Daikin C0 fault codewhat it means and how to fix it
Flow sensor malfunction: the sensor sees water moving 45 seconds after Daikin’s pump has stopped. Low pressure, air and an external pump running are on the list before the sensor itself.
Take care
What C0 means
C0 looks like a broken sensor and often is not. Daikin’s table gives C0-00 as flow sensor malfunction and C0-01 and C0-02 as flow switch malfunction, and the service manual explains the trigger for the first: the water flow sensor detects water flow 45 seconds after the water pump has stopped. The unit stops and the reset is manual. Think about what that trigger means. The Altherma has switched its own pump off and expects the water to stand still; instead, the sensor says water is still moving.
Daikin’s checklist begins on the water side — check the water pressure, check the water flow, purge the water circuit — and then names the cause that catches installers out: check the water circuit for an external pump, because the detected flow may be caused by a pump that is not the Altherma’s. Underfloor heating manifolds, secondary hot water circulation and buffer arrangements all have pumps of their own, and one that keeps running when the heat pump has stopped will push water through the flow sensor and produce C0. Air is the other non-fault cause: a pocket of air moving through the sensor after the pump stops reads as flow, which is why purging is on the list and why the code follows recent work on the system. Only after those does Daikin reach the sensor’s electrical check and the hydro PCB.
For C0-01 and C0-02 the trigger is the equivalent on a flow switch — flow detected while the pump is not running — and C0-01 resets automatically. The owner’s contribution is context: what other pumps exist on the system, what work has been done recently, and whether the code coincides with underfloor or hot water circulation running.
What causes it
| Cause | What is actually happening | How likely |
|---|---|---|
| An external pump moving water through the sensor | Daikin’s fourth check and the one most often missed. An underfloor manifold pump or secondary circulation pump running after the Altherma stops produces flow the sensor faithfully reports. | Common |
| Air in the water circuit | Daikin’s third check. Air moving through the sensor after the pump stops reads as flow, and it follows any recent work on radiators or the cylinder. | Common |
| Water pressure or flow too low | Daikin’s first two checks. A circuit short of pressure behaves oddly around the sensor, and the checks cost nothing. | Occasional |
| Flow sensor or hydro PCB faulty | The last two items on Daikin’s list, reached after the water side and any external pump have been ruled out. | Occasional |
What to try yourself
Read the system pressure gauge and note it. Water pressure is Daikin’s first check for this code.
List the other pumps on your system — underfloor manifold, hot water circulation, buffer or boiler pumps — and notice whether any runs when the heat pump has stopped. That is Daikin’s named cause and it is the one only you can observe.
Think about recent work. Air after a drain-down or radiator job is Daikin’s third check.
Book your installer with the code and what you found. The sensor’s electrical check and the hydro PCB are theirs, and an external pump that needs interlocking with the heat pump is a wiring job.
Do not purge the primary circuit yourself unless your installer has shown you how on your system. It may contain glycol, and the indoor unit carries mains.
Get an F-Gas certified heat pump engineer to look at your Daikin C0
We do not pass enquiries to a repair firm, so there is no form here. When you book an F-Gas certified heat pump engineer, tell them the make, the model and the exact code or message on the display, and ask whether they carry parts for this make — that is what turns one visit into a repair rather than a quote.
What it costs to fix
Typical repair cost
£90–£400
Time on site
15 minutes
An installer visit is typically £90 to £180 and a purge fits inside it. Interlocking an external pump is a modest wiring job; a replacement flow sensor or hydro PCB is where the range widens.
Typical UK prices including parts and labour, updated September 2026. Your quote may differ.
Own this appliance and had this code? . Nothing is published until a person has read it.
Related Daikin fault codes
- 7HGet it checkedA water flow problem. The sub-code matters more than the code: 7H-01 clears itself, 7H-04 to 7H-06 need a manual reset, 7H-08 is the pump, and 7H-07 is the unit freeing a stuck pump.Related: water flow
- A5Get it checkedHigh pressure peak cut in heating, freeze protection in cooling. On an Altherma, Daikin’s first four checks are all water-side: pressure, flow, stop valves and air. Not the indoor filter of an air conditioner.Related: water flow
- Bill has jumpedGet it checkedThe most important heat pump fault has no code. If your electricity cost has stepped up with no change in habits, the backup heater is probably doing the work.Related: same brand
- Ice on the outdoor unitGet it checkedNormal in cold damp weather, and the unit clears it itself. Daikin specifically forbid helping — no hot water, no de-icer, no scraping.Related: same brand
- Water at the tap is too coldGet it checkedDaikin give two causes and a remedy that costs money. Before using it, check what your schedule is actually asking the cylinder to do.Related: same brand
- U0Get it checkedShortage of refrigerant in the Altherma outdoor unit. Daikin’s own checklist ends with a leak test, the circuit is sealed, and on an R32 unit this is F-Gas certified work by law.Related: same brand
Own a Daikin heat pump? Save it and its codes come first next time.
Related guides
Common questions about Daikin C0
Is the sensor broken?
Possibly, but Daikin put it fifth on a six-item list. Before the sensor come water pressure, water flow, air, and an external pump moving water through it after the Altherma has stopped. A sensor that reports flow when there genuinely is flow is not faulty.
What is the difference between C0-00 and C0-01?
The device. C0-00 is the flow sensor, which measures how much water is moving; C0-01 and C0-02 are a flow switch, which only knows whether water is moving. Both trigger when flow is detected with the pump off, and Daikin publish C0-01 as resetting automatically.
Is this the same as 7H?
The opposite. 7H is too little flow when the pump is running. C0 is flow detected when the pump is stopped. Both involve the same sensor and the same water circuit, which is why air and pressure appear on both lists.
Technical review
Not reviewed by an engineer
No engineer has read this page. Severity follows what the manufacturer publishes, and we would rather tell you that than let a byline imply a sign-off.
Sources and how we verified this
We establish the facts from the sources below and then write the explanation ourselves. We do not copy manufacturer wording and we do not host their manuals.
- Manufacturer technical and service documentation
This page has not been reviewed by an engineer, and we would rather tell you that than imply otherwise. The severity rating follows what the manufacturer publishes: where they say isolate the appliance and call someone, so do we.