A Vaillant boiler that won’t work but shows no fault code has usually lost its power or gas, isn’t being asked for heat by the controls, or is showing a status code rather than a fault. The one to take seriously is comfort protection mode, where the boiler has already found a fault and is quietly working around it. On an older Vaillant that keeps going quiet on you, a new Vaillant boiler is usually the better long-term choice for efficiency and lower repair bills.
We’re Clever Energy Boilers: Gas Safe registered (no. 554138), accredited Worcester Bosch installers and based in Bingley. We’ve completed more than 25,000 installations in 15 years across Yorkshire, the North West and the Midlands. Below you’ll find the status codes we could confirm in Vaillant’s own manuals, the causes worth ruling out, the checks that are safe to do yourself and the point where an older boiler is better replaced.
Why is my Vaillant boiler not working with no fault code?
An F code means the boiler has found a fault and locked out. With no F code, it often thinks there’s nothing for it to do, or it’s limiting itself without telling you why. These are the usual reasons:
- No power, from a tripped breaker, RCD or fused spur
- Controls not calling for heat, whether that’s the timer, the thermostat, flat batteries or an eBUS controller
- Summer mode, or the heating switched off at the boiler itself
- A low pressure warning that hasn’t yet turned into a lockout
- No gas, or no credit on a prepayment meter
- A frozen condensate pipe
- Comfort protection mode hiding a real fault
- A failed PCB or display
Most of these take a couple of minutes to rule out. Our guide to a boiler not firing up covers the same ground for any make.
Vaillant status codes at a glance
These are the status codes we could confirm in Vaillant’s manuals:
| Status code | Meaning | Models | What it tells you |
|---|---|---|---|
| S.40 | Comfort protection mode | Newer ecoTEC plus | The boiler has detected a fault and is running with reduced comfort. An engineer needs to read the fault memory. |
| S.0 | No heat demand | Older ecoTEC plus | Nothing is asking the boiler for heat, so look at the timer, thermostat and controls. |
| Any other S code | Varies by model | All | Check the manual for your exact model before acting on a list you’ve found online. |
What’s the difference between a Vaillant status code and a fault code?
F codes, such as F.22 or F.28, are faults. S codes are status codes. They tell you what the boiler is doing right now and aren’t faults in themselves, with S.40 the big exception.
The catch is that the numbers change between model generations. A code on an older ecoTEC plus can mean something else on a newer one, which is why online lists so often disagree. Summer mode, frost protection and a low pressure warning all have their own status, but the number for each varies, so look it up in your own manual.
Seeing an egg timer rather than a code? Our guide to the egg timer on a Vaillant boiler covers it.
What stops a Vaillant working without showing a fault?
No power to the boiler
A blank display nearly always means no power. Check the consumer unit for a tripped breaker or RCD, then the fused spur beside the boiler. If a new fuse blows again, leave it alone and get an engineer.
Controls not calling for heat
The boiler only fires when something asks it to. Check the timer is in an on period, the thermostat is set above room temperature and a wireless stat has fresh batteries. If you’ve got a Vaillant eBUS controller, make sure it’s powered and set correctly. On older ecoTEC plus models, S.0 on the display confirms there’s no heat demand, which points straight at the controls. Our guide to a boiler thermostat not working goes further.
Summer mode or heating switched off
With the heating turned off at the boiler, or the boiler in summer mode, the radiators stay cold however high you turn the thermostat. You’ll usually still have hot water. If it’s the other way round, read our guide to a Vaillant boiler with no hot water.
Low pressure warning
A Vaillant can warn you about low pressure before it locks out. Let it fall further and you’ll get the Vaillant F22 fault code. Aim for 1 to 2 bar on the gauge with the boiler cold, and follow our guide on how to repressurise a boiler if it’s low.
No gas reaching the boiler
Try the hob. If nothing gas-powered works, or the prepayment meter has run out, the problem is the supply rather than the boiler.
Frozen condensate pipe
In a hard frost, the plastic condensate pipe outside can freeze and stop the boiler. Boilers on the higher ground around Haworth and Oxenhope tend to suffer first, as their outside pipe runs take the full force of the weather. Thaw it with warm water, never boiling, using our guide to defrosting a condensate pipe. A frozen pipe can also bring up the Vaillant F29 error code.
