Daikin AC-00 Fault Code: Causes, Fixes & Repair Costs
What does the Daikin AC-00 fault code mean?
The AC-00 (or simply '00') code on Daikin heat pumps has two distinct meanings depending on how and when it appears. If you see '00' after entering the diagnostic or inspection mode — for example, by holding the Cancel button for around five seconds — it simply confirms that no fault has been detected and the system is operating normally. However, if '00' appears as an active error during normal operation, it typically signals a communication breakdown between the indoor and outdoor units, meaning the two parts of the system have lost the ability to exchange data correctly. Always establish which context applies before taking any action.
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
- Temporary power disruption or surge Common
A brief power cut, voltage spike, or brownout — often associated with a storm or grid fluctuation — can interrupt the communication signal between the indoor controller and the outdoor unit. In many cases, the units simply lose sync and need to re-establish their connection after power is restored.
- Loose or degraded inter-unit wiring Common
The communication cable that runs between the indoor and outdoor units can develop loose terminations over time due to vibration from the unit or minor movement of the building. Poorly crimped connections from the original installation are a particularly common culprit. Corrosion on terminals, especially in exposed outdoor runs, can also cause intermittent signal loss.
- Faulty or failing control PCB Sometimes
If the wiring is found to be intact and no power event has occurred, one of the control boards — either in the indoor unit or the outdoor unit — may be failing. Engineers checking this will typically measure the logic supply voltages at test points on the board; a reading that drops below around 4.75 V DC on the logic rail suggests a failed onboard voltage regulator rather than a wiring issue.
- Environmental stress on electronic components Rare
Prolonged exposure to high humidity, condensation ingress, or temperature extremes can degrade the sensitive electronic components responsible for handling communication signals. This tends to manifest gradually rather than as a sudden fault.
How to fix it
- Check whether '00' is a real fault or a diagnostic-mode indicator DIY safe
Before doing anything else, confirm the context. If you pressed a button combination to enter inspection or self-diagnosis mode, '00' simply means no fault is present — no further action is needed. Only proceed with the steps below if '00' appeared spontaneously during normal operation.
- Power-cycle the heat pump from the isolator DIY safe
Switch off the heat pump at the dedicated isolator (and at the indoor controller if it has a separate power switch). Wait at least two minutes to allow the system to fully discharge, then restore power. This gives both units the chance to re-establish communication from a clean state and clears transient faults caused by power fluctuations. Check whether the '00' code returns.
- Check the consumer unit for any tripped MCB DIY safe
Head to your consumer unit and look for any circuit breakers that have tripped. If the heat pump MCB has tripped, reset it once. If it trips again immediately, do not reset it a second time — this indicates an electrical fault that needs professional investigation.
- Clear the area around the outdoor unit DIY safe
Ensure there is no debris, leaf build-up, snow, or vegetation blocking the outdoor unit. While this is unlikely to be the direct cause of a communication error, restricted airflow can cause wider system stress and should be ruled out as a contributing factor.
- Visually inspect the communication cable connections at accessible terminals DIY safe
If you are comfortable doing so, with the unit fully powered off at the isolator, visually inspect the terminals where the communication cable connects to the indoor unit. Look for any wires that are clearly loose, corroded, or pulled away from their terminals. Do not probe inside the outdoor unit — terminal access on the outdoor unit involves working near high-voltage components and must be left to a qualified engineer.
- Contact an MCS-certified heat pump engineer for further diagnosis heat pump engineer
If the fault code returns after a power cycle, or if you could not identify an obvious loose connection at the indoor unit, the fault requires professional diagnosis. An engineer will use a multimeter to test communication line continuity and resistance, check control board supply voltages, and inspect all connectors for corrosion or backed-out pins. If a PCB in either unit has failed, they will arrange replacement. Note that while this fault does not involve the refrigerant circuit, any wider investigation that touches sealed-circuit components must be carried out by an F-Gas registered engineer.
Parts you may need
- Outdoor unit main control PCB · from £320
- Indoor unit control PCB · from £210
- Inter-unit communication cable (per metre, installed) · from £35
- Terminal block / wiring connector set · from £25
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 £150–£450, depending on the underlying cause.
Frequently asked questions
Does AC-00 on my Daikin heat pump definitely mean something is wrong?
Not necessarily. On Daikin systems, '00' is also the code that appears when you enter the self-diagnosis or inspection mode to confirm that no active faults are stored. If you (or someone else) recently pressed a button combination to access diagnostics, the '00' display is simply the system's way of saying everything is fine. If it appeared on its own during normal heating or hot water operation, then it does indicate an active communication fault between the indoor and outdoor units and warrants investigation.
Can I fix a Daikin AC-00 communication error myself?
A power cycle from the isolator is a safe and sensible first step that resolves a significant proportion of AC-00 faults, particularly those triggered by power fluctuations. You can also visually check the communication cable at the indoor unit terminals (with the power off) for obvious loose connections. Beyond that, diagnosing the communication lines properly requires a multimeter and knowledge of the expected voltage readings, so further investigation should be left to an MCS-certified heat pump engineer.
How much does it cost to fix a Daikin AC-00 fault in the UK?
If the fault clears with a simple power cycle, there is no cost. If an engineer visit is required, expect to pay roughly £150–£450 including labour for the most common repairs such as reseating wiring connections or replacing a terminal block. If a control PCB in the indoor or outdoor unit has failed, parts alone can add £200–£350 or more on top of labour, pushing the total higher — this is worth bearing in mind, but it applies only to a minority of cases. If your system is still within its manufacturer warranty period, log the fault with Daikin first, as an authorised repair may be covered free of charge.
Will a Daikin AC-00 fault damage my heat pump if I leave it?
A persistent communication fault will prevent the system from operating correctly — it may lock out heating or hot water until the error is cleared. It is unlikely to cause physical damage to the heat pump itself in the short term, but leaving the system non-operational during cold weather can cause discomfort and, in some circumstances, risk to pipework if temperatures drop significantly. It is worth addressing the fault promptly rather than leaving it.