Daikin C0-02 Fault Code: Causes, Fixes & Repair Costs
What does the Daikin C0-02 fault code mean?
The C0-02 fault code appears on Daikin Altherma heat pumps when the system's flow sensor or flow switch reports a critical error — specifically, the sensor is detecting water movement in the circuit while the pump is not being commanded to run, or the sensor reading is otherwise inconsistent with what the controller expects. Unlike the milder C0-00 and C0-01 variants in the same family, C0-02 is the most severe: the heat pump will shut down completely and will not restart until the underlying fault has been addressed and the unit manually reset.
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
- Faulty water flow sensor Common
The flow sensor itself has failed or drifted out of calibration, causing it to report flow conditions that don't match the actual pump state. This is the most frequently diagnosed root cause when pressure and valve checks come back normal. The sensor is a relatively low-cost component but fitting it requires an MCS-certified engineer to drain, replace, and recommission the hydraulic circuit.
- Low or zero system water pressure Common
If system pressure has dropped — typically below 1.0 bar — the unit cannot circulate water properly and the flow sensor reading becomes unreliable or drops out entirely. Pressure loss can happen gradually through normal operation or suddenly if a leak develops. UK homeowners should check the pressure gauge on the indoor unit or buffer vessel; the target range is usually 1.0–1.5 bar when cold.
- Closed or partially closed isolator valves Sometimes
If any isolator valve in the heating circuit — including valves serving underfloor heating manifolds, radiator zones, or the heat pump itself — is partially or fully closed, water flow is restricted. The flow sensor will either detect no flow or an inconsistent signal, triggering the lockout. Air trapped at high points in the pipework can have the same effect.
- Faulty hydro PCB (A1P) Sometimes
The hydraulic module's main control board (A1P) interprets signals from the flow sensor and commands the circulation pump. If the PCB develops a fault — whether due to a component failure, power irregularity, or moisture ingress — it may misread sensor signals or fail to power the sensor correctly, producing a spurious C0-02 code even when the sensor itself is fine.
- Damaged or degraded wire harness Rare
The wiring loom connecting the flow sensor to the hydro PCB can develop intermittent faults over time — particularly at connector points subject to vibration or moisture. A damaged harness can cause the sensor signal to drop or fluctuate, mimicking a sensor failure.
- External or secondary pump interference Rare
Where an additional circulation pump has been installed on the same water circuit (for example, to serve a large underfloor heating system), it can create unexpected pressure differentials that confuse the flow sensor. This is an uncommon cause but worth investigating if the system has been modified since original installation.
How to fix it
- Check the system water pressure DIY safe
Find the pressure gauge on your indoor unit, buffer vessel, or filling loop assembly. The reading should sit between 1.0 and 1.5 bar when the system is cold. If it reads below 1.0 bar, use the filling loop to top up the pressure carefully — add water slowly and stop when you reach about 1.2–1.3 bar. Refer to your Daikin Altherma user manual for the exact location of the filling loop on your model. Do not overfill beyond 2.0 bar.
- Inspect the pipework and connections for visible leaks DIY safe
Walk around the indoor unit, any buffer cylinder, and accessible pipework looking for damp patches, staining, or corrosion around joints. If you find an active leak, do not attempt to repair it yourself — contact an MCS-certified installer. If pressure drops repeatedly after topping up, a hidden leak is likely and a professional trace-and-repair visit is needed.
- Check that all system valves are fully open DIY safe
Locate any isolator valves on the pipework serving the heat pump — these are typically lever-type ball valves. A lever parallel to the pipe means open; perpendicular means closed. Check valves at the underfloor heating manifold, any zone valves, and the connections into and out of the indoor unit. If your system has manual air bleed valves at high points, check that they are open and bleed any trapped air.
- Restart the heat pump after resolving any pressure or valve issues DIY safe
Switch the heat pump off at the controller, then turn off the external isolator switch and the indoor unit isolator. Wait at least three minutes, then restore power in reverse order. Navigate to the fault reset option on the user interface (typically under Settings or Diagnostics). If C0-02 clears and does not return within an hour, the issue was likely low pressure or an air lock. If the fault returns promptly, the underlying cause has not been resolved.
- Check the outdoor unit and surrounding area DIY safe
Ensure there is no debris — leaves, snow, or ice — blocking the outdoor unit's casing or any water pipework connections at the outdoor section of a split system. Restricted airflow or frozen external pipework can contribute to unusual operating conditions, though this is a secondary check for C0-02.
- Arrange a visit from an MCS-certified heat pump engineer for sensor and PCB diagnosis heat pump engineer
If the fault persists after all homeowner checks, the flow sensor, wire harness, and hydro PCB (A1P) all require hands-on testing with diagnostic equipment. An engineer will use the Daikin service interface to read live sensor values, test continuity of the harness, and confirm whether the PCB is powering the sensor correctly. Flow sensor replacement involves draining part of the hydraulic circuit and recommissioning — this must be carried out by a qualified engineer. If your system is within its 5–7 year standard Daikin warranty (or up to 12 years if registered and annually serviced), contact Daikin UK first to log the fault and have an authorised service partner carry out the repair under warranty at no charge. Attending a warranted system without prior Daikin authorisation may make the visit chargeable.
Parts you may need
- Water flow sensor (Daikin Altherma) · from £45
- Hydro PCB A1P (model-specific) · from £380
- Wire harness / sensor loom · from £55
- Filling loop assembly (for pressure top-up) · 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–£500, depending on the underlying cause.
Frequently asked questions
Can I fix the Daikin C0-02 fault myself?
Partially. You can safely check and restore system pressure, look for visible leaks, confirm all valves are open, and attempt a manual reset — and in some cases that is enough to clear the fault. However, if the fault returns, the flow sensor, wiring, or hydro PCB needs professional diagnosis. Working on internal electrical components or draining the hydraulic circuit without the right qualifications is not safe and risks voiding your warranty.
Will my Daikin warranty cover a C0-02 repair?
It may well do. Standard Daikin Altherma warranties cover 5–7 years on parts and labour, and this extends to up to 12 years if you registered the product and have kept up annual services by a Daikin-approved partner. The key step is to contact Daikin UK before booking any engineer visit — warranty repairs must be pre-authorised, and sending out a non-approved engineer without that authorisation can result in costs falling on you even if the system is within warranty.
How much does it cost to replace a Daikin Altherma flow sensor in the UK?
Flow sensor replacement — including the engineer's time, the part, and recommissioning — typically falls in the range of £200–£350 when carried out outside of warranty. The sensor itself is relatively inexpensive (around £40–£60 for the part), so the majority of the cost is the labour and call-out charge. If the fault turns out to be the hydro PCB rather than the sensor, costs rise considerably; PCBs for Altherma systems typically cost £300–£500 for the part alone, with total repair bills potentially reaching £600–£800 in that scenario.
Why does the C0-02 fault keep coming back after I top up the pressure?
If pressure drops repeatedly, it points to a slow leak somewhere in the closed heating circuit — either at a joint, a valve, a radiator connection, or the heat pump's own internal components. The leak may be too small to see easily as a drip but will cause pressure to fall over days or weeks. An engineer can pressure-test the system to locate it. Additionally, if the flow sensor itself is faulty, topping up the pressure won't resolve the root cause and the fault will return regardless.