Viessmann Boiler Fault Codes: F2, F4, F9 and More

Updated July 2026 · Emergency Plumber Chelsea

Start with safety. If you smell gas, hear hissing near the boiler or gas pipework, or your carbon monoxide alarm is sounding, do not reset anything. Turn the boiler off if you can do so safely, open windows, leave the property and call the National Gas Emergency Service on 0800 111 999. A fault code on its own, with none of those signs, is a repair to arrange rather than an emergency, and our guide to carbon monoxide warning signs explains what to look out for.

Before you look anything up, write down the exact code and your model. The badge on the front or the data plate under the casing will say Vitodens 050-W, 100-W, 111-W, 200-W or similar. This matters because Viessmann uses the same code letters in slightly different ways across its controls, and its own UK fault code checker warns that the diagnosis can differ between boilers. Note also whether the code appeared with no heating, no hot water, or both, and whether the pressure gauge looks normal. An engineer can diagnose far faster with that information.

F2 is the one most Chelsea households see. It means the fixed high-limit safety device has operated because the water inside the boiler got too hot. The boiler has not failed; it has protected itself. The usual cause is that water is not moving through the heat exchanger fast enough, which in turn usually comes down to low system pressure, air in the system, a sticking or failing pump, closed valves, or sludge restricting flow. Check the pressure gauge first. On most sealed systems it should sit around 1 to 1.5 bar when the heating is cold, and our guide to what pressure your boiler should be covers the readings. If it is low, top it up with the filling loop as the manual describes, then try one reset.

If F2 keeps returning after the pressure is correct, the problem is circulation or a component. The pump may be seized or weak, a zone valve may be stuck shut so there is nowhere for the water to go, or the system may be heavily sludged, which is common in older Chelsea and Fulham properties where a new condensing boiler has been fitted to decades-old pipework. If the pressure keeps dropping and you keep having to top it up, there is a leak somewhere, and our guide to a boiler losing pressure explains how to track it down. Repeated overheating is not something to keep resetting through.

F4 means the boiler went through its ignition sequence and could not confirm a flame. It is the Viessmann equivalent of an ignition lockout. Check the simple things first: that other gas appliances such as the hob are working, that your prepayment meter has credit if you have one, and that the gas isolation valve under the boiler is open. In winter, check the condensate pipe too. A frozen or blocked condensate pipe can stop a condensing boiler lighting, and our guide to a frozen condensate pipe shows how to thaw it safely. If the gas supply is fine and one reset does not clear it, the likely causes are the ignition electrode, the flame sensing electrode, the ignition lead or the gas valve, which all need a Gas Safe registered engineer. Our guide to boiler ignition lockout explains the start-up sequence in more detail.

F9, and the related FA on some models, points to the fan. The boiler checks that the fan reaches the right speed before it lights, so that flue gases are cleared properly. If the fan is slow, noisy or not reaching speed, the boiler refuses to fire. You may hear the fan whine, rattle or run up and down before the code appears. Fan faults need the casing removed and the combustion side of the boiler opened, so this is always an engineer job. Viessmann also groups some fan codes, such as F6 and 58, with burner faults where the fan is not stopping when it should.

F1 means the flue gas temperature has gone above its safe maximum. Like F2, it is often linked to poor circulation or system conditions rather than a failed part, but it can also point to a dirty heat exchanger or a sensor problem. F3 means the boiler detected a flame signal before the ignition sequence began, which suggests a flame sensing or gas valve fault. F8 relates to the gas valve not closing correctly. All three involve the gas and combustion side of the boiler, so switch off, do not keep resetting, and book an engineer.

Some Viessmann codes are numbers rather than F codes. Code 10 and code 18 relate to the outdoor temperature sensor, which weather-compensated Vitodens systems use to adjust flow temperature. Viessmann describes 10 as a short circuit in that sensor and 18 as a broken sensor cable. The boiler usually keeps running in a fixed mode, so you are not left without heat, but it runs less efficiently until the sensor or wiring is repaired. Codes in the 30s and 20s generally relate to boiler water or flow temperature sensors, and codes such as 0C relate to the electrical supply or flame signal. Treat any code you cannot find in your manual as a reason to call rather than guess.

Resetting a Viessmann is simple, but do it once, not repeatedly. Most Vitodens models have a reset button marked with a flame symbol or the word reset on the control panel. Press it once after fixing anything obvious, such as low pressure. If the code clears and stays clear, keep an eye on it. If it returns, the boiler is telling you something is still wrong, and repeated resets can hide a developing fault or, on the gas side, make it worse. Our guide on how to reset a boiler covers when a reset is appropriate.

In larger Chelsea townhouses, mansion flats and refurbished mews houses, a Viessmann is often one part of a bigger system: paired with an unvented cylinder or a weather compensation sensor, and sometimes installed in a basement or cupboard where the flue and condensate pipe run a long way. That means a fault code can have a cause outside the boiler itself, such as a long condensate run, a pump on a separate zone, or a cylinder control. A good engineer checks the whole system rather than just swapping the part the code points to.

If your Viessmann is showing a code you cannot clear, Emergency Plumber Chelsea sends Gas Safe registered engineers across Chelsea, Knightsbridge, Sloane Square, South Kensington and Fulham. We diagnose the fault, explain what we have found and your options before any work starts, and there is no call-out fee on jobs over 3 hours. See our boiler repair and boiler service pages, or call 020 3002 6667 with the code and model. If you have a different make, our guides to Worcester, Baxi and Ideal fault codes cover those brands.

Quick tips

• Write down the exact code and model (Vitodens 050-W, 100-W, 111-W, 200-W) before you call. Code meanings vary between models.
• For F2, check the pressure gauge first. Low pressure is a common and cheap cause.
• For F4, check the gas is on at the hob and the meter, and in cold weather check the condensate pipe for ice.
• Press reset once. If the code comes back, stop and book an engineer rather than resetting again.
• Gas smell, hissing or a CO alarm with any code means switch off, ventilate, leave and call 0800 111 999.

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