. If the fault is recurring, it is worth having an engineer check signal voltage at the P1/P2 terminals and inspect the wiring run for any sections exposed to moisture or physical movement.