A Vaillant fault code is the boiler telling you what's wrong before it shuts down to protect itself. On most Vaillant ecoTEC models the code shows on the digital display as an “F” followed by a number. Some boilers also flash a letter.
Here's the short version. Note the code down. Try one reset. If it comes straight back, stop resetting and get a Gas Safe engineer to look at it. Repeated resets on a locked-out boiler hide the real fault, and on gas faults they can make the problem worse.
This page lists the Vaillant fault codes we see most often, what each one usually points to, and where to get the full fix. Every code links to a dedicated guide. If your boiler needs a hands-on repair, we can help with boiler repair in London.
How Vaillant Fault Codes Work
Not every code on the screen is a fault. Vaillant boilers show three kinds of code, and it helps to know which is which before assuming the worst.
“F” codes are faults — the boiler has locked out and stopped for a reason. Status codes, usually a letter such as “S” or “C”, tell you what the boiler is doing right now, like running the heating or filling a hot water demand. They are not faults. Diagnostic codes starting with “d.” are for engineers reading live values off the board.
To reset a Vaillant boiler, hold the reset button for a few seconds until the boiler restarts. One reset is fine. If the same code returns, that's the boiler telling you the fault is still live — don't keep resetting it. Book an engineer.
A quick word on safety: anything involving gas, the burner, the flue or the sealed heat exchanger is Gas Safe work by law. The checks below are for understanding the fault, not for opening the boiler up. If in doubt, call someone.
Pressure and Water Flow Faults
This is the most common family of faults we're called out for. If the boiler can't see enough water, or can't move it around, it shuts down quickly to protect the heat exchanger.
F22 — Low Water Pressure
F22 is a dry-fire warning. The boiler detects there isn't enough water in the system and locks out to protect itself. Usually the pressure gauge is sitting below 1 bar. Repressurise using the filling loop, then look for the reason it dropped — if it keeps falling, there's a leak somewhere. Our full guide to the Vaillant F22 fault takes you through repressurising and leak checks step by step.
F24 — Temperature Rising Too Fast
F24 shows when the flow temperature climbs too quickly, which almost always means water isn't moving properly. Think trapped air, low pressure, a seized pump or a closed valve. Our guide to the Vaillant F24 fault code covers circulation, air and low pressure in order.
F73 — Water Pressure Sensor Signal
F73 means the water pressure sensor is sending a signal that doesn't add up, usually reading too low. Sometimes the pressure really has dropped; sometimes the sensor or its wiring has failed. Our guide to the Vaillant F73 fault code covers the water pressure sensor signal and how to tell the two apart.
F75 — Pump Runs But Pressure Doesn't Change
F75 is a common one. The pump starts but the pressure sensor sees no change, so the boiler assumes the pump isn't moving water. Common causes are air in the system, a worn pump, a stuck sensor or a failed expansion vessel. Our guide to the Vaillant F75 fault covers the pump, the sensor and the expansion vessel.
The Pressure Sensor Itself
Several of the faults above trace back to one component. The pressure sensor is what the boiler relies on to know how much water is in the system. When it drifts or fails, you can get F22, F73 or F75 even when the pressure is actually fine. Our guide to the Vaillant pressure sensor explains how it drives the pressure faults above.
Mass Flow Sensor Fault (F52 / F53)
The mass flow sensor sits near the fan and measures the air passing into the boiler for combustion, not water. When it plays up, the boiler can't maintain the correct air-to-gas mixture, which shows as an F52 or F53 code and can throw up related lockouts and hot water problems. Our guide to the Vaillant mass flow sensor fault is worth checking when the fault points to F52 or F53.
F61 – No Hot Water With No Fault Code At All
Sometimes the boiler runs normally, shows no error, and still gives you no hot water. That points to the diverter valve, the pump, an airlock or a slow pressure drop rather than a single lockout. Our full guide to a Vaillant boiler with no hot water and no fault code walks through the pressure, diverter valve and airlock checks in order.
Ignition, Flame and Gas Faults
This family is about getting a flame lit and keeping it lit. These are Gas Safe jobs — try one reset, and if the code returns, leave it to an engineer.
F28 — No Ignition On Start-Up
F28 means the boiler tried to light at start-up and failed. Gas supply, the gas valve, ignition leads and a blocked condensate pipe are all common causes. Our guide to the Vaillant F28 fault covers gas supply, ignition and condensate.
F29 — Flame Lights Then Goes Out
F29 is when the flame establishes and then drops out mid-cycle. Interrupted gas supply, flue recirculation, a frozen or blocked condensate pipe, or a poor earth can all cause it. Our guide to the Vaillant F29 error code covers the gas, flue and condensate checks.
F27 — Phantom Flame Detected
F27 is a flame simulation fault: the boiler detects a flame signal when the gas valve is shut, which shouldn't be possible. It usually means moisture on the board or a gas valve that isn't closing cleanly. This is a stop-and-call-an-engineer code. Our guide to the Vaillant F27 fault code explains why this one needs an engineer rather than a reset.
F54 — Gas Supply and Pressure
F54 points to a gas pressure or supply problem, and it often shows up alongside ignition faults like F28 and F29. Our guide to the Vaillant F54 fault code covers gas supply and pressure.
F61 — Gas Valve Fails to Open
F61 is a fault in how the board controls the gas valve during ignition — the valve doesn't open when it should, so wiring or the PCB is usually behind it. This is not a DIY fix. Our guide to the Vaillant F61 fault code covers the gas valve control.
