Daikin E4-00 Fault Code: Causes, Fixes & Repair Costs
What does the Daikin E4-00 fault code mean?
The E4-00 fault code indicates that the low-pressure switch (LPS) on your Daikin heat pump's outdoor unit has activated due to an abnormal drop in refrigerant-side pressure. The '-00' sub-code confirms the fault is being reported from the primary outdoor unit. When this threshold is breached, the heat pump shuts itself down automatically to protect the compressor from serious damage. This is a refrigerant-circuit fault and should be treated as urgent — do not attempt to restart and run the system repeatedly, as doing so risks turning a manageable repair into a costly compressor failure.
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
- Refrigerant leak Common
A leak somewhere in the sealed refrigerant circuit causes suction pressure to fall below the safe operating threshold, tripping the low-pressure switch. This is by far the most frequent root cause of an E4-00 and will require an F-Gas registered engineer to locate the leak, carry out a repair, and recharge the system to the correct specification. Running the unit with low refrigerant risks destroying the compressor.
- Blockage or restriction in the refrigerant pipework Sometimes
A partial or full blockage within the refrigerant circuit — such as a contaminated expansion valve or debris within the pipework — can restrict flow sufficiently to cause an abnormal pressure drop on the low-pressure side, triggering the LPS. An engineer will need to diagnose and clear or replace the affected component.
- Faulty low-pressure switch (LPS) Sometimes
The LPS itself can fail mechanically or electrically and begin sending a false trip signal even when actual refrigerant pressure is within normal limits. If refrigerant levels and pipework are confirmed to be healthy, the engineer may determine the switch itself needs replacing.
- Outdoor unit PCB fault or wiring issue Rare
A defective control board on the outdoor unit, or a loose, corroded, or damaged connector between the PCB and the LPS, can produce a spurious E4-00 reading. The board may misinterpret pressure sensor signals or fail to process them correctly.
How to fix it
- Stop using the heat pump immediately DIY safe
Switch the system off at the controller and leave it off. Repeatedly restarting a heat pump with low refrigerant pressure dramatically increases the risk of compressor damage — which can turn a £350 repair into a £1,500+ replacement. Do not attempt to clear the fault by cycling the power multiple times.
- Check the system pressure gauge (if fitted) DIY safe
Many Daikin Altherma installations have a pressure gauge on the heating-circuit (water side) pipework inside the home. Check whether it reads within the normal range — typically 1.0 to 1.5 bar when cold. Note that this is the heating circuit pressure, not the refrigerant circuit, so a normal reading here does not rule out a refrigerant-side fault. If the heating circuit pressure is low (below 0.8 bar), top it up via the filling loop following your system's instructions, but be aware this is unlikely to be the primary cause of E4-00.
- Clear any obstructions around the outdoor unit DIY safe
Check that the outdoor unit's air inlet and outlet grilles are clear of leaves, debris, snow, or ice. Poor airflow can worsen refrigerant-side conditions. Do not use a pressure washer or sharp tools near the fins. Gentle removal of debris by hand is fine.
- Check the indoor isolator and consumer unit (fuse board) DIY safe
Confirm the heat pump's dedicated MCB on your consumer unit has not tripped, and that the local isolator switch near the outdoor unit is in the ON position. If an MCB has tripped, reset it once — but if it trips again immediately, leave it and call an engineer, as a repeated trip suggests an underlying electrical fault.
- Contact an MCS-certified, F-Gas registered heat pump engineer heat pump engineer
Because the most likely cause of E4-00 is a refrigerant leak or restriction in the sealed circuit, the repair must be carried out by an engineer who holds both MCS certification and F-Gas registration. Under the UK F-Gas Regulations 2015, topping up a refrigerant charge without first locating and repairing any leak is unlawful. The engineer will perform a leak test, identify the source, carry out the repair, and recharge the system to the manufacturer's specified pressure. If the refrigerant circuit is found to be intact, they will then investigate the LPS switch, PCB, and wiring as potential causes.
Parts you may need
- Low-pressure switch (LPS) · from £45
- Outdoor unit PCB (control board) · from £280
- Refrigerant recharge (R32 or R410A — labour and gas) · from £180
- Expansion valve · from £95
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 £250–£600, depending on the underlying cause.
Frequently asked questions
Is it safe to keep running my Daikin heat pump when E4-00 is showing?
No — you should switch the unit off and leave it off. The E4-00 code means the refrigerant-side pressure has dropped to an unsafe level. Running the compressor with insufficient refrigerant causes it to overheat and can cause permanent damage. What might be a £300 sensor or regas repair can become a £1,500 or more compressor replacement if the system is run repeatedly in this condition.
Can I top up the refrigerant myself to fix E4-00?
No. In the UK, handling refrigerants is restricted to F-Gas registered engineers under the F-Gas Regulations 2015. Beyond the legal requirement, simply adding more refrigerant without first finding and fixing a leak is both unlawful and pointless — the gas will escape again. An engineer must perform a full leak test, repair the fault, and then recharge to the correct specification.
How much does it typically cost to fix a Daikin E4-00 fault in the UK?
For the most common repairs — a refrigerant leak fix and regas, or a sensor and minor component swap — most homeowners in the UK pay somewhere between £250 and £600 including parts and labour. A straightforward sensor replacement or connector repair tends to be towards the lower end, while a leak repair with regas typically falls in the £350 to £600 range. If a PCB replacement is needed, costs can rise to £450 to £900. In the less common scenario where compressor damage has already occurred due to the unit being run in a low-pressure condition, replacement costs can exceed £1,500 — which is why acting quickly matters.
Does E4-00 mean my Daikin heat pump has a refrigerant leak?
A refrigerant leak is the single most common cause, but it is not the only possibility. The fault can also be triggered by a blocked expansion valve or refrigerant pipework, a low-pressure switch that has failed and is giving a false reading, or a wiring or PCB fault producing an incorrect signal. An F-Gas registered engineer will use pressure testing and diagnostic equipment to identify the actual root cause before carrying out any repair.