Daikin E9-00 Fault Code: Causes, Fixes & Repair Costs
What does the Daikin E9-00 fault code mean?
The E9-00 code signals that the heat pump's outdoor unit has detected a fault with the Electronic Expansion Valve (EEV) or its associated control circuit. The EEV is a precision component inside the refrigerant circuit that regulates how much refrigerant flows through the system at any given moment — fine-tuning performance as heating demand changes. When the controller detects no continuity through the valve, or spots abnormal superheat readings (suction superheat below 4K and discharge superheat below 5K) alongside a valve stuck at minimum opening, it raises E9-00 and locks the system out to protect the compressor. The fault applies primarily to Daikin Altherma split and monobloc heat pump models in the EBLA and EDLA ranges, though the E9 designation appears across other Daikin heat pump and VRV product lines too.
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
- EEV coil or motor failure Common
The electronic expansion valve is driven by a small stepper motor and coil assembly. If the coil develops an open circuit, short, or winding fault, the outdoor PCB loses the ability to position the valve, triggering E9-00. This is the single most frequently identified root cause when engineers attend this fault.
- Loose, corroded or damaged EEV wiring connector Common
The multi-pin connector linking the EEV coil to the outdoor PCB is exposed to condensation, vibration and temperature cycling. A poor contact at this connector produces the same 'no continuity' signal as a failed coil and is worth checking before condemning the valve itself.
- Faulty refrigerant-side thermistor giving false readings Sometimes
The suction and discharge thermistors feed the superheat calculations that inform the E9-00 trigger condition. A drifting or open-circuit thermistor can cause the controller to see impossible superheat values and falsely log E9-00, even when the EEV itself is healthy. Daikin's own service procedure recommends ruling this out first.
- Outdoor unit PCB defect Sometimes
The main PCB in the outdoor unit generates the drive signals that step the EEV motor. If the EEV output circuit on the board has suffered a component failure or heat damage, the valve cannot respond and E9-00 is raised. This is typically only confirmed after the EEV coil and wiring have been cleared as causes.
- Power supply irregularity Rare
A voltage imbalance greater than 4% or a sustained under-voltage condition can disrupt the precision control signals sent to the EEV drive circuit and cause spurious E9-00 trips, particularly on first start-up in cold weather.
- Mechanically seized expansion valve Rare
In older units or systems with refrigerant contamination, debris or wear can cause the EEV needle to seize in position. The motor and coil may still show correct resistance but the valve itself will not move, leading to the superheat conditions that trigger E9-00.
How to fix it
- Check the system's power supply and reset the unit DIY safe
Confirm the heat pump's isolator switch and indoor controller are both on, and check your consumer unit for any tripped MCB associated with the heat pump circuit. Restore power if interrupted, wait two minutes, and attempt a restart from the controller. A transient power disturbance can occasionally cause a spurious E9-00 trip that clears on restart. If the fault reappears immediately, do not keep resetting — move on.
- Check system water pressure DIY safe
While not a direct cause of E9-00, low system pressure (below roughly 1.0 bar when cold) can accompany other faults and should be corrected before an engineer visits. Top up via the filling loop to approximately 1.2–1.5 bar cold, following your system's specific guidance.
- Clear any obstruction around the outdoor unit DIY safe
Ensure the outdoor unit's air inlet and outlet are free from leaves, snow, ice or debris that could cause the unit to work harder and contribute to abnormal operating conditions. Do not attempt to access the unit's internal components.
- Note the exact fault code and any secondary codes shown on the controller DIY safe
Take a photo of the fault display and note down when the fault first appeared, whether it happens immediately on start-up or after a period of running, and any recent changes to the system (e.g. recent servicing, unusual weather). This information will help the engineer diagnose the fault faster and reduce billable time on site.
- Have an F-Gas registered, MCS-certified engineer inspect the EEV coil resistance heat pump engineer
Using a multimeter, the engineer will measure the resistance across each winding of the EEV coil and compare readings. On multi-circuit systems, a coil reading significantly differently from the others indicates the faulty unit. This is a refrigerant-circuit-adjacent task requiring access to the outdoor unit's sealed electrical compartment and should only be carried out by a qualified engineer.
