Daikin L5-00 Fault Code: Causes, Fixes & Repair Costs
What does the Daikin L5-00 fault code mean?
The L5-00 code is triggered when the outdoor unit's inverter board detects an instantaneous current spike on its output to the compressor — one that exceeds the built-in protection threshold, even if only for a fraction of a second. The moment this happens, the board cuts power to the compressor and locks the system out completely. Unlike a gradual overcurrent event, L5 points to a hard electrical fault: the inverter is essentially detecting that something is drawing far more current than it should, right from the moment it tries to start the compressor. The indoor controller will display L5-00 and the unit will not respond to the remote or restart itself. You may hear the outdoor fan spin up briefly, followed by a thump or hum from the compressor, and then silence — that sound is the compressor attempting and failing to start.
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
- Degraded or shorted compressor windings Common
On units that are more than five years old, the internal insulation on the compressor's motor windings can break down over time. This causes a phase-to-phase short or a winding-to-earth fault, which immediately pulls far more current than the inverter can safely supply. This is the most frequently diagnosed root cause of L5-00 on older Daikin heat pumps, and it often means the compressor itself needs replacing.
- Faulty inverter PCB or Intelligent Power Module (IPM) Common
The IPM is the power stage of the inverter board that actually drives the compressor. If the IPM or the wider inverter PCB has failed — whether from age, a power surge, or moisture ingress — it may produce unbalanced or incorrect three-phase output, which trips the overcurrent protection. An engineer will check for burned components and test the output waveform with the compressor disconnected.
- Mechanically seized compressor Sometimes
If the compressor's internal moving parts have jammed — sometimes called a mechanical lock — the motor cannot turn freely. The inverter tries to force it to spin, the current surges instantly, and L5-00 fires. A seized compressor typically cannot be repaired and requires full replacement along with a refrigerant circuit service.
- Wiring short-circuit between inverter and compressor Sometimes
The three wires (U, V, W) running from the inverter PCB to the compressor can suffer insulation damage from heat, chafing, or rodent activity. If two or more wires touch, or if a wire contacts the chassis, the inverter sees a dead short and trips immediately. An engineer will inspect and resistance-test these wires as part of diagnosis.
- Service valves left closed after installation Sometimes
On a newly commissioned or recently serviced unit, if the Allen-key service valves on the outdoor unit have not been fully opened, the compressor tries to pump against a sealed system. This creates an almost instant mechanical overload and current spike, triggering L5-00. This is only relevant in the period immediately following installation or a refrigerant circuit service.
- Outdoor fan not operating Sometimes
If the outdoor unit's fan has failed or is blocked, heat cannot be dissipated from the refrigerant circuit. Pressure builds rapidly, the compressor has to work far harder to overcome it, and the resulting current draw can exceed the inverter's protection threshold. You may notice the fan is not spinning when the fault occurs.
- Low refrigerant charge Rare
A significantly undercharged refrigerant circuit forces the compressor to run harder to achieve the same heat transfer, which can push current draw beyond normal limits. While not the most common cause of L5-00 specifically, it is a contributing factor worth investigating — particularly if the unit has a history of refrigerant loss. Refrigerant work must only be carried out by an F-Gas registered engineer.
How to fix it
- Do not repeatedly attempt to restart the unit DIY safe
Each restart attempt when a hard electrical fault is present risks causing further damage to the inverter PCB. Limit yourself to a single reset attempt as described below. If L5-00 returns immediately or within minutes, stop and arrange an engineer visit.
- Check that the outdoor unit service valves are fully open DIY safe
On the outdoor unit, locate the refrigerant pipe connections. Each pipe has a cap covering an Allen-key valve. If the unit has been recently installed or serviced, check that these valves are in the fully open position (Allen key turned fully anti-clockwise). If you are unsure or cannot safely access them, leave this to an engineer.
- Check for obvious obstructions around the outdoor unit DIY safe
Walk around the outdoor unit and look for anything that could be blocking airflow — compacted snow, fallen leaves, garden debris, or items stored too close. Clear any obstructions and ensure there is at least 30 cm of clear space around all sides of the unit.
