Viessmann 1F Fault Code: Causes, Fixes & Repair Costs
What does the Viessmann 1F fault code mean?
The F1 fault code on Viessmann boilers — sometimes displayed as '1F' depending on the control panel orientation or model variant — signals that the boiler's flue gas temperature has climbed beyond its safe operating limit. A dedicated sensor in the flue circuit detects this excessive heat and triggers an automatic lockout, shutting the boiler down before any internal damage can occur. Crucially, the root cause is almost never a problem with the flue itself. Instead, it typically points to a circulation issue: when water cannot flow through the heat exchanger at the correct rate, heat that should be carried away into the heating system instead accumulates inside the boiler and vents through the flue at abnormally high temperatures. Viessmann's own diagnostic guidance directs engineers to check three things first: system fill level (pressure), pump operation, and whether air is trapped in the system.
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
- Low system pressure Common
This is the single most common trigger for the F1 fault. Viessmann systems should sit between 1.0 and 1.5 bar when hot and around 0.5–1.0 bar when cold. Below those thresholds there simply is not enough water in the circuit to absorb heat efficiently, so temperatures build up rapidly inside the heat exchanger and spike at the flue sensor. A dropping pressure gauge reading alongside the F1 code is a strong indicator of this cause.
- Faulty or seized circulation pump Common
The pump is responsible for moving heated water continuously around the system. When it fails, sticks, or runs at insufficient speed, hot water stagnates inside the boiler body, causing localised overheating that the flue gas sensor picks up. On Vitodens 100-W and 111-W models, engineers can also check whether the pump speed has been left on a lower setting and increase it to speed III as a first measure.
- Airlocks in the heating circuit Common
Air pockets accumulate over time — especially after radiator bleeding or recent system work — and block the smooth circulation of water through the heat exchanger. The result is the same as a failing pump: heat builds up and the flue gas temperature climbs. Airlocks can be subtle and are easily missed without a systematic bleed of the full circuit.
- Faulty flue gas temperature sensor (NTC thermistor) Sometimes
If system pressure, pump operation, and circulation all check out, the sensor itself may be sending inaccurate signals to the PCB. The NTC thermistor's electrical resistance changes with temperature; as it ages or degrades, it can report temperatures far higher than reality and trigger a false lockout. An engineer will test the sensor's resistance with a multimeter and compare it against Viessmann's specified range.
- Scaled-up plate heat exchanger (combi boilers) Sometimes
On combi models, the secondary plate heat exchanger that produces domestic hot water can fur up with limescale in hard-water areas. Heavy scaling restricts water flow through the primary circuit whenever a tap is opened, pushing the boiler into an overheating condition that registers as an F1 fault. Viessmann recommends descaling every 3–5 years in hard-water regions.
- Jammed diverter valve Sometimes
A motorised diverter valve that jams in an intermediate position forces the boiler to attempt simultaneous heating of the central heating circuit and domestic hot water. The resulting flow imbalance causes the heat exchanger to overheat and can trigger the F1 lockout repeatedly.
- Incorrectly positioned flue gas sensor Rare
Viessmann's own fault data lists sensor positioning as a discrete cause. If the sensor has been dislodged — for example during a service or a flue modification — it may sit outside the correct measurement point and produce misleading readings. This is relatively uncommon but worth checking if the fault appeared shortly after boiler work was carried out.
How to fix it
- Let the boiler cool down before doing anything else DIY safe
An F1 lockout means the boiler has detected overheating. Give the system at least 10–15 minutes to cool before attempting any checks or a reset. Acting on a hot system can produce misleading pressure readings and increases personal risk.
- Check the system pressure gauge DIY safe
Look at the pressure gauge on the boiler front panel. It should read between 1.0 and 1.5 bar. If it reads below 1.0 bar, low pressure is very likely the cause of your F1 fault. Locate the filling loop (a braided or plastic connector usually beneath the boiler), open both valves slowly, and watch the gauge rise. Stop at 1.2–1.3 bar and close both valves firmly. If pressure regularly drops, there may be a leak somewhere in the system that a Gas Safe engineer needs to investigate.
- Reset the boiler — but no more than twice DIY safe
Once the system has cooled and pressure is correct, press and hold the reset button (labelled R or shown as a flame icon on most Viessmann panels) for approximately three seconds until the display changes. The boiler will attempt to restart. Watch the pressure gauge and listen for the pump running. If the F1 code reappears within a short time, do not keep resetting — repeated resets on a boiler with an underlying fault can cause heat exchanger damage. Stop after two attempts and arrange for an engineer.
