Daikin C0-00 Fault Code: Causes, Fixes & Repair Costs
What does the Daikin C0-00 fault code mean?
The C0-00 fault code appears on Daikin Altherma air-source heat pumps when the flow sensor detects water movement approximately 45 seconds after the internal water pump has stopped running. Because the system expects to see no flow at that point, it interprets the sensor reading as an error and shuts down as a precaution. The result is typically a complete loss of heating and hot water until the underlying 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
- Low system pressure or restricted water flow Common
If the heating circuit pressure has dropped below the recommended operating level (typically 1.0–1.5 bar), or flow through the system is restricted by a partly closed valve, the flow sensor can behave erratically and log a C0-00 error. Low pressure caused by a slow leak is a very common trigger for this fault in UK homes.
- Air trapped in the heating circuit Common
Air pockets in the pipework or within the heat pump's internal heat exchanger can disrupt the flow sensor's ability to read conditions accurately. This is especially common after a recent system pressure top-up, a drain-down, or any work carried out on the pipework.
- Faulty or worn flow sensor Sometimes
The flow sensor itself can develop an electrical fault or become worn over time, causing it to report incorrect readings — including detecting flow when the pump has stopped. The Daikin Altherma uses a SIKA-manufactured flow switch, and these can fail without any obvious external sign.
- Secondary or external pump causing unexpected flow Sometimes
If an additional circulation pump exists on the same water circuit (for example, on an underfloor heating zone), it may continue pushing water through the primary circuit after the heat pump's own pump stops, causing the sensor to trigger a false C0-00 alarm.
- Faulty hydro PCB (A1P board) Rare
The hydro PCB processes the signals from the flow sensor. If the board develops a fault, it may misinterpret valid sensor data and generate a C0-00 code even when the sensor and water circuit are functioning correctly. This is a less common but more costly cause.
How to fix it
- Check the system pressure gauge on the heat pump or buffer vessel DIY safe
Locate the pressure gauge — usually on the front panel of the indoor unit or on an associated buffer cylinder. The normal operating pressure for most Daikin Altherma installations is between 1.0 and 1.5 bar. If the reading is below 1.0 bar, top up the system using the filling loop according to your installer's instructions until it reaches approximately 1.2–1.5 bar. If pressure drops again within days, there is likely a leak that will need a professional inspection.
- Reset the heat pump from the controller DIY safe
Once the pressure is confirmed as correct, switch the heat pump off from the Daikin controller or indoor unit interface. Wait two to three minutes, then switch it back on. Some C0-00 faults caused by a momentary pressure dip or sensor glitch will clear after a proper reset. Observe whether the fault returns within the next heating cycle.
- Inspect the outdoor unit and surroundings for obvious issues DIY safe
Walk around the outdoor unit and check that nothing is blocking the airflow — snow, leaves, or debris can cause the unit to work harder. Also visually check accessible pipework and connections for any signs of water leaks (damp patches, corrosion marks, or dripping). Do not remove any panels or access covers.
- Check that all system valves are fully open DIY safe
Locate the isolation valves and zone valves on the heating circuit (usually near the indoor unit, manifold, or buffer vessel). Ensure every valve that should be open during normal operation is fully open. A partly closed valve can restrict flow sufficiently to cause false sensor readings.
- Bleed air from radiators or underfloor heating manifold DIY safe
Using a standard radiator bleed key, carefully bleed each radiator in the property — especially those on upper floors. If you have an underfloor heating manifold, check whether its air bleed valve or automatic air vent needs attention. After bleeding, recheck the system pressure and top up if needed.
- Have an MCS-certified engineer inspect and test the flow sensor heat pump engineer
If the fault returns after the above steps, the flow sensor will need an electrical check. An engineer will test the sensor's resistance and signal output against the values stated in the Daikin service manual. If the sensor is faulty, it will be replaced — this is not a DIY task as it involves draining part of the sealed water circuit and working inside the unit's casing.
- Have an engineer check for an external pump conflict and review the hydraulic configuration heat pump engineer
If a secondary circulation pump is installed on the same circuit, an engineer needs to verify that the controls and sequencing are set up so this pump cannot push water back through the primary circuit after the heat pump's own pump stops. Incorrect hydraulic configuration is a genuine cause of persistent C0-00 faults and requires professional diagnosis.
- Have the hydro PCB (A1P board) inspected if the fault persists heat pump engineer
If the sensor tests as healthy but the fault code keeps appearing, the hydro PCB that processes the sensor signal may be failing. PCB diagnosis and replacement must be carried out by a qualified engineer. This is one of the more expensive repairs associated with this fault code, so it is important to rule out simpler causes first.
Parts you may need
- Daikin / SIKA flow switch sensor (e.g. EKFLSW2) · from £65
- Hydro PCB (A1P board) for Daikin Altherma · from £420
- Automatic air vent / bleed valve · from £15
- Filling loop assembly (system pressure top-up kit) · 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 £200–£450, depending on the underlying cause.
Frequently asked questions
Can I fix a Daikin C0-00 error myself?
A homeowner can safely carry out a few initial checks: verifying and topping up the system pressure, bleeding air from radiators, ensuring all valves are open, and performing a power reset. These steps resolve the fault in a meaningful number of cases — particularly when a slow pressure drop is to blame. However, if the fault reappears, the flow sensor or internal components will need testing by an MCS-certified heat pump engineer. Opening the unit casing, working on internal pipework, or replacing parts is not a DIY job.
How much does it cost to fix a Daikin C0-00 fault in the UK?
For most households, the repair involves either a flow sensor replacement or an engineer callout to address low pressure and air in the system. Flow sensor replacement including parts and labour typically costs in the region of £200–£450. If the hydro PCB (A1P board) turns out to be the cause, costs can rise to £600–£800 or more depending on the model and engineer. Daikin's own direct non-contracted callout charge starts at around £311, to which any parts costs are added separately. Taking out an annual maintenance contract can reduce per-visit costs considerably.
What is the difference between Daikin fault codes C0-00, C0-01 and C0-02?
All three codes are part of the same flow-related fault family. C0-00 specifically means the flow sensor has detected water movement around 45 seconds after the pump stopped — essentially flow at an unexpected time. C0-01 is a flow switch warning that fires when flow is detected while the pump is confirmed as not running. C0-02 relates to the measured flow rate being too low during operation. They often share the same underlying causes — low pressure, air in the system, or a faulty sensor — but the precise trigger condition differs between them.
Will a Daikin C0-00 fault cause me to lose hot water as well as heating?
Yes, in most cases the C0-00 error causes the heat pump to shut down entirely as a protective measure, which means both space heating and domestic hot water production will stop until the fault is cleared. If you have a separate immersion heater in your cylinder, this can be used as a temporary backup for hot water, but the heat pump itself will remain locked out until the fault is resolved and the unit is reset.