Smell Gas?
If you think there is a gas leak or a carbon monoxide emergency, call the National Gas Emergency Service on 0800 111 999. Meanwhile, open the windows, put out any naked flames and leave the property for your own safety.
What a Vaillant fault code means
Vaillant boilers watch their own sensors constantly. When a reading stops making sense, the boiler stops the burner and puts a code on the display rather than carrying on. That is the boiler protecting itself and the house, not a sign it has broken beyond repair.
Codes that start with F are faults, and the boiler stays locked out until the cause is dealt with. Newer ecoTEC and ecoFIT displays write them with a full stop in the middle, so F.20 on the screen is the same fault as F20 in this list. Vaillant writes the two lowest codes as F.0 and F.1, with a single digit; we list them as F00 and F01 because that is how people search for them. Anything shown as an S or status code is not a fault at all: it is the boiler telling you which part of a normal cycle it is in.
A code narrows the problem down, it does not name the broken part. F23, F24 and F73 all point at water moving badly, but the culprit could be a pump, a stuck valve, sludge or a leak. That is why the per-code guides below matter more than the one-line meaning.
How to read the code on the display
The code sits on the boiler's digital display, usually as F and two digits. Write it down before you touch anything, because the display clears the moment the boiler restarts, and the code is the single most useful thing you can give an engineer over the phone.
Two things are worth noting alongside it. First, what the boiler was doing when it locked out: heating only, hot water only, or both. Second, the reading on the pressure gauge, the round dial or bar on the front. Those two facts plus the code often tell an engineer which part to bring.
Your model's own user guide names the reset button and confirms the code wording for that boiler. Vaillant publishes the guides for current models on vaillant.co.uk. Where a code is worded differently between an ecoTEC plus and an ecoFIT pure, we say so on that code's own page.
What to do first, safely
There are only four checks a homeowner should make. Everything past this point is gas work, and gas work is legally an engineer's job.
- Check the pressure gauge. With the heating off and the system cold, most Vaillant boilers want roughly 1 to 1.5 bar. If the needle is on or near zero, that alone explains several codes. Our guide on a boiler that keeps losing pressure covers what to do and what it usually means.
- Check the controls are actually asking for heat. A flat battery in a wireless thermostat, a programmer stuck on an old schedule, or a room stat turned down will stop a healthy boiler firing and can look like a fault.
- Check your other gas appliances. If the hob and the fire are also weak or dead, the problem is the gas supply to the house, not the boiler. If you have a prepayment meter, check it has credit.
- Reset once, using the button in your manual. One reset is fine on a sensor, pressure or electronics code. Do not reset a code about gas, combustion, the flue or overheating: that covers F20, F25, F26, F27, F28, F29, F32, F54, F56, F61, F62, F67, F68, F76 and F77. Leave the boiler switched off and book a Gas Safe registered engineer instead. On any other code, if the same code comes straight back, stop. Repeated resetting on a locked-out boiler fixes nothing and can make the fault harder to trace.
Never remove the boiler's front casing, touch the gas valve, adjust the gas pressure or work on any sealed part. If you smell gas at any point, call 0800 111 999 first and read the rest of this later.
Let us fix your Vaillant.
At Smart Plan we work with a national network of thousands of Gas Safe registered engineers. We aim to get an engineer out to you within two working days, with cover-plan call-outs booked Monday to Friday, 08:00 to 18:00. A £95 call-out fee applies.
Vaillant fault codes, with the full fix
One line each on what the code means. Where we have written a full guide, the last column links to it, and that page carries the real causes, the safe checks and what an engineer will do on the day. Go straight to the code you are seeing rather than reading the whole table.
This is not the whole of Vaillant's fault code list. If your code is not in the table, Vaillant's own published fault code list is the authority on what it means, and any code you cannot find still needs a competent person to look at it.