- Check your consumer unit for a tripped MCB DIY safe
Go to your consumer unit (fuse board) and check whether the MCB or RCD protecting the heat pump circuit has tripped. If it has, reset it once. If it trips again immediately, do not attempt a further reset — contact an engineer, as repeated tripping indicates a persistent electrical fault.
- Perform a single power reset DIY safe
Switch off the heat pump at its dedicated isolator (usually a red or grey switch near the outdoor unit or in the plant room). Wait a full two minutes to allow the inverter's DC bus capacitors to discharge safely, then switch the isolator back on. Attempt to restart the unit from the controller. If L5-00 returns, do not reset again.
- Arrange for an F-Gas registered, MCS-certified heat pump engineer to carry out a full diagnosis heat pump engineer
If the fault persists after a single reset, professional diagnosis is required. An engineer will disconnect the compressor and resistance-test the U, V, and W windings, perform insulation testing with a megohmmeter, inspect the IPM and inverter PCB for damage, check the U-V-W wiring harness for shorts, and test inverter output waveform balance. This work involves potentially lethal DC voltages and specialist test equipment — it must not be attempted by anyone other than a qualified engineer.
- Allow the engineer to replace the faulty component heat pump engineer
Depending on diagnosis, the repair will typically be either an inverter PCB/IPM replacement or a compressor replacement. Compressor replacement involves recovering the existing refrigerant charge (F-Gas registered engineer required), replacing the compressor and filter-drier, evacuating the circuit, and recharging with a measured refrigerant quantity. Neither repair is suitable for DIY.
Parts you may need
- Daikin outdoor unit inverter PCB / IPM module · from £420
- Compressor (model-specific, hermetically sealed) · from £850
- U-V-W wiring harness (compressor leads) · from £45
- Filter-drier (required with compressor replacement) · 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 £450–£950, depending on the underlying cause.
Frequently asked questions
Is it safe to keep resetting the unit when L5-00 appears?
No. A single reset is reasonable to rule out a transient fault, but if L5-00 returns, further resets risk damaging the inverter PCB. Each time the inverter tries to start a compressor with shorted windings or a seized motor, it puts the power module under extreme stress. Stop resetting and book an engineer.
Does L5-00 always mean I need a new compressor?
Not necessarily. While a failing compressor is a common cause, L5-00 can also result from a faulty inverter PCB or IPM module, a wiring short, or even closed service valves on a recently serviced unit. An engineer needs to carry out winding resistance tests and PCB diagnostics to confirm which component has failed before any parts are ordered. Inverter PCB replacement is significantly cheaper than a compressor swap.
How much does it typically cost to fix L5-00 on a Daikin heat pump in the UK?
For most people, the repair will be either an inverter PCB replacement — typically £450–£950 including parts and labour — or a compressor replacement, which usually falls in the range of £1,500–£2,000 when you factor in parts, labour, refrigerant recovery and recharge, and a new filter-drier. On units that are eight years old or more, some engineers and homeowners decide that a full system replacement represents better long-term value than a costly compressor repair, particularly if the unit is out of warranty.
My Daikin heat pump is still under warranty — what should I do?
Contact Daikin UK or your installing MCS-certified engineer before arranging any third-party repair. Daikin typically logs the fault and arranges for an approved service partner to attend. Having the unit worked on without authorisation during the warranty period could result in the warranty being voided and any visit being chargeable. Daikin heat pumps generally carry a 5–7 year warranty, with extensions of up to 12 years available when the product is registered and serviced annually by an approved engineer.
Why does L5-00 cause a complete lockout rather than just a temporary shutdown?
L5-00 is a hard protection event, not a recoverable alarm. The inverter board is designed to cut output instantly and latch the fault because the underlying cause — a current spike of this magnitude — indicates a serious electrical problem. Allowing the system to keep attempting to restart could cause irreparable damage to the inverter's power module or create a safety hazard. The lockout forces the fault to be properly investigated before the system runs again.