Daikin L1-00 Fault Code: Causes, Fixes & Repair Costs
What does the Daikin L1-00 fault code mean?
The L1-00 code indicates that the heat pump's outdoor unit has detected a fault within its inverter printed circuit board (PCB). The inverter PCB is the electronic controller that regulates the compressor speed and overall power delivery in the outdoor unit. When Daikin's diagnostics log this code, it means the system's self-monitoring has identified an abnormal condition within that inverter circuit — whether the board itself has failed, a protective fuse has blown, or a connected component such as the outdoor fan motor or compressor wiring has caused the inverter's protection circuit to trip. This is a high-urgency fault: shut the unit down and do not continue running it, as doing so risks escalating a moderate repair into a far more costly compressor replacement.
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
- Blown fuse on the inverter PCB Common
A protective fuse on the inverter board is often the first and simplest explanation for an L1-00 trip. Electrical surges, brief overloads or an aging fuse can cause it to blow, cutting power to part of the inverter circuit and triggering the fault code. An engineer can identify this quickly during inspection.
- Defective outdoor unit fan motor Common
If the outdoor fan motor windings have degraded or the motor has seized, the inverter PCB detects the abnormal load or loss of feedback signal and logs an L1 fault. A common presentation is that the unit appears to run normally at first, but after roughly 15 minutes the fan stops and the L1-00 code appears. Fan motor failures are among the most frequently reported root causes of this code in the field.
- Failed inverter PCB Sometimes
The inverter board itself can fail due to component aging, moisture ingress, power surges or heat stress. In some cases the fan motor or compressor wiring is the underlying trigger that then damages the board — so both components may need replacing at the same time. Field reports have noted the inverter control board and filter board failing together.
- Damaged or loose compressor wiring Sometimes
The wiring harness connecting the compressor to the inverter board can develop faults — loose terminals, chafed insulation or broken conductors — that cause the inverter's protection circuit to trip. This is typically found during a full electrical inspection of the outdoor unit.
How to fix it
- Switch off and isolate the outdoor unit immediately DIY safe
Stop using the heat pump as soon as the L1-00 code appears. Continued operation under this fault can cause further damage to the compressor, significantly increasing repair costs. Turn the system off at the controller and then at the outdoor unit's isolator switch.
- Clear the area around the outdoor unit DIY safe
While waiting for an engineer, check that the outdoor unit is free from snow, ice, leaves or any debris blocking airflow around or over the unit. Restricted airflow can compound electrical stress on the inverter. Do not remove any panels or touch any internal components.
- Attempt a single power-cycle reset DIY safe
Switch off the outdoor unit isolator and wait at least five minutes — the inverter capacitors need this time to fully discharge. Then restore power, wait two minutes for the system to initialise, and observe whether the fault clears. If L1-00 returns within an hour of restarting, there is a genuine underlying fault and further operation should be avoided.
- Engineer: inspect inverter PCB fuses heat pump engineer
An MCS-certified engineer will open the outdoor unit and visually inspect the fuses on the inverter PCB for signs of blowing, burning or discolouration. A blown fuse is quick and inexpensive to resolve, but the engineer will also check what caused it to blow before simply replacing it.
- Engineer: test the outdoor fan motor heat pump engineer
With power isolated and capacitors discharged, the engineer will measure the fan motor winding resistance (typically 10–50 ohms on Daikin units) and check for continuity and absence of shorts to earth. A faulty motor should be replaced at this stage rather than after a PCB replacement, to avoid repeat failure.
- Engineer: test and diagnose the inverter PCB heat pump engineer
Using a multimeter and, where appropriate, an oscilloscope, the engineer will measure DC voltage outputs and PWM signal lines on the inverter board. Absent or abnormal readings confirm the board has failed and requires replacement. Daikin replacement PCBs must be configured to match the outdoor unit model.
- Engineer: inspect all compressor wiring and harnesses heat pump engineer
The full wiring run between the compressor terminals and the inverter board is checked for loose connections, damaged insulation and broken conductors. Any compromised wiring is repaired or replaced before the system is powered back up.
- Contact an MCS-certified heat pump engineer heat pump engineer
If the reset does not clear the fault, arrange a visit from an MCS-certified engineer. Any work inside the outdoor unit's electrical cabinet or on the sealed refrigerant circuit must be carried out by a qualified professional — refrigerant-side work specifically requires an F-Gas registered engineer. Do not run the heat pump again until the fault has been properly diagnosed and repaired.
Parts you may need
- Daikin outdoor unit inverter PCB · from £320
- Daikin outdoor fan motor · from £180
- Inverter PCB fuse kit · from £12
- Compressor wiring harness · from £65
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–£900, depending on the underlying cause.
Frequently asked questions
Is the Daikin L1-00 fault dangerous to ignore?
Yes — you should stop the unit as soon as this code appears. Running the heat pump through an inverter PCB fault puts sustained electrical stress on the compressor, which is the most expensive component in the system. What might be a £300–£450 fan motor or PCB repair can become a compressor replacement costing £1,400 or more if the unit is left running. Switch off at the isolator and call an engineer promptly.
Can I reset a Daikin L1-00 fault myself?
You can attempt a single power-cycle reset: isolate the outdoor unit, wait five full minutes for the internal capacitors to discharge, restore power and observe the unit for up to an hour. If the code returns, do not reset again — a recurring L1-00 is a confirmed hardware fault, not a glitch, and the unit should stay off until an engineer has diagnosed it. Do not open the outdoor unit casing or touch any internal components.
How much does it typically cost to fix a Daikin L1-00 fault in the UK?
Most L1-00 repairs fall in the range of £250–£900 depending on the root cause. A blown fuse found during a callout is usually resolved for £100–£180 including labour. An outdoor fan motor replacement typically costs £250–£450 all-in. An inverter PCB replacement is usually £450–£900 including parts and labour. If both the PCB and fan motor need replacing together, expect £600–£1,200. In rare cases where the compressor has been damaged by prolonged operation under fault conditions, costs can exceed £1,400 — which is why acting quickly matters.
Will my Daikin warranty cover an L1-00 fault?
It depends on the cause and your service history. Daikin's standard residential warranty covers manufacturer defects on parts for three years, but claims are typically rejected without annual service records from an F-Gas certified engineer. Wear-and-tear failures — such as a fan motor that has degraded over time — are generally not covered. Contact your installer or Daikin's UK support line with your installation date and service records before authorising any paid repair, so you can establish whether a warranty claim applies.