The fan speed is too low
The C1 error code on Bosch-Junkers boilers indicates the fan speed is too low.
Bosch-Junkers C1 means: the fan speed is too low. On this component the documented failure modes are fan blocked, worn or its speed signal wrong or the fan has failed — short circuit or open circuit. You can safely reset the boiler once — if C1 returns, do not keep resetting. A qualified heating engineer should diagnose the root cause.
What Does C1 Mean
The C1 code is shown by the boiler’s control board when it detects the fan speed is too low. The boiler then stops instead of continuing to run in that condition: the code is the appliance reporting a problem, not proof that the boiler is broken.
| What you see on the boiler |
|---|
| The display shows C1 and the boiler stops until the fault is cleared |
Common Causes
The manufacturer does not publish a cause list for this specific code. The causes below are the failure modes documented for the same component on other boilers, confirmed by the service literature of 7 different manufacturers.
| Cause |
|---|
| Fan blocked, worn or its speed signal wrong |
| The fan has failed — short circuit or open circuit |
Safety Warning
C1 involves electrical or pressure-related components. Do not attempt repairs without proper qualifications. Ensure the boiler is electrically isolated before any inspection. If you notice water leaks, unusual sounds, or pressure anomalies, contact a qualified heating engineer immediately.
Parts & Prices
Prices shown are indicative. Actual prices and availability vary by region.
How to Fix It
Check the fan, its connector and the wiring, replace them if necessary
Check the fan turns freely, then check its connector, wiring and speed signal; replace the fan if the bearings are worn or the signal is absent.
Measure the resistance of the fan with a multimeter and compare it against the manufacturer’s resistance-versus-temperature table; replace it if it is open circuit, short circuit or out of range.
Refer to the Bosch-Junkers service documentation for the detailed procedure. Only a qualified engineer should carry out these repairs.
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Affected Models
Bosch-Junkers models that can display fault code C1:
| Model Family | Output Range | Frequency |
|---|---|---|
| All Bosch-Junkers models | — | Possible |
Frequently Asked Questions
Check the fan, its connector and the wiring, replace them if necessary. According to the technical documentation, this is the first check on the list for C1.
C1 is not classified as a high-severity fault, so the boiler can be reset once. If C1 comes back, stop resetting: the underlying cause is still there and needs a qualified heating engineer.