Daikin E3 Fault Code
The Daikin E3 fault code has two distinct meanings depending on your product type. On Daikin gas boilers and heating systems, E3 signals a circulation problem — typically an airlock or flow restriction preventing water from moving properly around the heating circuit. On Daikin air conditioning and heat pump units (including Altherma), E3 indicates that the high-pressure switch on the outdoor unit has tripped, pointing to an over-pressure condition in the refrigerant circuit. Identifying which type of system you have is the essential first step before attempting any diagnosis.
General guidance only — not a substitute for professional advice. Any gas work must be carried out by a Gas Safe registered engineer. If you smell gas or suspect carbon monoxide, leave the property and call the National Gas Emergency line on 0800 111 999.
What does the Daikin E3 fault code mean?
The Daikin E3 fault code has two distinct meanings depending on your product type. On Daikin gas boilers and heating systems, E3 signals a circulation problem — typically an airlock or flow restriction preventing water from moving properly around the heating circuit. On Daikin air conditioning and heat pump units (including Altherma), E3 indicates that the high-pressure switch on the outdoor unit has tripped, pointing to an over-pressure condition in the refrigerant circuit. Identifying which type of system you have is the essential first step before attempting any diagnosis.
Common causes
- Trapped air in the heating circuit (boilers) Common
Air that has worked its way into the system — often after a period of low pressure, recent maintenance, or radiator bleeding — can block the flow of hot water around the circuit. This is one of the most frequently seen triggers for an E3 on Daikin heating systems and is usually straightforward to address.
- Low system pressure (boilers) Common
If the system pressure has dropped below roughly 1 bar, the boiler may struggle to circulate water effectively. Pressure can fall gradually through minor leaks or normal expansion vessel wear. Checking the pressure gauge on the boiler front panel is a quick first check.
- Blocked or dirty outdoor heat exchanger (AC/heat pump) Common
On air conditioning and heat pump units, a clogged outdoor condenser coil is the leading cause of high-pressure switch trips. Dust, leaves, and other debris restrict airflow across the coil, causing refrigerant pressure to climb until the safety switch cuts in.
- Sludge or limescale in the heat exchanger (boilers) Sometimes
In hard water areas — particularly across the South East of England — limescale can accumulate inside the primary heat exchanger over time. Neglected systems can also develop sludge from corroding pipework, both of which reduce flow and trigger circulation faults.
- Circulation pump failure (boilers) Sometimes
The pump that drives hot water around the system can seize or wear out, especially in older installations. If the pump is not moving water at sufficient speed, the boiler detects a flow problem and raises an E3 fault.
- Refrigerant overcharge (AC/heat pump) Sometimes
If the refrigerant circuit contains more gas than the manufacturer specifies — perhaps after an incorrect re-gas — system pressure will run high and repeatedly trip the high-pressure switch. This must be diagnosed and corrected by an F-Gas certified engineer.
- Faulty high-pressure switch or PCB (AC/heat pump) Rare
If cleaning the outdoor unit and resetting the system does not resolve the fault, the high-pressure switch itself may have failed and be tripping incorrectly, or a faulty PCB or thermistor may be misreading pressure data.
How to fix it
- Identify your system type DIY safe
Before doing anything else, confirm whether you have a gas boiler/heating system or an air conditioning/heat pump unit. The correct diagnosis and safe actions differ significantly between the two. Check the model label on the appliance and cross-reference with your manual if needed.
- Check system pressure (boilers only) DIY safe
Look at the pressure gauge on the front of your boiler. For most Daikin heating systems the cold pressure should sit between 1 and 1.5 bar. If it reads below 1 bar, the system needs topping up via the filling loop — a task most homeowners can carry out safely by following the boiler's quick-start guide.
- Top up pressure via the filling loop if needed (boilers only) DIY safe
Locate the filling loop — usually a braided flexible hose or a built-in lever beneath the boiler. Slowly open the valve(s) and watch the gauge rise. Stop at around 1.2–1.3 bar and close the valve fully. Never overfill beyond 1.5 bar when cold.