| Code | What it means | Full guide |
|---|---|---|
| F00 | Flow temperature sensor interruption. The board has lost the signal from the sensor on the flow pipe. | See below |
| F01 | Return temperature sensor interruption. Vaillant writes this as F.1 and most people search it as F1. | F01 explained |
| F10 | Flow temperature sensor short circuit. | F10 explained |
| F11 | Return temperature sensor short circuit. | See below |
| F20 | Overheat safety switch-off. The boiler got too hot and cut the burner. | F20 explained |
| F22 | Low water pressure, or too little water reaching the heat exchanger. | F22 explained |
| F23 | Temperature gap between the flow and return pipes is too wide, usually poor circulation. | F23 explained |
| F24 | Flow temperature climbing too fast, again pointing at circulation or the pump. | F24 explained |
| F25 | Flue gas temperature too high. | F25 explained |
| F26 | Gas valve stepper motor fault, so the valve cannot be driven properly. | F26 explained |
| F27 | A flame is detected when the gas valve should be shut. | F27 explained |
| F28 | The boiler tried to light, no flame was detected during the ignition phase, and it locked out. | F28 explained |
| F29 | The flame went out while the boiler was running and did not come back. | F29 explained |
| F32 | Fan fault, so waste gases cannot be cleared safely. | F32 explained |
| F49 | eBUS fault, the wiring that lets the controls and the boiler talk to each other. | F49 explained |
| F54 | Gas supply pressure fault: no gas, or not enough of it. | F54 explained |
| F56 | Combustion safety lockout tied to the carbon monoxide limit. | F56 explained |
| F61 | Gas valve actuation fault: the board cannot control the valve. | F61 explained |
| F62 | Delay switching the gas valve off after the flame goes out. | F62 explained |
| F63 | The circuit board reports a fault with itself. | See below |
| F64 | Electronics or sensor fault. The exact wording differs between models. | F64 explained |
| F65 | The circuit board is running too hot to work safely. | F65 explained |
| F67 | Flame signal the board cannot trust, so it locks out. | F67 explained |
| F68 | Unstable flame signal, so the burner is shut down. | F68 explained |
| F70 | No valid Device Specific Number on the board, usually after a parts swap. | F70 explained |
| F71 | Flow sensor stuck at a fixed reading instead of tracking the water. | F71 explained |
| F72 | Invalid gap between the flow and return sensor readings. | F72 explained |
| F73 | Water pressure sensor signal outside its valid range, usually the sensor itself or its wiring rather than the water pressure. | F73 explained |
| F74 | Water pressure sensor fault. What it means depends on the model. | F74 explained |
| F75 | No pressure rise when the pump starts, so the sensor or the pump is at fault. | F75 explained |
| F76 | Heat exchanger overheat trip. | F76 explained |
| F77 | No response from the condensate pump or the flue flap. | F77 explained |
| F83 | Too little temperature change registered when the burner lights. | F83 explained |
| F84 | The flow and return sensors disagree with each other. | F84 explained |
| F85 | The flow and return sensors are fitted on the same pipe, or on the wrong one. | F85 explained |
Codes we have not written a full guide for yet
Three codes in the list above have no page of their own yet, so here is the longer version.
F00 - flow temperature sensor interruption
The control board has lost the signal from the temperature sensor on the flow pipe, the pipe carrying hot water out of the boiler. The usual causes are a sensor that has worked loose from its clip, a plug that is not seated on the board, or a broken wire in the harness. It is the same sensor that F10 reports as a short circuit, just failing the opposite way. An engineer tests the sensor's resistance against Vaillant's figures for your model before deciding whether it is the sensor or the wiring.
F11 - return temperature sensor short circuit
The mirror image of F00, on the return pipe instead. The board is reading a resistance far lower than the sensor should ever give, which usually means a chafed or trapped wire touching metal, water in a plug, or a sensor that has failed internally. Do not try to strip it back yourself: the wiring runs inside the sealed casing. The fix is a resistance test at the board, then either a new sensor or a repaired harness.
F63 - circuit board fault
The board has found a problem with itself rather than with anything it is measuring. Because the board drives the pump, the fan and the gas valve, nothing else can be trusted while F63 is showing, and there is no homeowner-side fix. Try one reset in case the fault was a one-off. If it returns, an engineer replaces or recodes the board. On an older boiler it is worth asking whether a new board is economic before you agree to it: our guide on repairing versus replacing a boiler sets out how to weigh that up.
When it is definitely an engineer job
Any code touching gas, combustion, the flue or overheating is a job for an engineer, and gas work is an engineer's job by law rather than by preference. That covers F20, F25, F26, F27, F28, F29, F32, F54, F56, F61, F62, F67, F68, F76 and F77. Only a Gas Safe registered engineer may work on those parts, and you can check anyone's registration on the Gas Safe Register before you let them start.
Electronics codes such as F63, F64, F65 and F70 also need an engineer, because the board sits behind the sealed casing. So does anything that keeps coming back after one reset, whatever the code. A lockout that returns within minutes is telling you the underlying fault is still there.
The exceptions are narrow. Low pressure showing as F22 can sometimes be corrected by topping up through the filling loop if your manual shows you how, and a control fault such as F49 occasionally turns out to be a flat thermostat battery. If topping up works for a day and the pressure drops again, that is a leak, and a leak is an engineer's job too.
If the boiler is dead and the house is cold, our page on no heating covers what to check while you wait, and boiler repair explains how a call-out works.
Cover for Vaillant boiler repairs
A Vaillant repair is rarely just a part. Diagnosis, labour and a second visit for something that was not on the van all add up, and a board or a gas valve sits at the expensive end. A Smart Plan boiler and central heating cover plan puts parts and labour behind that, up to £500 in a 12-month period if the boiler is under seven years old, or £200 if it is older. It is a service plan from UK Boiler Company Ltd, trading since 2014, not insurance, and you pick only the cover you want. Once you use the plan a 12-month agreement period applies.
Two things to know up front. A £95 call-out fee applies, paid in advance. It applies during your first 30 days (cover still applies in that window), and after that for issues your plan does not cover, for a missed appointment where the engineer cannot get access, and for an annual service requested early. Cover-plan call-outs are booked Monday to Friday, 08:00 to 18:00, and we aim to get an engineer out to you within two working days.
Still working out what your code means? Start with it in the table above, or browse the rest of our Vaillant help pages.
