Viessmann 52 fault codewhat it means and how to fix it
Flow sensor fault on a 100-W combi: 52 is a short circuit, 5A a lead break. The burner is blocked, because the boiler cannot tell whether a tap is running. Engineer job.
Take care
Get a Gas Safe registered engineer to look at your Viessmann 52
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What 52 means
52 and 5A belong to the Vitodens 100-W combi's flow sensor — the turbine in the cold-water inlet that tells the control unit a tap has been opened and how much water is passing. The rows read "Burner blocked — Short circuit, flow sensor" and "Burner blocked — Lead break, flow sensor", measure "Check connections and lead; replace sensor if required", on the WB1B, WB1C, B1HA/B1KA and B1HC/B1KC and the 050-W BPJD. The system characteristic is stricter than for the other DHW sensors: with 50/58 or 51/59 only hot water stops, but a combi that cannot see its flow sensor blocks the burner altogether, since a spurious flow signal could run the plate heat exchanger dry and a missing one could leave a tap running cold while the burner heats radiators.
The code is absent from the WB1A (which reports a flow problem as 0b), from the 111-W B1LA to B1LD (a storage combi meters the cylinder, not the tap), and from the Vitotronic 200-W tables. There is no householder step and no reset; the burner is released when the sensor reads plausibly. The 2020-on numeric table has no row for this sensor; its F.980 and F.981 are a different check — the internal circulation flow before and during cylinder heating — and have their own page.
What causes it
| Cause | What is actually happening | How likely |
|---|---|---|
| Flow sensor lead or plug | Viessmann put "check connections and lead" before the sensor. The plug sits in the wet lower part of the boiler. | Common |
| Flow sensor failed | The turbine's Hall sensor has gone short (52) or open (5A). Replaced with the cold supply isolated. | Common |
| Debris in the turbine | Grit from the mains jamming the impeller does not itself produce 52 or 5A, but a sensor that has been forced by debris often fails electrically soon after. | Occasional |
Can you fix it yourself?
Stop — this needs an engineer
What it costs to fix
Typical repair cost
£60–£140
Time on site
1 hour
A flow sensor within a visit, with the cold supply isolated.
Typical UK prices including parts and labour, updated September 2026. Your quote may differ.
Parts you may need
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Viessmann DHW flow sensor (combi)
£30–£80Authorised Viessmann parts distributor
Price checked 28 September 2026
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Related Viessmann fault codes
- 30Get it checkedBoiler water temperature sensor fault: 30 is a short circuit, 38 a lead break. The burner is blocked until the sensor reads plausibly again. Engineer replaces the sensor.Related: temperature sensors
- 50Usually simple to fixCylinder temperature sensor fault — 50 short circuit, 58 lead break. Heating continues; hot water from the cylinder stops until the sensor or its lead is replaced. Not a safety fault.Related: temperature sensors
- 10Usually simple to fixOutside temperature sensor fault — 10 short circuit, 18 lead break. The boiler keeps heating, in constant-temperature mode rather than weather-compensated. Not urgent; an engineer replaces the sensor.Related: temperature sensors
- 51Usually simple to fixDHW outlet temperature sensor fault on a combi: 51 short circuit, 59 lead break. Heating continues but hot water stops, because the boiler cannot see the tap temperature it is controlling.Related: temperature sensors
- F4Stop — call an engineerNo flame signal was detected during burner start on any two-character Vitodens, so the burner has locked out. Reset once; if it returns, call a Gas Safe engineer.Related: same brand
- F.74Get it checkedSystem pressure too low on a 2020-on Vitodens with the numeric display. The burner and pump stop until the pressure is back. Viessmann tell the householder to top up below 1.0 bar.Related: same brand
Own a Viessmann boiler? Save it and its codes come first next time.
Related guides
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.