Viessmann 20 Fault Code: Causes, Fixes & Repair Costs
What does the Viessmann 20 fault code mean?
Fault code 20 on a Viessmann boiler is a fan and burner control fault that causes the boiler to enter a lockout or emergency operating mode. Exactly what triggers it depends on your specific model. On most Viessmann boilers, the burner shuts down because the fan has not returned to a standstill after an auto-reset attempt, or because a plug-in wiring connection to the fan or burner control unit is unstable. On Vitodens 200-W, 222-F, and models running Vitotronic 200, 300, or 050 controllers, the boiler may instead switch to a fixed 'constant mode' output because the outdoor temperature sensor has developed a short-circuit. On the Vitodens 100-W specifically, the fault is often linked to DIP switch S2 being set incorrectly. If your display shows code 20, identify your boiler model first — the underlying problem and the repair route can differ significantly between series.
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
- Fan motor fault or incorrect fan speed Common
The flue fan is responsible for drawing combustion air through the burner. If the fan motor fails, runs at the wrong speed, or does not return to standstill after a shutdown, the boiler's burner control detects the discrepancy and locks out. This is the most frequently reported cause of code 20 across Viessmann models.
- Loose or damaged wiring connection to the fan Common
The connector or wiring loom between the fan motor and the PCB can work loose over time, particularly in boilers subject to vibration. An intermittent or open-circuit connection produces the same signal to the burner controller as a failed fan, triggering the fault.
- Outdoor temperature sensor short-circuit Sometimes
On Vitodens 200-W, 222-F, and models with Vitotronic weather-compensating controllers, a short-circuit in the outdoor sensor causes the boiler to fall back to a fixed constant-mode output and log code 20. The sensor is mounted outside and can be damaged by moisture ingress or physical impact.
- DIP switch misconfiguration (Vitodens 100-W) Sometimes
On the Vitodens 100-W, internal DIP switch S2 controls certain burner operating parameters. If this switch is left in or moved to the ON position when it should be OFF, the burner enters fault mode and code 20 is displayed. This is typically a commissioning or post-service error rather than a component failure.
- PCB or burner control communication fault Rare
Internal communication errors between the main PCB and the burner control module can mimic a fan fault. On Vitotronic 300-K combined with Vitotronic 200 systems, a break in the communication link between the two control units is a specific documented cause of code 20.
- Gas valve or modulation valve fault Rare
A defective gas valve or modulation valve can prevent normal burner operation and, in some cases, contribute to code 20 being logged alongside other faults. Engineers investigating a persistent code 20 should also check valve operation if the fan and wiring appear sound.
How to fix it
- Check your boiler model and note the full fault display DIY safe
Before doing anything else, identify your exact Viessmann model (printed on the data plate inside the boiler front panel) and note whether the display shows anything alongside the '20'. The appropriate fix differs between Vitodens 100-W, 200-W, and Vitotronic-controlled models, so this step prevents unnecessary work.
- Attempt a single boiler reset DIY safe
On most Vitodens models, press and hold the reset button (labelled R) on the front panel for approximately two seconds until the boiler attempts to restart. On touchscreen-equipped models, follow the on-screen reset prompt. If the boiler fires up and runs normally, monitor it over the next few hours. Viessmann allows up to three resets within a 24-hour window before a safety lockout engages, but repeatedly resetting without addressing the root cause accelerates wear on the igniter and PCB — if the fault returns after one or two resets, stop and call an engineer.
- Call a Gas Safe registered engineer to inspect the fan assembly and wiring Gas Safe engineer
An engineer will check the fan motor for correct operation and speed, inspect the fan cable and plug-in connectors for looseness or damage, and test electrical continuity throughout the loom. On Viessmann-specific systems, specialist diagnostic equipment allows precise measurement of fan RPM signals rather than relying on guesswork or parts-swapping.
- Have the engineer test the outdoor temperature sensor (where applicable) Gas Safe engineer
On Vitodens 200-W, 222-F, and Vitotronic weather-compensating systems, the engineer should measure the resistance of the outdoor sensor and check for a short-circuit. A faulty sensor is a straightforward replacement once confirmed and is significantly cheaper than a fan or PCB repair.
- Have the engineer verify DIP switch settings (Vitodens 100-W) Gas Safe engineer
On the Vitodens 100-W, the engineer should confirm that DIP switch S2 is configured correctly for the installation. An incorrectly set switch is a quick, low-cost fix if caught early.
- Have the engineer test the PCB, burner control module, and gas valve Gas Safe engineer
If the fan, wiring, sensor, and DIP switches are all found to be in order, the engineer will move on to testing the PCB and burner control unit for internal communication errors, and the gas valve and modulation valve for correct operation. Replacing a PCB or gas valve is a Gas Safe-only task and should only be done once other causes are ruled out to avoid unnecessary expense.
Parts you may need
- Flue fan motor assembly · from £180
- Fan wiring harness / connector · from £35
- Outdoor temperature sensor · from £45
- PCB / burner control board · from £320
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
Can I fix Viessmann fault code 20 myself?
The one thing you can safely do yourself is a single boiler reset — press and hold the reset button for two seconds and see if the boiler runs normally. Beyond that, code 20 involves the fan motor, wiring, control board, or gas components, all of which must be inspected and repaired by a Gas Safe registered engineer. Attempting to access or adjust these parts yourself is unsafe and will invalidate your warranty.
Why does fault code 20 mean different things on different Viessmann boilers?
Viessmann uses a shared fault-code numbering system across its range, but the boiler's control unit interprets the code according to the components it is monitoring. On a Vitodens 100-W the code points to a fan lockout or DIP switch issue; on a Vitodens 200-W or a model with a Vitotronic weather-compensating controller it more often indicates an outdoor sensor short-circuit causing the boiler to run in constant mode. Always identify your exact model before diagnosing — your boiler's data plate is inside the front panel.
How much does it cost to repair Viessmann fault code 20 in the UK?
Most repairs for code 20 fall in the range of £120 to £380, covering the engineer's callout and diagnostic fee (typically £60–£100) plus parts and labour. A wiring connector or outdoor sensor replacement sits at the lower end; a fan motor swap is mid-range at roughly £150–£300 all-in. A PCB replacement is the costliest common repair, often £300–£600 including labour. Costs rise by roughly 30–50% for out-of-hours or weekend callouts.
Will my Viessmann warranty cover fault code 20?
Viessmann's standard guarantee on models such as the Vitodens 100-W and 050-W lasts five years, extendable to ten years when installed and serviced by a Viessmann-approved engineer. If your boiler is within the guarantee period and has been annually serviced as required, the repair may be covered. Skipping a service can void your cover. Contact Viessmann customer services on 01952 675000 to check your entitlement before booking an independent engineer.