Daikin PJ-00 Fault Code: Causes, Fixes & Repair Costs
What does the Daikin PJ-00 fault code mean?
The PJ-00 fault code flags a configuration or hardware problem with the outdoor unit's main PCB. Specifically, it points to one of two closely related issues: either the capacity setting adaptor (a small plug-in resistor that tells the board what size unit it's controlling) is absent or incorrect, or there is a mismatch between the inverter board and the fan driver board installed in the outdoor unit. Either way, the outdoor unit cannot verify its own identity or configuration, so it locks out to prevent damage. This code sits within the 'P' family of outdoor unit errors and the sub-code '00' identifies the master outdoor unit as the source.
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
- Missing or incorrect capacity setting adaptor after a PCB replacement Common
The capacity setting adaptor is a small plug-in resistor that identifies the unit's kilowatt or HP rating to the new board. When a main PCB is swapped out, this adaptor must be transferred from the old board or a correctly rated new one must be fitted. If the engineer forgot to transfer it, or fitted the wrong rating, PJ-00 will appear immediately on first power-up. This is by far the most frequent trigger of this code.
- Wrong replacement PCB fitted (inverter/fan driver mismatch) Common
Daikin outdoor units use a combination of control, inverter, and fan driver boards that must be matched to the specific model series and capacity. If any one of these boards is replaced with an incompatible variant — even one that looks physically similar — the boards cannot communicate correctly and PJ-00 is raised. This commonly happens when a repair board is sourced from the wrong model family.
- Field settings not applied after PCB swap Sometimes
After fitting a new main PCB, Daikin requires specific field settings to be programmed via the service menu. If this step is skipped or completed incorrectly, the board will not recognise the system configuration and will fault. The capacity setting adaptor and the field settings work together; both must be correct.
- Electrical damage to the outdoor PCB (surge, moisture, or age) Sometimes
Power surges, water ingress into the outdoor unit enclosure, or long-term component degradation can corrupt the PCB's EEPROM or damage onboard components. In these cases PJ-00 may appear on a board that has never been replaced. Connector corrosion or loose wiring causing intermittent signals can produce similar symptoms.
How to fix it
- Note the exact fault code displayed and perform a single power-cycle reset DIY safe
Before touching anything, take a photo of the fault code on the controller display. Switch the heat pump off at the indoor controller, wait 30 seconds, then switch it back on. If PJ-00 clears and does not return, monitor the unit closely over the next few hours. If the fault reappears after just one reset, do not keep cycling the power — repeated resets will not resolve a configuration fault and may mask the issue for an engineer.
- Check the outdoor unit for obvious physical issues DIY safe
Walk around the outdoor unit and look for signs of water pooling inside the casing, storm damage, or debris blocking the cabinet. Do not open the electrical enclosure or touch any internal components. If you can see that a connector has physically come away from a board through the vents, report this to your engineer but do not attempt to reconnect it yourself.
- Check whether a PCB or any component was recently replaced DIY safe
If your system has had any recent service work — particularly a main PCB replacement — this is almost certainly the cause. Contact the engineer who did the work and tell them PJ-00 has appeared; the likely culprit is a missing or incorrect capacity setting adaptor, which is a straightforward and inexpensive fix when caught early.
- Have an engineer verify and fit the correct capacity setting adaptor heat pump engineer
An MCS-certified engineer will open the outdoor unit electrical enclosure, identify the main PCB, and check whether the capacity setting adaptor (a small resistor plug specific to the unit's rated output) is present and matched to the unit. If it is missing or wrong, fitting the correct one is a quick and low-cost fix. The part itself costs only a few pounds, though you will still pay the engineer's callout and labour time.
- Have the engineer confirm PCB compatibility and board identity heat pump engineer
If the adaptor is correct, the engineer should cross-reference the part numbers on all installed boards (control PCB, inverter PCB, fan driver PCB) against the unit's model number. A board sourced from the wrong series will need to be returned and the correct one ordered. Daikin UK's parts warehouse in Weybridge stocks common boards and can often deliver next working day.
- Have the engineer apply the correct field settings heat pump engineer
After any PCB replacement, Daikin requires specific field settings to be programmed through the service interface. The engineer should work through the field setting checklist in the unit's service manual to confirm all parameters — including capacity, connected indoor units, and sensor assignments — are correctly configured.
- Have the engineer inspect for electrical damage and test the board heat pump engineer
If no configuration error is found, the engineer should inspect the PCB for signs of surge damage (scorching, swollen capacitors), moisture ingress, or corroded connectors. Wiring continuity and fuse integrity should also be checked. If the board is confirmed faulty and is not repairable, it will need to be replaced with the correct matched board and adaptor.
- If the fault persists, contact an MCS-certified heat pump engineer for a full diagnostic heat pump engineer
PJ-00 always requires an engineer to resolve correctly — there is no homeowner fix for a PCB configuration or hardware fault. Ensure your engineer is MCS-certified and, if refrigerant circuit work becomes necessary during the repair process, F-Gas registered. If the unit is still within Daikin's warranty period (typically 5–7 years via a certified installer), log the fault with Daikin UK before authorising any paid repair work.
Parts you may need
- Capacity setting adaptor (plug-in resistor, model-specific) · from £10
- Daikin outdoor unit main control PCB (model-specific) · from £380
- Daikin inverter PCB (outdoor unit, model-specific) · from £320
- Daikin fan driver PCB (outdoor unit, model-specific) · from £210
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 £280–£750, depending on the underlying cause.
Frequently asked questions
Can I fix the Daikin PJ-00 fault myself?
No — PJ-00 is a PCB configuration or hardware fault inside the outdoor unit's electrical enclosure, which is not safe or appropriate for a homeowner to open. The only DIY steps are noting the fault code, trying a single reset, and checking the outside of the unit for obvious damage. Everything else requires an MCS-certified engineer.
Is PJ-00 always caused by a failed PCB, or could it be something cheaper?
Quite often it is something cheaper. If a PCB has been replaced recently, the most likely cause is simply a missing or incorrectly rated capacity setting adaptor — a part that costs around £5–£10. You still pay for the engineer's callout and time, but the repair itself is straightforward. Only if the adaptor is correct and the boards are properly matched should a full PCB replacement be considered. For reference, a new outdoor unit PCB with labour typically costs £400–£750; a board refurbishment service can be £150–£400. For rare cases involving multiple failed boards or severely water-damaged units, costs can exceed these figures.
My heat pump is still under warranty — does Daikin cover PJ-00?
Potentially yes. Daikin offers 5–7 year parts and labour warranties through MCS-certified installers. Log the fault with Daikin UK before authorising any paid repair work, as an unauthorised repair could affect your warranty. If the fault has appeared after a recent service by your installer — for example because a PCB was swapped without fitting the correct adaptor — that installer may be liable for the cost of putting it right.
Will PJ-00 damage my heat pump if I leave it running?
The unit has locked itself out specifically to prevent damage, so it will not run normally while PJ-00 is active. Do not attempt to repeatedly reset the unit to force it to run — this will not resolve a configuration fault and may cause additional wear on components that try to start without valid configuration data. Book an engineer promptly, particularly in cold weather when losing your heating and hot water has an immediate impact.