Daikin E6-00 Fault Code: Causes, Fixes & Repair Costs
What does the Daikin E6-00 fault code mean?
The E6-00 code on a Daikin heat pump signals that the outdoor unit has detected a compressor lock or start-up failure. The 'E6' portion tells you this is a compressor overcurrent or lock fault originating in the outdoor unit, while the '00' suffix simply identifies it as the primary fault stored in the system's diagnostic memory. In plain terms, the heat pump controller is reporting that it attempted to start the compressor but could not complete the start-up sequence successfully. This can point to anything from a mechanical problem inside the compressor itself through to a refrigerant shortage, a faulty expansion valve, or an issue with the outdoor unit's control electronics. Because this fault directly involves the compressor — the heart of the heat pump — it should be treated as urgent and not 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
- Seized or mechanically locked compressor Common
The compressor motor has attempted to start but cannot turn. This can happen after a prolonged shutdown, following liquid refrigerant flooding back into the compressor, or simply due to age-related mechanical wear. When the compressor is physically locked, the inverter detects the resulting overcurrent condition and trips the fault. This is the most serious outcome and typically requires compressor replacement.
- Refrigerant shortage due to a leak Common
If the refrigerant charge has dropped — through a slow leak in the sealed circuit — the compressor struggles to build enough pressure to start and run correctly. The inverter registers abnormal operating conditions and raises the E6 alarm. Refrigerant work must only be carried out by an F-Gas registered engineer.
- Defective outdoor unit control PCB Common
The outdoor unit's control board manages the inverter drive signals sent to the compressor. If the PCB develops a fault — damaged capacitors, failed transistors, or corrupted firmware — it may send incorrect start-up signals, causing the compressor to lock up even when the compressor itself is mechanically sound.
- Defective electronic expansion valve (EEV) Sometimes
The electronic expansion valve regulates refrigerant flow into the heat exchanger. A stuck-closed or poorly calibrated EEV can starve the compressor of properly conditioned refrigerant at start-up, creating conditions that trigger a lock fault.
- Service stop valve left closed after maintenance Sometimes
Following a service visit or refrigerant top-up, the technician must reopen the system's stop valves. If a valve is inadvertently left closed, refrigerant cannot circulate and the compressor will trip immediately on start-up. This is a straightforward fix but requires an engineer to identify and correct it.
- Inverter circuit failure Sometimes
The inverter board converts mains electricity into the variable-frequency drive signal the compressor needs. Failing capacitors or power transistors on the inverter can produce incorrect waveforms that prevent clean compressor start-up, mimicking a mechanical compressor lock.
- Loose or damaged wiring between indoor and outdoor units Rare
Poor connections or damaged communication cables between the indoor controller and the outdoor unit can cause the system to receive garbled signals during the start-up handshake, triggering protective shutdown and logging an E6 code.
How to fix it
- Switch the heat pump off at the controller and note the fault code DIY safe
Before doing anything else, write down the full fault code displayed (E6-00) and the time it appeared. This information is useful for the engineer. Turn the system off using the indoor controller or room thermostat.
- Isolate power at the outdoor unit isolator and wait five minutes DIY safe
Locate the dedicated isolator switch — usually a small grey or red switch mounted on the wall near the outdoor unit. Switch it off. Wait a full five minutes to allow the internal capacitors to discharge safely before restoring power. Do not open any panels.
- Restore power and attempt a single restart DIY safe
Switch the isolator back on, wait two minutes for the system to complete its start-up handshake, then turn the heating on from the controller as normal. Observe whether the fault clears. If E6-00 returns within an hour, do not reset again — repeated restarts through a genuine compressor fault can cause additional damage.
- Check the area around the outdoor unit for obvious obstructions DIY safe
Walk around the outdoor unit and make sure there is at least 30 cm of clear space on all sides. Remove any leaves, debris, or accumulated snow from around the casing and the fan grille. Do not insert anything into the unit or touch any internal components.
- Check system water pressure at the pressure gauge DIY safe
On the heating system side, locate the pressure gauge — typically on the indoor unit or buffer cylinder pipework. The reading should normally sit between 1.0 and 2.5 bar when cold. If it is below 1.0 bar, top up via the filling loop following your system's instructions. Low pressure is not a direct cause of E6 but is worth correcting before the engineer arrives.
- Do not attempt any further resets or internal inspection — contact an MCS-certified heat pump engineer heat pump engineer
If the E6-00 code returns after a single reset attempt, the fault requires professional diagnosis. An MCS-certified engineer will check the stop valves, test the inverter and control PCB, inspect wiring, and assess refrigerant charge. Any work on the refrigerant circuit must be carried out by an F-Gas registered engineer. Do not open the outdoor unit casing, handle refrigerant pipework, or test electrical components inside the unit yourself.
Parts you may need
- Outdoor unit control PCB · from £320
- Electronic expansion valve (EEV) · from £180
- Inverter board / power module · from £280
- Compressor (model-dependent) · from £850
- Communication cable (indoor–outdoor) · 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 £200–£900, depending on the underlying cause.
Frequently asked questions
Is it safe to keep using my heat pump when E6-00 is showing?
No — you should not continue running the system with an active E6-00 fault. Unlike sensor codes that can sometimes be monitored for a short period, a compressor lock fault can cause serious internal damage if the unit keeps attempting to start against a real mechanical or refrigerant problem. Switch the system off and arrange an engineer visit promptly.
Could the E6-00 code just be a one-off glitch, or does it always mean a serious fault?
Occasionally a transient voltage dip or a communication hiccup during start-up can produce a spurious E6 code that clears with a single controlled reset. That is why one careful reset — with a proper five-minute power-down — is reasonable. However, if the code returns within an hour of restarting, you have a genuine fault that will not resolve itself. Do not keep resetting.
How much does it cost to fix E6-00 on a Daikin heat pump in the UK?
The typical repair bill for most E6-00 faults falls between £200 and £900. At the lower end, a closed stop valve or a loose wiring connection may be corrected for little more than a standard call-out fee of around £100–£200. A refrigerant leak repair with recharge typically costs £350–£600, and PCB or inverter board replacement usually comes to £450–£900 all-in. A full compressor replacement is a much bigger job — expect £1,400 or more depending on the model — but this is the worst-case outcome, not the most common one.
Do I need an F-Gas engineer or an MCS engineer — and are they the same person?
They can be, but not always. MCS certification covers the installation and commissioning of heat pump systems in the UK. F-Gas registration (under the UK F-Gas Regulations) is the separate qualification required to handle, recover, or recharge refrigerant. Many heat pump engineers hold both, but it is worth confirming before booking. For an E6-00 fault that turns out to involve a refrigerant leak, you specifically need someone who is F-Gas registered to legally work on the sealed circuit.