The following articles discusses boiler faults and codes. If you are in any doubt, please call a qualified gas safe heating engineer to look and inspect your boiler.
Identifying Your Boiler Fault Codes
When your boiler stops working, the first sign of trouble is often a flashing fault code on the display. These codes are designed to guide engineers or homeowners toward the root of the problem. But if you’re not familiar with what they mean, they can feel more like a source of stress than a useful diagnostic tool.
The good news is that many boiler faults are straightforward. Some issues — like topping up the water pressure or resetting the system — can be safely handled by homeowners. But others may point to deeper problems within your central heating system, including invisible leaks that cause pressure to drop and lead to recurring faults.
In this guide, we’ll explain what boiler fault codes are, outline the most common examples, and show you what they could be telling you – especially if standard fixes aren’t solving the issue.
What are boiler fault codes?
Boiler fault codes (also known as error codes) are messages programmed by the manufacturer to indicate problems inside the system. These can range from minor faults, such as a frozen condensate pipe or low water pressure, to more complex issues like sensor failures or loss of circulation. According to Viessmann UK, nearly one-fifth of UK residents have experienced a broken boiler in the last 12 months.
Each code corresponds to a specific condition. Although different brands use different coding formats, many of the underlying problems are universal. For example, a code that signals low water pressure on one boiler may appear as “E119”, “F22”, or “E1” on others – but the underlying issue remains the same.
Understanding these codes allows homeowners to respond appropriately – and helps avoid unnecessary boiler resets or emergency callouts for issues that might be simple to fix.
What are the common boiler fault codes and what do they mean?
Here are some of the most frequently encountered boiler fault codes in UK homes. These are typically seen across models by Worcester Bosch, Vaillant, Baxi, Ideal, and others:
| Fault Code | Likely Cause | Leak-Related? | Recommended Action |
| E119 | Low water pressure | Yes | Repressurise using filling loop. If pressure drops again, a leak may be present. |
| F75 | Faulty pressure sensor or pump | Yes | May suggest water loss or circulation problems. Call a professional if persistent. |
| E133 | Ignition failure or no gas | No | Check gas supply and reset. If issue continues, contact a Gas Safe engineer. |
| F22 | Dry fire (boiler operating with no water) | Yes | Indicates water loss — potentially from a leak. Do not keep resetting. |
| E1 | General system water error | Possible | Pressure and water flow should be checked. Could indicate a slow leak. |
| L2 | Flame detection fault | No | May relate to frozen condensate or blocked flue. Likely not leak-related. |
| E118 | Pressure too low (Ideal Boilers) | Yes | Check for repeat pressure loss — a key sign of hidden leakage. |
| F28/F29 | Ignition failure or flame loss | No | Could be sensor or flue related. Not typically linked to leaks. |
What should I do when a fault code appears?
When a fault code displays on your boiler, follow these steps:
- Consult the manual: This will help you identify what the code refers to and whether it’s homeowner resolvable.
- Check the pressure: If it’s below 1.0 bar (when cold), this usually indicates low water pressure.
- Try a safe reset: Many minor issues can be resolved by resetting the boiler as per the manual.
- Top up the system: Use the filling loop to increase pressure, but only if you’re confident doing so.
- Monitor closely: If the same code returns or pressure drops again, don’t just keep resetting – this may hide a more serious issue.
At this point, a qualified gas-safe heating engineer should be your first call. They can check the system for blockages, failed valves, or airlocks. But what happens when everything appears intact – yet the problem persists?
What Should I Do When Fault Codes Suggest a Hidden Leak?
Some fault codes hint at more than just wear and tear. Codes that relate to low pressure, dry firing, or faulty pressure sensors are particularly important, as they may indicate an undetected leak within the sealed central heating system.
Here are the warning signs:
- You top up the boiler pressure, but it drops again within days
- You see fault codes return repeatedly, especially ones like E119, F22, or E118
- There are no visible signs of a leak on radiators or pipes
- Your engineer has checked the boiler but can’t find the cause
If a qualified gas safe heating engineer has ruled out common causes and the issue continues, it’s likely that the leak is hidden under flooring, behind plaster, or within concrete — places not easily accessible with standard checks.
How do you find a leak that’s not visible?
This is where specialist leak detection becomes essential. Using traditional trial-and-error methods like lifting floors or cutting into walls are both disruptive and expensive. Modern leak detection uses non-invasive technology to identify even the smallest leaks without damaging your home.
These methods include:
- Acoustic Detection: Uses ultra-sensitive microphones to listen for leak sounds through concrete, floors, and walls
- Tracer Gas: A harmless gas is introduced into the system and detected wherever it escapes — ideal for pinpointing tiny leaks
- Thermal Imaging: Highlights changes in surface temperature that reveal where hot water is leaking or pooling invisibly
Why does identifying the leak matter?
Left unresolved, even a slow leak in your central heating system can:
- Cause long-term pressure loss and boiler failure
- Increase your heating costs as the system works inefficiently
- Damage subfloors, insulation, and internal finishes over time
- Result in mould growth or structural deterioration
When do I escalate beyond routine checks?
You don’t need to call a leak detection company at the first sign of a fault code. Most issues can be checked and resolved by a qualified gas-safe heating engineer.
But you should consider leak detection when:
- Your heating engineer cannot find the cause of low pressure
- You’ve repaired visible joints or valves, but the issue continues
- There’s no water pooling, yet fault codes keep returning
- You need formal documentation for insurance purposes
Harmuns is here to support you when the usual routes don’t provide answers. We work quickly, cleanly, and professionally – and provide a full report that your engineer or insurer can act on.
How can Harmuns help
Harmuns Leak Detection specialises in finding leaks that others can’t. If a qualified gas safe heating engineer has exhausted their checks but you’re still seeing unexplained fault codes or ongoing pressure loss, we provide targeted, non-destructive investigation. Our highly experienced engineers are all gas-safe qualified, which is important if you have a central heating leak.
We don’t replace boilers or carry out heating repairs, but we do help professionals and homeowners find the missing piece: the precise source of the leak and can then also access and fix it too.
Speak to Harmuns (If You’ve Tried Everything Else)
Your boiler’s fault codes are more than just a flashing nuisance; they’re trying to tell you something. If you’ve gone through the standard steps and still don’t have clarity, a hidden leak could be the missing link.
Call Harmuns Leak Detection on 01753 378569
Email: info@harmuns.co.uk