F62 — Gas Valve Delayed Shutdown
F62 means the gas valve is closing too slowly, or not closing fully, after the burner should have switched off — the reverse problem to F61. The gas valve or the board is usually behind it. Our guide to the Vaillant F62 fault code covers the gas valve and PCB.
Temperature and Sensor Faults
These codes are about the boiler getting temperature readings it doesn't like, or a safety limiter tripping to stop it overheating.
F20 — Overheat Safety Switch-Off
F20 is the safety temperature limiter tripping because the boiler got too hot. Poor circulation, a failing pump, low water or a faulty sensor are the usual reasons. Our full guide to the Vaillant F20 fault code runs through the checks an engineer makes.
F23 — Flow and Return Difference Too Large
F23 shows when the gap between the flow and return temperatures is too wide, which points to poor circulation or a pump problem, and occasionally sensors that have been swapped over. Our guide to the Vaillant F23 fault code covers circulation and the flow/return sensors.
F25 — Flue Gas Temperature Too High
F25 is a safety shutdown on high flue gas temperature, and a loose flue-gas thermostat plug is a common trigger. It's closely related to F24 — F24 is the earlier warning, F25 is the more serious shutdown. Our guide to the Vaillant F25 fault code covers the flue gas temperature checks.
F72 — Flow or Return Sensor Fault
F72 means the flow and return temperature sensors are disagreeing in a way that doesn't make sense, so one of them is likely faulty. Our guide to the Vaillant F72 fault code covers the flow and return sensors.
F76 — Heat Exchanger Safety Cut-Out
F76 is the overheat protection on the main heat exchanger, often the safety fuse or cable on the exchanger itself. Our guide to the Vaillant F76 fault code covers the heat exchanger safety cut-out.
F83 — Temperature Barely Moves On Start
F83 shows when the flow and return temperature hardly changes as the burner fires, which usually means low water, trapped air or a stuck pump. Our guide to the Vaillant F83 fault covers the water, air and pump checks.
Fan and Airflow Faults
F32 — Fan Fault
F32 means the fan isn't running at the speed the boiler expects, or the air-pressure signal is off. A blocked flue, a failing fan, a Hall sensor issue or a loose plug are the usual suspects. Our guide to the Vaillant F32 fault covers the fan, the flue and the wiring.
Board, Configuration and Status Codes
The last group covers faults tied to the boiler as a whole, plus one code that isn't a fault at all.
F70 — Invalid Device Number
F70 nearly always shows up after a board or display has been changed and the device-specific number wasn't set correctly. It's a configuration fault, not a mechanical one, and it needs an engineer with the correct data for your exact model to programme it — not a homeowner reset. Read our fix for the Vaillant F70 fault code before anyone touches the boiler again.
Heat Generator Fault
A heat generator fault is the boiler flagging a problem with its core heat-producing unit rather than a single component. It's a broad one, and the right next step depends on what else the boiler is showing. Our guide to a Vaillant heat generator fault explains what the boiler is flagging and what happens next.
Boiler Flashing “C”
A flashing “C” is a status code, not a fault — it tells you the boiler is in a particular operating state, most commonly Comfort Mode. It's worth checking what your specific model's flashing C means before assuming anything is broken, since the exact meaning can vary. Our guide to a Vaillant boiler flashing C explains what the status is telling you.
Code Not Listed Here?
Vaillant uses a lot of codes and this hub covers the ones we see most. If yours isn't above, start with our overview of common Vaillant boiler faults and work from there.
The Same Few Causes Sit Behind Most Codes
Once you've looked at enough Vaillant boilers, the codes start to repeat a pattern. A handful of underlying problems account for most of the faults on this page, so it's worth knowing them.
Low water pressure and trapped air are the big two. Between them they feed F22, F24, F75 and F83, because the boiler can't heat water it can't move. Keep the pressure between 1 and 1.5 bar cold, and bleed your radiators if the heating is patchy.
Limescale is the other major one, and it matters more in hard-water areas. Hard water can coat the heat exchanger and narrow the waterways over time, which drives overheat and circulation faults like F20 and, on some units, F28, along with slow, inconsistent hot water. In hard-water areas, a scale reducer and an annual service are worth having.
Then there's the boiler simply getting older. Ageing pumps, worn expansion vessels, drifting sensors and moisture reaching the board are behind a lot of the intermittent, comes-and-goes faults. A code that clears and returns a week later usually means a component nearing the end of its life rather than a one-off glitch.
When to Stop and Call an Engineer
One rule covers most of these codes. If a single reset clears it and it stays clear, that's the fault gone. If the code comes back, or it's a gas, flame, fan or board fault, that's your cue to stop.
Gas work is Gas Safe work by law, and a boiler that keeps locking out is trying to tell you something. Continuing to run it risks the heat exchanger, and on gas faults it risks a lot more than that.
If your Vaillant is throwing any of the codes above and won't clear, we can send a Gas Safe engineer out. Book a Vaillant boiler repair in London and we'll diagnose the fault properly rather than guess at it.
Next Steps
Note your fault code. Try one reset. If it returns, don't keep going.
Then get it looked at. Our engineers handle Vaillant faults across London, so book a boiler repair in London and we'll get your heating and hot water back on.
