Viessmann 93 Fault Code: Causes, Fixes & Repair Costs
What does the Viessmann 93 fault code mean?
Fault code 93 tells you that the boiler's burner control unit has triggered a protective lockout because it detected the fan was still spinning at a point when it should have come to a complete stop — or because it cannot get a reliable signal from the fan due to a wiring or connector problem. The boiler enters a controlled error mode to prevent unsafe operation. Code 93 sits within the 90–9F family of burner-control faults that apply across all Viessmann boiler ranges, including the popular Vitodens series. In plain terms: the boiler expects the fan to be idle (at standstill) at a specific point in the startup or shutdown sequence, but is either seeing residual rotation or losing contact with the fan entirely, so it refuses to attempt a normal burner cycle until the root cause is investigated.
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
- Faulty or worn fan motor Common
The fan motor is the most frequent culprit. If the motor bearings are worn or the motor itself is starting to fail, the fan may not decelerate to a standstill quickly enough after shutdown, or it may run at an incorrect RPM during startup. The burner control unit monitors fan speed and will lock out if the readings don't match expected values.
- Loose, damaged, or corroded wiring connector to the fan Common
The electrical plug-in connector between the fan and the PCB or burner control board can work loose over time due to vibration, or the terminals can corrode. A poor connection causes intermittent or incorrect speed signals, which the control unit interprets as a fan-standstill error. Engineers should inspect the fan cable and all associated connectors as a first step.
- PCB or burner control board fault Sometimes
If the control board itself is misreading the tachometer signal from the fan — or if the board's fan-control circuitry has developed a fault — it may falsely report that the fan isn't at standstill even when it is. This is less common than a physical fan or wiring issue but should be considered if the fan and connections check out.
- Blocked or restricted flue Sometimes
A partially blocked flue terminal (caused by debris, a bird nest, or ice in cold weather) can create back-pressure that affects how the fan spins and how quickly it responds to control signals. While this is a less direct cause of code 93, flue restrictions can contribute to abnormal fan behaviour and are worth ruling out early.
How to fix it
- Check the flue terminal outside for obvious obstructions DIY safe
Go outside and look at the flue outlet (usually on an external wall or roof). Clear away any debris, leaves, or ice you can safely reach. A blocked terminal can affect airflow and fan behaviour. Do not attempt to access the flue from inside the boiler.
- Attempt a single boiler reset DIY safe
On most Vitodens models, press and hold the reset (R) button on the front panel for approximately two seconds until the boiler attempts to restart. On touchscreen variants, follow the on-screen reset prompt. If the boiler fires up and runs normally, monitor it over the next hour or two. If fault code 93 returns straight away, do not keep resetting — repeated lockouts can stress ignition components and won't resolve an underlying fault.
- Check system pressure is within the normal range DIY safe
While not a direct cause of code 93, confirm the pressure gauge reads between 1.0 and 1.5 bar when the system is cold. If it's below 0.8 bar, top up via the filling loop following your boiler manual. This rules out any compounding issues before an engineer arrives.
- Engineer to inspect the fan, wiring, and connectors Gas Safe engineer
A Gas Safe registered engineer should access the boiler casing and inspect the fan motor for signs of wear, unusual resistance, or physical damage. All plug-in connectors and the fan cable harness should be checked for looseness, corrosion, or broken terminals, and reseated or replaced as needed.
- Engineer to test fan motor speed and RPM output Gas Safe engineer
Using diagnostic equipment, the engineer will verify that the fan accelerates to the correct RPM during startup and decelerates fully to standstill at shutdown. If the readings are outside Viessmann's specified tolerances, the fan motor should be replaced rather than repaired (repair costs are typically comparable to replacement on these units).
- Engineer to inspect and test the PCB Gas Safe engineer
If the fan and wiring are confirmed to be in good condition but the fault persists, the engineer should test the burner control board or PCB for faults in the fan-speed sensing circuit. A faulty board will need replacing — this is a job strictly for a qualified professional.
- Engineer to carry out a full combustion analysis after repair Gas Safe engineer
Once any component has been replaced or adjusted, Viessmann requires a full combustion analysis (not just a visual check) to verify safe and efficient operation. This is especially important on Vitodens models with the Inox-Radial stainless steel heat exchanger, and is a condition of maintaining any extended warranty. The engineer should also check ionisation readings and the burner gasket.
- Call a Gas Safe registered engineer if the fault persists or returns Gas Safe engineer
If the reset does not clear the fault, or if code 93 keeps returning after a short period of normal operation, stop resetting the boiler and arrange a visit from a Gas Safe registered engineer. Continuing to run a boiler in repeated lockout can mask a worsening fault.
Parts you may need
- Viessmann boiler fan motor / fan assembly · from £120
- Fan wiring harness / plug-in connector set · from £25
- PCB (printed circuit board) / burner control board · 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 £180–£380, depending on the underlying cause.
Frequently asked questions
Can I fix Viessmann fault code 93 myself?
Beyond a single reset and a visual check of the external flue terminal, fault code 93 requires a Gas Safe registered engineer. The fault involves the fan motor, its electrical connections, and potentially the PCB — all internal components that must not be touched by an unqualified person. Attempting DIY repairs on these parts is dangerous and will void any remaining Viessmann warranty.
How much does it cost to fix a Viessmann boiler showing fault code 93?
Most homeowners pay between £180 and £380 to resolve code 93. A fan replacement (parts and labour) typically falls in the £200–£300 range, while a wiring or connector fix at a callout visit might come in closer to £100–£150 all in. PCB replacement is the more expensive scenario and can add £300–£500 on top of the callout charge — if your engineer suspects the PCB is at fault, it's worth getting a written quote before proceeding, especially on older boilers.
Why does code 93 appear alongside codes 90, 91, 92, 94, and others on Viessmann boilers?
Viessmann groups fault codes 90 through 9F (and AB) together as burner-control or emergency-operation codes, all of which cause the boiler to enter a controlled lockout or safe mode. Code 93 specifically points to the fan not reaching standstill or a connection error on the fan circuit. Other codes in the same family relate to different burner-control conditions — for example, ignition failures or other sensor faults. If your display shows a code in this range, the specific number tells the engineer exactly which subsystem triggered the shutdown.
Will fault code 93 damage my Viessmann boiler if I keep resetting it?
Repeated resets without fixing the underlying fault won't resolve the problem and can put additional stress on ignition components over time. Limit yourself to one or two resets at most. If the fault returns, leave the boiler in lockout and call a Gas Safe engineer — running a boiler in repeated lockout is not safe and could worsen the fault.
Does fault code 93 affect my Viessmann warranty?
Viessmann's extended warranties (up to 10 or 12 years on some Vitodens models) require annual servicing by a Gas Safe registered engineer and a full combustion analysis at each visit. If you've missed services, a warranty claim related to the fan or PCB may be at risk. Always check your warranty documentation and use a Viessmann-registered installer if possible to protect your cover.