- Bleed your radiators (boilers only) DIY safe
Working from the radiator furthest from the boiler and moving towards it, use a radiator bleed key to open each bleed valve a quarter turn. Hold a cloth underneath. When a steady stream of water (rather than hissing air) appears, close the valve. Re-check system pressure afterwards and top up again if it has dropped.
- Reset the boiler (boilers) or power-cycle the outdoor unit (AC/heat pump) DIY safe
Once you have addressed pressure or bled the radiators, press the reset button on your Daikin boiler and allow it to restart. For AC/heat pump units, switch the system off at the isolator or mains for at least five minutes before powering back on. Do not reset more than two or three times — repeated resets without fixing the underlying cause can mask a more serious problem.
- Clear obstructions from the outdoor unit (AC/heat pump only) DIY safe
Switch the unit off before approaching it. Remove any leaves, dirt, or debris from the grille and the area immediately surrounding the outdoor unit. Gently rinse the condenser fins with a low-pressure garden hose if visibly dirty. Ensure there is at least half a metre of clear space around the unit for airflow.
- Call a Gas Safe registered engineer (boilers) or F-Gas certified engineer (AC/heat pump) Gas Safe engineer
If the fault persists after the DIY steps above, the problem is likely to involve the circulation pump, heat exchanger, refrigerant circuit, high-pressure switch, or PCB — all of which require a qualified professional. For gas boilers, the engineer must be Gas Safe registered. For any work on the refrigerant circuit of an AC or heat pump unit, F-Gas certification is a legal requirement in the UK. Do not attempt to open refrigerant lines or gas connections yourself.
Parts you may need
- Circulation pump (heating system) · from £95
- High-pressure switch (AC/heat pump outdoor unit) · from £45
- PCB (boiler or outdoor unit) · from £220
- Thermistor/temperature sensor · from £30
- Magnetic system filter (e.g. Magnaclean) · from £75
- Filling loop assembly · from £20
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–£400, depending on the underlying cause.
Frequently asked questions
Is Daikin E3 always a boiler fault?
No — E3 means something different depending on your Daikin product. On gas boilers and heating systems it typically points to an airlock or circulation restriction. On Daikin air conditioning and heat pump units (including Altherma), E3 means the high-pressure switch on the outdoor unit has tripped. Always check your specific model's documentation to confirm the meaning before taking any action.
Can I fix a Daikin E3 fault myself?
Some initial checks are safe for homeowners: topping up system pressure, bleeding radiators, clearing debris from the outdoor unit, and resetting the appliance are all reasonable first steps. However, if those actions do not clear the fault, the repair will likely involve the circulation pump, heat exchanger, refrigerant circuit, or PCB — all of which require a Gas Safe registered engineer (for boiler work) or an F-Gas certified engineer (for refrigerant work). Attempting these yourself is unsafe and, in the case of refrigerant handling, illegal.
How much does it typically cost to fix a Daikin E3 fault in the UK?
Most homeowners pay somewhere between £150 and £400 for an E3-related repair, covering the engineer call-out plus common fixes such as a power-flush to clear sludge, a pump replacement, or high-pressure switch renewal. If the fault turns out to be a failed heat exchanger or a PCB replacement, costs can climb to £500–£700 or beyond — but these outcomes are less common. An annual boiler or AC service (usually £90–£120) is the best way to catch the conditions that lead to E3 before they escalate.
Why does my Daikin E3 fault keep coming back?
A recurring E3 strongly suggests the root cause has not been fully resolved. On heating systems, repeated airlocks often point to a slow system leak that keeps allowing air in, or a failing automatic air vent. A persistent circulation fault can indicate a pump on its way out, or significant sludge and scale build-up requiring a power-flush. On AC and heat pump units, a recurring high-pressure trip usually means the outdoor coil needs a professional deep clean, or the refrigerant charge is incorrect. In either case, a qualified engineer should carry out a thorough inspection rather than continuing to reset the unit.