Daikin C0-01 Fault Code: Causes, Fixes & Repair Costs
What does the Daikin C0-01 fault code mean?
The C0-01 code appears on Daikin Altherma heat pumps when the water flow sensor continues to detect movement in the water circuit roughly 45 seconds after the internal pump has been commanded to stop. In simple terms, the system expects stillness but the flow switch is still registering activity — signalling either a genuine flow problem, a faulty sensor, or interference from an external pump on the same circuit. Unlike a hard lockout, the unit typically keeps running when C0-01 is first triggered, but the underlying issue should be investigated promptly to prevent escalation and potential shutdown.
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 water pressure Common
If system pressure has dropped below the recommended operating range (typically 1.0–1.5 bar), the flow switch can give erratic or misleading readings. Pressure drops gradually over time due to small leaks, trapped air escaping, or a lack of routine maintenance top-ups. This is the first thing to check and is extremely straightforward to diagnose by looking at the pressure gauge on the hydronic unit.
- Air trapped in the water circuit Common
Air pockets lodged in pipework or at high points in the heating circuit can cause inconsistent flow, meaning the sensor sees turbulence when it should see stillness (or vice versa). Newly commissioned systems or those that have recently had pipework work done are especially prone to this.
- Blocked or stuck flow switch Common
The flow switch sits in the pipework between the water inlet and the heat exchanger. Sediment, scale, or debris can cause the switch mechanism to stick in the open position, making the PCB believe flow is still present even after the pump has stopped. A stuck switch is a common root cause once pressure and air have been ruled out.
- External or secondary pump not electrically interlocked Sometimes
If a secondary circulation pump — for example, on an underfloor heating manifold or a buffer vessel circuit — is running independently, it can push water past the flow switch even when the Daikin unit's own pump has stopped. Without an electrical interlock to synchronise the pumps, this creates a false reading that reliably triggers C0-01.
- Faulty flow switch or sensor Sometimes
The flow switch itself may have developed an internal fault, causing it to report incorrectly regardless of what is actually happening in the circuit. This is diagnosed by the engineer performing an electrical check of the sensor against the values specified in the installer reference guide.
- Faulty hydro PCB or wiring harness Rare
In rare cases, the signal from a functioning flow switch is misinterpreted by the PCB due to a PCB fault, damaged wiring, or a loose connector on the harness. External vibration near the unit can also trigger a spurious flow reading in borderline cases. This is typically only considered once all other causes have been eliminated.
How to fix it
- Check the system water pressure gauge DIY safe
Locate the pressure gauge on the indoor hydronic unit. The reading should typically sit between 1.0 and 1.5 bar when the system is cold. If it reads below 1.0 bar, top up the system using the filling loop until the pressure is within range, then close the filling loop valve securely. Refer to your system documentation for the exact target pressure for your installation.
- Confirm all zone and isolation valves are fully open DIY safe
Walk around the heating circuit and check that every isolation valve, zone valve, and manifold port is in the open position. A partially closed valve can restrict flow and cause the sensor to behave unpredictably. Ensure all manual air bleed valves are also accessible and ready for the next step.
- Purge trapped air from the circuit DIY safe
Open the manual air bleed valves (automatic air vents if fitted) at the highest points of the heating circuit to release any trapped air. Work systematically from the highest point downwards. You may need to briefly run a circulation pump to move air towards the bleed points. Re-check system pressure after purging and top up again if needed, as releasing air often causes a small pressure drop.
- Clear the area around the outdoor unit DIY safe
Check that the outdoor unit's air inlet and outlet grilles are free from leaves, debris, snow, or any obstruction. While this step does not directly cause C0-01, it ensures the overall system is in good working order before performing a restart.
- Power cycle the heat pump DIY safe
Switch off the heat pump at both the indoor controller and the external isolator (or the relevant MCB in your consumer unit). Wait at least three minutes before switching back on in reverse order. Once restarted, monitor the display to see whether C0-01 reappears. If it clears and does not return, the issue was likely a transient pressure or air problem that has now been resolved.
- Check for evidence of system leaks DIY safe
If pressure keeps dropping despite repeated top-ups, there is likely a slow leak somewhere in the closed heating circuit — commonly at compression fittings, radiator valves, or underfloor heating manifold connections. Inspect accessible pipework and joints for damp patches, white mineral deposits, or corrosion staining. A persistent leak will cause C0-01 to recur and must be repaired by a qualified heating engineer.
- Arrange professional diagnosis of the flow switch and circuit heat pump engineer
If the fault persists after the DIY checks above, an MCS-certified heat pump engineer should carry out an electrical test of the flow switch against manufacturer specifications, inspect the wiring harness and connectors, and confirm whether the switch needs to be replaced. The engineer will also verify whether any secondary pumps on the circuit require an electrical interlock to prevent false flow readings. If the PCB is suspected, further diagnostics will determine whether it needs replacement. Any work on electrical components or the hydronic circuit beyond basic pressure checks should be left to a qualified professional.
Parts you may need
- Daikin flow switch / water flow sensor (e.g. EKFLSW1 or equivalent) · from £50
- Wiring harness for hydro unit flow sensor · from £35
- Hydro PCB (hydronic controller board) · from £280
- Automatic air vent (for high points in circuit) · from £12
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 £180–£450, depending on the underlying cause.
Frequently asked questions
Will my Daikin heat pump keep running with a C0-01 fault?
Yes — unlike some fault codes that trigger an immediate shutdown, C0-01 is classified as a warning, so your Daikin Altherma will usually continue operating when the code first appears. However, you should not ignore it. If the underlying cause is not resolved, the fault can escalate and cause the unit to stop entirely. Address the basic checks (pressure, air, power reset) promptly, and call an engineer if those do not clear it.
How much does it cost to fix a Daikin C0-01 fault in the UK?
For most homeowners, the repair involves either resolving a low-pressure or trapped-air issue (which you may be able to do yourself at no cost) or replacing the flow switch, which is a relatively low-cost part (around £20–£80 for the sensor itself). When you factor in an engineer's callout and labour, the typical total sits between roughly £180 and £450. Daikin's own UK callout rates run from around £239 (with an active maintenance plan) to £311 without one. In rarer cases where the fault is traced to a faulty PCB, costs can rise significantly — potentially towards £600 or more — but PCB replacement is not the common outcome for this code.
Do I need an F-Gas engineer for a C0-01 fault?
Not necessarily for the most common fixes. Checking system pressure, topping up the system, purging air, and power cycling the unit are all homeowner-safe tasks. If the flow switch or wiring harness needs replacing, an MCS-certified heat pump engineer can carry this out — F-Gas registration is only specifically required for work involving the refrigerant (sealed) circuit, such as the compressor or expansion valve, which are not typically involved in a C0-01 fault.
Why does my Daikin Altherma keep getting C0-01 after I top up the pressure?
If pressure keeps dropping and C0-01 keeps returning, there is almost certainly a slow leak somewhere in the closed heating circuit. Common culprits include corroded or loose compression fittings, radiator valve packings, or joints on an underfloor heating manifold. A one-off pressure loss is normal (air escaping during early use), but repeated drops mean water is escaping. You should arrange for an engineer to trace and repair the leak, as no amount of topping up will permanently solve the fault if water is leaving the system.