Viessmann 98 Fault Code: Causes, Fixes & Repair Costs
What does the Viessmann 98 fault code mean?
Fault code 98 on a Viessmann boiler (most commonly the Vitodens 200-W) indicates that the control system has lost a valid signal from the hot water cylinder (tank) temperature sensor, or has detected a break in the sensor's wiring. The boiler enters a fallback 'control mode', meaning the burner can continue to operate, but the system can no longer accurately monitor or regulate the cylinder temperature. Because the boiler doesn't lock out entirely, this fault is sometimes overlooked — but it should still be investigated promptly to ensure correct hot water regulation, particularly on systems fitted with a Viessmann solar thermal control module (such as the Vitosolic). Note: if you have a Viessmann model other than the Vitodens 200-W, fault code 98 may carry a slightly different meaning — check your specific model's documentation or consult a Viessmann-accredited 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
- Failed tank temperature sensor Common
The cylinder temperature sensor itself has developed an open circuit or otherwise stopped producing a readable signal. This is the most frequent cause — sensors degrade over time, particularly in systems where the hot water cylinder runs at higher temperatures or has seen years of thermal cycling.
- Damaged or disconnected sensor cable Common
The wiring between the cylinder sensor and the solar control module can become loose at a plug-in connection, corroded, or chafed against pipework or the cabinet interior. Even a partial break in the cable is enough to trigger the fault.
- Faulty plug-in connection at the solar control module Sometimes
The connector joining the sensor cable to the solar control module can work loose or develop a poor contact over time. A short circuit at this connection can also produce the same fault code as a broken sensor.
- Defective solar control module Rare
If the sensor and all associated wiring test as electrically sound, the solar control module itself may have failed internally and is misreporting the sensor circuit. This is the least common cause but does occur in older or heavily used systems.
How to fix it
- Check the boiler's gas supply and other basic utilities DIY safe
Before assuming a component fault, confirm the gas supply to the property is on and that other gas appliances are working normally. This rules out a wider supply issue, though for fault code 98 this is unlikely to be the root cause.
- Inspect visible wiring and plug connections (visual check only) DIY safe
Without opening any sealed boiler panels, look at any accessible wiring running to the hot water cylinder sensor and the solar control module. Check for obviously loose plugs, visible cable damage, or signs of corrosion. Do not attempt to probe, disconnect, or repair any wiring — this is for visual assessment only.
- Reset the boiler DIY safe
Press and hold the reset button (refer to your boiler's user manual for the exact method on your Vitodens model). If the fault clears and does not return within a short period, monitor the system closely. If fault code 98 reappears, do not continue resetting — a reset will not fix an underlying sensor or wiring problem and repeated resets can mask a developing fault.
- Arrange a diagnostic visit from a Gas Safe registered or Viessmann-accredited engineer Gas Safe engineer
A qualified engineer will use electrical test equipment to measure the sensor's resistance and confirm whether it has failed, inspect the full wiring loom and all plug connections at the solar control module, and identify any short circuits. Viessmann-accredited engineers have access to dedicated diagnostic tools designed for Viessmann's control architecture, which can speed up diagnosis considerably compared to a general boiler engineer.
- Replace the faulty tank temperature sensor or cable Gas Safe engineer
If testing confirms the sensor or its cable has failed, the engineer will replace the defective component with a compatible part. This is the most common repair outcome for fault code 98 and typically resolves the fault completely.
- Replace the solar control module if sensor and wiring are found to be intact Gas Safe engineer
Should the sensor and all cabling test as fully functional, the engineer may conclude the solar control module itself is at fault. Replacement of this module is a more involved and costly repair, and the engineer should confirm the diagnosis clearly before proceeding.
Parts you may need
- Cylinder (tank) temperature sensor — Vitodens compatible · from £45
- Sensor cable / wiring harness · from £30
- Vitosolic / solar control module · from £180
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 £100–£300, depending on the underlying cause.
Frequently asked questions
Will my boiler still heat my home and hot water with fault code 98 showing?
In most cases, yes — fault code 98 places the boiler in a fallback control mode rather than locking out the burner entirely, so heating and hot water may continue to function. However, the system can no longer accurately regulate the cylinder temperature, which means your hot water could be under-heated or, in some scenarios, overheated. You should still arrange an engineer visit promptly rather than running the boiler indefinitely in this state.
Is fault code 98 on a Viessmann boiler a Gas Safe job, or can I fix it myself?
The initial visual check of accessible wiring and a single boiler reset are safe for a homeowner to carry out. Beyond that, diagnosing and replacing a temperature sensor or control module requires electrical testing and work on the boiler's internal components — this must be done by a Gas Safe registered engineer. Viessmann also strongly recommends using a Viessmann-accredited installer, as they carry specialist diagnostic equipment for Viessmann's control systems, which reduces guesswork and can lower your overall repair bill.
How much does it cost to fix Viessmann fault code 98 in the UK?
For the most common repair — replacing the cylinder temperature sensor — most homeowners pay in the region of £100 to £180, covering parts and labour. If the wiring loom needs attention the cost is similar. If the solar control module itself needs replacing, expect to pay £220 to £350 or more, depending on the specific module and the engineer's call-out rate. These figures assume a single visit; if the fault proves difficult to trace, additional diagnostic time will add to the bill.
My Viessmann boiler isn't a Vitodens 200-W — does fault code 98 mean the same thing?
Not necessarily. Fault code 98 is specifically documented as a tank temperature sensor or cable fault on the Vitodens 200-W. On other Viessmann models the same code number can refer to a different fault entirely. If you have a different model, check your specific boiler's user or service documentation, or call a Viessmann-accredited engineer who can confirm the correct interpretation for your system.