Comfort protection mode
This is the one that catches people out. On newer ecoTEC plus boilers, S.40 means comfort protection mode: the boiler has found a fault but keeps running with reduced comfort instead of locking out. Your heating or hot water may be weaker than usual, with no F code anywhere. The fault is stored in the boiler’s memory for a Gas Safe registered engineer to read.
PCB or display failure
If the power is on but the display stays blank, or the boiler ignores every control, the PCB or display may have failed. Our guide to boiler PCB faults explains what’s involved. A boiler that starts and then cuts out is a different problem, covered in our guide to a boiler that keeps turning off.
What can you safely check yourself?
None of these involve opening the boiler.
- Power. Check the breaker or RCD at the consumer unit and the fused spur by the boiler.
- Thermostat and timer. Set the stat above room temperature and check the timer is on. Swap the batteries if it’s wireless.
- Is the heating switched on at the boiler? Make sure it isn’t set to off or summer mode.
- Pressure. The gauge should read 1 to 2 bar when cold. Top up if it’s low.
- Gas. Check another gas appliance works and a prepayment meter has credit.
- Condensate pipe. In freezing weather, thaw any frozen section with warm water.
- Reset. Vaillant’s manual allows up to three resets. If the boiler still won’t run after the third, stop.
Is a Vaillant with no fault code dangerous?
Not usually. Most causes come down to power, controls or settings. Comfort protection mode needs looking at because there’s a genuine fault behind it, but it isn’t an emergency on its own.
If you smell gas, open the windows, don’t touch any switches, turn the gas off at the meter and call the National Gas Emergency Service on 0800 111 999.
When should you stop and call a Gas Safe engineer?
After the third reset. Get a Gas Safe registered engineer straight away if the display shows S.40, if it’s blank with the power confirmed on, or if a fuse keeps blowing. They’ll read the fault memory, check the wiring and controls and test the PCB. Never take the casing off yourself.
Is it time to replace your Vaillant boiler?
It depends what’s behind the problem. A flat thermostat battery isn’t a reason. Comfort protection mode is different, because the boiler has already logged a component failure and is working around it. On an older boiler, treat that as an early warning, and replace it on your own terms before it locks out on the coldest night of the year. Think about a new boiler if:
- It’s more than 8 years old, as boilers lose efficiency with age
- It’s in comfort protection mode, or has been recently
- You’ve been quoted for a new PCB
- More than one engineer visit has been needed in the past year
As a rough guide from a third-party source (247homerescue), here’s what the repairs behind a silent boiler tend to cost:
| Vaillant repair | Typical UK cost |
|---|---|
| PCB | £250 to £500 |
| Wiring or controls repair | Depends on the fault found |
Our boiler replacement service is fixed price, and a long, market-leading warranty is available through Clever Energy. If you’d rather spread the cost, see our boiler finance options. Our guides on when a boiler needs replacing and new boiler costs will help you decide.
Get a new Vaillant boiler with Clever Energy
We install Vaillant boilers in Bingley, Bradford, Leeds, Keighley, Huddersfield and Halifax, and across the North West and the Midlands, from Manchester and Liverpool down to Birmingham and Leicester. We take care of everything from removing the old boiler to setting up your controls. Take a look at our Vaillant boiler installation page, get a fixed price online in a few minutes, or request a callback. A Clever Care plan then keeps your new boiler protected.
Vaillant boiler not working with no fault code FAQs
Why is my Vaillant on but not heating?
Usually because nothing is calling for heat or the heating is switched off at the boiler. On older ecoTEC plus models, S.0 on the display confirms there’s no heat demand.
Is my Vaillant in summer mode?
Possibly, if you’ve got hot water but cold radiators whatever the thermostat says. The status number for summer mode differs between models, so check your manual.
What is comfort protection mode on a Vaillant?
It’s S.40 on newer ecoTEC plus boilers. The boiler has found a fault and keeps running with reduced comfort instead of locking out, and an engineer needs to read the fault memory.
Why is my Vaillant display blank?
Nearly always no power. If the breaker, RCD and fused spur are all fine, the PCB or display may have failed.
How do I reset a Vaillant boiler?
Press the reset button on the boiler. Vaillant’s manual allows up to three resets, and if it still won’t run after that, stop and get a Gas Safe registered engineer.
Showing an F code after all? Our Vaillant boiler error codes guide lists them, including F28 when the boiler won’t light, F62, F75 and F83. For any make that tries to light and gives up, see our guide to boiler ignition lockout.