Daikin H3-00 Fault Code: Causes, Fixes & Repair Costs
What does the Daikin H3-00 fault code mean?
The H3-00 fault on a Daikin heat pump indicates that the high pressure switch (HPS) has detected an open circuit condition while the compressor is stopped. In plain terms, the system's safety switch — which monitors refrigerant pressure on the high-pressure side — has either tripped due to genuinely excessive pressure, or there is an electrical fault (such as a wiring break or connector issue) that is making the control board believe the switch is open. The '00' suffix simply identifies the unit number in multi-split or VRV/S installations and does not alter the underlying fault meaning. Because this switch exists to protect the compressor and refrigerant circuit from dangerously high pressures, the system will lock out until the root cause is identified and resolved.
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
- Faulty or worn high pressure switch Common
The switch itself has reached end of life or developed an internal fault, causing it to report an open circuit even when system pressure is within normal operating range. This is the single most frequent root cause engineers encounter on H3-00 call-outs.
- Restricted airflow across the outdoor unit coil Common
Leaves, debris, overgrown vegetation, snow or ice blocking the outdoor unit's coil prevent adequate heat exchange. This forces refrigerant head pressure upward until the safety switch trips. Even a partially blocked coil is enough in warm or humid conditions.
- Loose, corroded or broken wiring in the HPS circuit Sometimes
A damaged wire, corroded spade connector or loose plug between the high pressure switch and the outdoor unit PCB can create an open-circuit reading without any actual overpressure event. The board cannot distinguish a wiring fault from a tripped switch.
- Refrigerant overcharge or blockage in the refrigerant circuit Sometimes
Too much refrigerant in the sealed circuit, or a restriction such as a partially blocked expansion valve or dirty condenser coil, will cause head pressure to rise above the switch's trip point under load. This genuinely trips the switch rather than simulating a fault.
- Defective outdoor unit PCB Rare
If the control board's input circuit for the HPS signal has failed, it may read the switch as open regardless of actual switch condition or refrigerant pressure. This is confirmed only after wiring and the switch itself have been ruled out.
How to fix it
- Clear the area around the outdoor unit and check for visible obstructions DIY safe
Walk around the outdoor unit and remove any leaves, dirt, plant growth, snow or ice that is covering the coil grille or blocking airflow above and around the unit. Manufacturers typically recommend at least 30 cm of clear space on the air-intake sides and 1 m or more in front of the discharge. A garden hose on a gentle setting can rinse the coil fins from the outside if they are visibly dirty — do not use a pressure washer.
- Check the system water pressure on the indoor unit display or pressure gauge DIY safe
While the H3-00 is a refrigerant-side fault, low system (heating circuit) water pressure can sometimes cause secondary pressure-related alarms. Confirm the heating circuit pressure reads between 1.0 and 1.5 bar when cold. If it is below 1.0 bar, top up via the filling loop following the instructions in your homeowner manual.
- Reset the unit from the controller and observe whether the fault returns DIY safe
Navigate to the fault reset function on your Daikin controller or indoor unit interface (refer to your user manual for the exact key sequence — commonly holding the Mode or Reset button for several seconds). Allow the unit to attempt a restart. If the fault clears and does not return within the first heating cycle, monitor closely over the next 24 hours. If it returns immediately or within minutes, the fault is persistent and requires an engineer — do not keep resetting, as repeated resets without addressing the cause risk compressor damage.
- Check the electrical isolator and consumer unit MCB for the heat pump DIY safe
Confirm the outdoor unit isolator (usually a red rotary switch on the wall near the outdoor unit) is in the ON position, and check your consumer unit (fuse board) for any tripped MCB labelled 'heat pump' or 'ASHP'. If an MCB has tripped, reset it once. If it trips again immediately, do not reset a second time — contact an engineer, as repeated tripping indicates an electrical fault.
- Arrange for an MCS-certified engineer to inspect the high pressure switch and wiring heat pump engineer
A qualified engineer will isolate the unit safely, then check continuity across the high pressure switch terminals and inspect the wiring harness and connectors for corrosion, chafing or breaks. If the wiring is intact and the switch tests open-circuit at normal pressure, the switch will be replaced. This is not homeowner-serviceable work as it involves working inside the outdoor unit's electrical enclosure.
- Have an F-Gas registered engineer check refrigerant pressure and circuit integrity heat pump engineer
If the wiring and switch are found to be sound, an F-Gas registered engineer will connect refrigerant manifold gauges to measure actual high-side pressure. Overpressure can indicate refrigerant overcharge, a blocked expansion valve or a fouled heat exchanger. Any refrigerant handling — whether recovering, recharging or leak-testing the sealed circuit — is legally restricted to F-Gas registered technicians in the UK.
- Have the outdoor unit PCB tested and replaced if required heat pump engineer
If pressure, switch and wiring all test correctly, the engineer will assess the outdoor unit PCB. A faulty input circuit on the board can produce a false H3-00 reading. PCB replacement must be carried out by a qualified engineer and, for Daikin, ideally by a Daikin-approved service partner to ensure the correct firmware and part number are used.
Parts you may need
- Daikin high pressure switch (HPS) · from £45
- Outdoor unit PCB (model-specific) · from £220
- HPS wiring harness / connector set · from £25
- Refrigerant (R-32 or R-410A per kg, F-Gas registered supply) · from £35
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
Can I reset the H3-00 fault myself and carry on using the heat pump?
You can attempt a single reset from the controller to see whether the fault was a one-off transient event — for example caused by a brief airflow restriction. However, if the H3-00 returns after the reset, do not keep cycling the power. The high pressure switch is a safety device, and repeatedly overriding it without addressing the root cause risks serious compressor damage, which is an expensive repair. Treat a recurring H3-00 as a call-an-engineer situation.
Is the H3-00 fault dangerous — do I need to evacuate the property?
The H3-00 is a compressor-protection lockout, not a carbon monoxide or combustion hazard (heat pumps have no burner or flue). The unit will simply stop operating until the fault is cleared. There is no immediate safety risk to occupants, though you will lose heating or hot water while the fault is active. If you notice any unusual smells (refrigerant has a faint, slightly sweet odour) or hissing from the outdoor unit, keep people away from the unit and call an F-Gas engineer promptly, as this could indicate a refrigerant leak.
How much does it typically cost to fix a Daikin H3-00 fault in the UK?
Most H3-00 repairs fall in the range of £150–£450 all-in. A straightforward high pressure switch replacement including parts and an engineer's time typically costs £150–£350. A coil clean or connector repair will be at the lower end of that range. If the fault turns out to involve the refrigerant circuit — for example an overcharge or a leak requiring recharge — costs rise to £300–£600 depending on the amount of refrigerant needed and how accessible the leak is. In the relatively rare scenario that the outdoor unit PCB has failed, expect £350–£700 including the board and labour. Always check whether your Daikin unit is still under manufacturer warranty before booking a repair, as covered repairs are authorised free of charge.
Why does the H3-00 fault keep coming back after I reset it?
A recurring H3-00 points to an underlying problem that a reset does not fix. The most common reasons for repeat trips are a failing high pressure switch that is becoming intermittently open-circuit, a partially blocked outdoor coil that is only an issue under load, or a slow refrigerant overcharge condition. An engineer visiting with manifold gauges and a multimeter can usually identify which of these is responsible within a single diagnostic visit. Continuing to reset the unit accelerates wear on the compressor start circuit and can eventually damage the compressor itself.