Your boiler has gone quiet and there’s a code on the display — something like EA, F.22, or E119. You don’t know what it means, whether it’s serious, or what to do next.
This guide gives you the answers. Below you’ll find a full explanation of the most common fault codes for the four most popular boiler brands in the UK — Worcester Bosch, Baxi, Vaillant, and Ideal — along with clear guidance on which you can fix yourself and which require a Gas Safe engineer.
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What Is a Boiler Fault Code?
Modern boilers are fitted with a range of sensors that continuously monitor temperature, pressure, gas flow, and flue operation. When one of those sensors detects a reading outside safe parameters, the boiler’s printed circuit board (PCB) instructs the boiler to shut down — a process called lockout.
At the same time, it displays a fault code on the screen. That code acts as a shorthand for the diagnostic: it tells you (and an engineer) which part of the system triggered the shutdown. Without it, diagnosing a fault would mean working through every component one by one. With it, an experienced engineer can often identify the likely culprit before they’ve even looked inside the casing.
Two types of fault:
- Lockout codes — the boiler has shut down completely and will not restart until the fault is resolved (or a reset clears it temporarily)
- Warning codes — the boiler is still running but flagging a developing issue; action should be taken before it escalates to lockout
Most codes you encounter in a domestic setting are lockout codes. A warning code that is ignored will almost always become a lockout.
Most Common Fault Codes at a Glance
| Brand | