Viessmann 94 Fault Code: Causes, Fixes & Repair Costs
What does the Viessmann 94 fault code mean?
Fault code 94 on Viessmann boilers signals that the solar control module has detected a short circuit on the domestic hot water (DHW) cylinder temperature sensor (referred to in Viessmann documentation as sensor %). When this sensor's electrical resistance collapses, the control module can no longer trust the temperature readings coming from the storage cylinder, so it suspends solar DHW heating as a precaution. Importantly, this is not necessarily a full boiler lockout — the main burner and central heating may continue to operate normally, but your solar thermal contribution to hot water will be inactive until the fault is cleared. Code 94 sits within a family of solar-related sensor fault codes: codes 90 and 91 relate to short circuits on other sensors at the solar control module, code 92 covers a short on the solar collector sensor, code 93 is a collector sensor short in control mode, and code 94 specifically points to the DHW tank sensor. If you see 94 on your display, the solar side of your system needs attention from a qualified engineer.
General guidance only — not a substitute for professional advice, and not certified by a Gas Safe engineer. Always have your specific appliance assessed by a qualified professional. Any gas work must be carried out by a Gas Safe registered engineer. If you smell gas or suspect carbon monoxide, leave the property and call the National Gas Emergency line on 0800 111 999.
Common causes
- Short circuit in the DHW cylinder temperature sensor Common
The most frequent culprit. The tank sensor's internal resistance has dropped to near zero, causing the solar control module to receive a false or invalid signal. This can happen through sensor ageing, moisture ingress, or physical damage to the sensor element itself.
- Damaged or corroded sensor wiring Common
The cable run between the cylinder temperature sensor and the solar control module can develop a short if the insulation is chafed, kinked, pinched behind pipework, or has suffered corrosion at the terminals. Even a small breach in the cable can produce the same fault symptom as a failed sensor.
- Loose or poor plug-in connections at the solar control module Sometimes
Vibration over time or an incomplete reconnection after previous servicing can cause the sensor plug to seat poorly at the solar control module, producing intermittent or permanent short-circuit readings.
- Faulty solar control module (Vitosolic or integrated) Rare
If the sensor and all wiring check out correctly, the solar control module itself may have developed an internal fault causing it to misread the sensor circuit as a short. This is a less common but possible outcome, particularly in older installations.
How to fix it
- Check that your boiler's main gas supply and mains power are both on DIY safe
Before anything else, confirm that the gas isolation valve near the boiler is open and that the boiler has mains power. These are basic checks that rule out unrelated supply issues.
- Note whether central heating is still working DIY safe
Fault code 94 targets solar DHW heating specifically. If your radiators are still warming up, the main boiler burner is likely unaffected. Make a note of what is and is not working — this helps the engineer diagnose quickly.
- Attempt a single boiler reset DIY safe
Press and hold the reset button on your Viessmann boiler as described in your user guide (typically 3–5 seconds). Do this no more than once or twice. If the fault clears and solar DHW resumes, monitor the boiler closely. If code 94 returns, do not keep resetting — a recurring fault needs professional investigation.
- Inspect the cylinder sensor and its wiring visually (engineer task) Gas Safe engineer
A Gas Safe registered engineer will locate the DHW cylinder temperature sensor (sensor %) and examine it for visible damage, corrosion, moisture ingress, or signs of heat stress. They will also trace the cable back to the solar control module looking for chafed or pinched insulation.
- Test plug-in connections at the solar control module (engineer task) Gas Safe engineer
The engineer will check that all connectors at the solar control module or Vitosolic unit are fully seated, clean, and free from oxidation. Loose connections are an easy fix but must be done carefully to avoid damage to the module.
- Electrically test the sensor and wiring (engineer task) Gas Safe engineer
Using a multimeter, the engineer will measure the resistance of the tank temperature sensor and continuity of the wiring loom. A shorted sensor will typically read close to 0 ohms rather than the expected resistance value for its temperature range.
- Replace the DHW cylinder temperature sensor if faulty (engineer task) Gas Safe engineer
If testing confirms the sensor has failed, the engineer will fit a compatible replacement sensor and recheck the solar control module's readings before resetting the fault.
- Replace the solar control module if sensor and wiring are both sound (engineer task) Gas Safe engineer
Should the sensor and cabling prove to be in good condition, the fault lies within the solar control module itself. The engineer will source and fit the correct replacement module for your Viessmann installation.
- Reset and verify the system (engineer task) Gas Safe engineer
After the repair, the engineer will perform a controlled reset of the boiler control unit, confirm that fault code 94 has cleared, and verify that solar DHW heating has resumed correctly. They should also check that no related fault codes (90–93) are present.
Parts you may need
- DHW cylinder temperature sensor (compatible with Viessmann solar systems) · from £45
- Sensor cable / wiring loom for solar control · from £30
- Vitosolic solar control module · from £220
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 £120–£380, depending on the underlying cause.
Frequently asked questions
Will my boiler still heat my home if fault code 94 appears?
In most cases, yes. Fault code 94 suspends solar DHW heating — meaning the solar thermal panel contribution to your hot water cylinder is disabled — but the main gas burner can usually continue providing central heating and conventional (non-solar) hot water. Check whether your radiators and taps are still working normally and report this to the engineer when you call.
Can I fix Viessmann fault code 94 myself?
Resetting the boiler once or twice is the only DIY action worth trying. All other work — testing the sensor, checking wiring, replacing components on the solar control module — must be carried out by a Gas Safe registered engineer. Solar thermal systems are specialist equipment, and incorrect work could damage the control module or create an unsafe situation.
How much does it cost to repair fault code 94 in the UK?
Most homeowners pay between £120 and £380 all in, depending on what has failed. A wiring fault or loose connection is at the cheaper end; replacing the DHW tank sensor typically costs £100–£200 including labour. If the Vitosolic or solar control module itself needs replacing, costs can rise to £350–£500 or more, since the module itself can cost upwards of £200 before labour. Rates are generally higher in London and the South East.
Why does fault code 94 keep coming back after I reset the boiler?
A recurring code 94 means the underlying fault — most likely a failing sensor, degraded wiring, or a deteriorating solar control module — has not been resolved. Repeated resets will not fix a hardware fault and may mask the problem temporarily. If the code returns more than once or twice, stop resetting and book a Gas Safe engineer to carry out a proper diagnosis before the fault worsens.
Is fault code 94 covered under my Viessmann warranty?
It may be, depending on the age of your boiler and whether you have kept up with annual servicing by a Viessmann-accredited engineer. Viessmann's extended warranty terms typically require regular professional servicing — skipping a service can invalidate your cover and leave you paying the full repair cost. Check your warranty documentation or contact Viessmann UK directly to confirm your entitlement before booking a repair.