- Check that your gas supply is working DIY safe
Confirm that other gas appliances in your home (hob, gas fire) are functioning normally. If nothing gas-powered is working, contact your gas supplier rather than a boiler engineer. If other appliances work fine, the gas supply to the boiler is not the issue.
- Bleed any radiators that feel cool at the top DIY safe
If some radiators have cool patches at the top, trapped air may be contributing to poor circulation. Use a radiator key to open each bleed valve briefly until water (not air) emerges, then close it. Recheck boiler pressure afterwards and top up if it has dropped below 1.0 bar.
- Have a Gas Safe engineer inspect the circulation pump Gas Safe engineer
If the F1 fault returns after correct pressure and bleeding, the circulation pump is the next most likely cause. A Gas Safe engineer will check whether the pump is running, test its speed settings, and determine whether it needs servicing or full replacement. On Vitodens 100-W and 111-W models they can also verify the pump speed setting and increase it if required.
- Have a Gas Safe engineer test the flue gas temperature sensor Gas Safe engineer
The engineer will use a multimeter to measure the NTC thermistor's resistance and compare the reading against Viessmann's published values for that sensor at ambient temperature. A sensor reading outside the acceptable range will be replaced. They will also confirm the sensor is correctly positioned in the flue assembly.
- Have a Gas Safe engineer inspect the plate heat exchanger and diverter valve Gas Safe engineer
On combi models, the engineer will assess the plate heat exchanger for scaling and either chemically clean it or replace it if too far gone. They will also operate the diverter valve through its full range to check for sticking or partial blockage, replacing the cartridge or motor head if necessary.
- Ask the engineer to review the boiler's fault history Gas Safe engineer
Viessmann boilers store the last 10 fault events in their memory. Pressing the info (i) button and scrolling through the log gives the engineer a clear picture of how frequently the F1 fault has occurred and whether any other codes have been recorded alongside it — valuable context for pinpointing the root cause quickly.
- Call a Gas Safe registered engineer if the fault persists or returns Gas Safe engineer
If the F1 code keeps coming back despite correct pressure and bleeding, or if you have any doubt about the cause, contact a Gas Safe registered engineer — ideally one with Viessmann training. Do not continue resetting a boiler that locks out repeatedly, as this risks damaging the heat exchanger or other internal components.
Parts you may need
- Flue gas NTC temperature sensor · from £55
- Circulation pump (Grundfos or Wilo equivalent) · from £95
- Diverter valve cartridge / motor head · from £65
- Plate heat exchanger (combi models) · from £120
- Pump speed controller / PCB relay · from £80
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–£350, depending on the underlying cause.
Frequently asked questions
Is the Viessmann F1 code the same as 1F — are they the same fault?
Yes, they refer to exactly the same fault. Viessmann officially labels it F1, but depending on how the control panel displays characters, some homeowners read it as 1F. Both indicate the flue gas overtemperature lockout described on this page. If you see either on your Viessmann display, follow the same diagnostic steps.
Can I fix the Viessmann F1 fault myself?
The most common cause — low system pressure — is something you can safely address yourself using the filling loop to repressurise to 1.2–1.3 bar. You can also bleed radiators and attempt a reset (no more than twice). Beyond those checks, the repair involves internal boiler components such as the pump, sensor, heat exchanger, or diverter valve, all of which must be worked on by a Gas Safe registered engineer. Attempting those repairs yourself is unsafe, illegal without Gas Safe registration, and will invalidate your Viessmann warranty.
Why does my Viessmann boiler keep showing F1 after I reset it?
If the F1 fault returns quickly after each reset, the underlying cause has not been resolved. Repeatedly resetting without fixing the root problem risks damaging the heat exchanger — an expensive component that can cost £500 or more to replace. Limit yourself to two resets, and if the fault returns, stop and book a Gas Safe engineer. Ask them to check the fault history log (accessible via the info button) to see how often the lockout has been triggered.
How much does it cost to fix a Viessmann F1 fault?
If low pressure is the cause, you can fix it yourself for free by topping up via the filling loop. If an engineer is needed, a pump replacement typically costs £180–£280 including parts and labour, sensor replacement around £100–£150 all in, a diverter valve repair £180–£260, and a plate heat exchanger clean or replacement £220–£350. Most F1 repairs therefore fall in the £100–£350 range. In rare cases where the PCB is involved or the heat exchanger is severely damaged, costs can exceed £500 — your engineer will advise once they have diagnosed the fault.