- Have the engineer inspect the EEV wiring harness and connector heat pump engineer
The engineer should visually inspect the connector body for corrosion, bent pins, or cracked insulation, and perform a continuity check through the harness. A poor connection here is often cheaper to rectify than a full component replacement.
- Have the engineer test the refrigerant-side thermistors heat pump engineer
The suction and discharge thermistors should be tested for correct resistance at a known temperature to rule out a false trigger. Replacement thermistors are relatively low-cost and quick to fit, so this is a good early step if the EEV coil checks out healthy.
- EEV coil or full valve replacement if confirmed faulty heat pump engineer
If the coil resistance is out of specification or the valve is found to be mechanically seized, the engineer will replace the coil assembly or the complete EEV. Because the valve sits within the refrigerant circuit, any work involving the refrigerant requires an F-Gas registered engineer. Refrigerant must be legally recovered, not vented.
- Outdoor PCB replacement if the drive circuit is identified as faulty heat pump engineer
Should the EEV, wiring and thermistors all prove healthy, the engineer will assess the outdoor PCB for damage to the EEV drive output stage. PCB replacement is an electrical task within the outdoor unit and must be carried out by a qualified engineer.
- Contact an MCS-certified heat pump engineer to attend site heat pump engineer
If the fault persists after the basic homeowner checks above, book an MCS-certified engineer with Daikin Altherma experience. Before booking an independent engineer, check whether your system is still within Daikin's 5–7 year parts and labour warranty — warranty repairs must be logged with Daikin UK first to be authorised and covered free of charge. Attending without prior authorisation may make the visit chargeable even if a fault is covered.
Parts you may need
- Daikin EEV coil assembly (outdoor unit) · from £130
- Electronic Expansion Valve (complete, OEM) · from £280
- Outdoor unit main PCB (Altherma 3 M series) · from £320
- Suction thermistor (refrigerant-side) · from £35
- Discharge thermistor (refrigerant-side) · from £35
- EEV wiring harness / connector · from £25
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 £220–£650, depending on the underlying cause.
Frequently asked questions
Can I reset the E9-00 fault myself and carry on using the heat pump?
You can try a single reset from the controller after checking the power supply, but if E9-00 returns straight away the underlying problem has not gone away. Continuing to reset and run the system risks driving abnormal refrigerant conditions that can ultimately damage the compressor — an expensive component. Reset once, and if the fault reappears, leave the unit off and call an engineer.
Is E9-00 always caused by a failed expansion valve, or could it be something cheaper?
Not always. A faulty refrigerant-side thermistor can produce the superheat readings that trigger E9-00 even when the EEV itself is perfectly healthy. Thermistors typically cost £30–£50 in parts and are quick to replace, making this one of the first things a good engineer will rule out. A loose or corroded EEV connector is another low-cost fix. The more expensive repairs — a full EEV or outdoor PCB — are only necessary when those cheaper possibilities have been eliminated.
My system is still under Daikin's warranty — what should I do?
Do not book an independent engineer before contacting Daikin UK directly. Daikin offers a 5–7 year parts and labour warranty on Altherma heat pumps installed by approved partners, but warranty jobs must be authorised by Daikin before an engineer attends, otherwise the visit can become chargeable regardless of the outcome. Call Daikin UK or your original installer first, quote the E9-00 code, and let them guide you through the warranty claim process.
Why does refrigerant work on a heat pump require a special qualification?
The refrigerants used in heat pump systems (such as R32 or R410A) are potent greenhouse gases regulated under UK F-Gas legislation. It is a legal requirement that anyone handling, recovering or charging these refrigerants holds an F-Gas certificate. Releasing refrigerant to atmosphere is both illegal and harmful to the environment. Always confirm that any engineer working inside the refrigerant circuit of your heat pump holds a current F-Gas qualification, in addition to being MCS-certified for heat pump work.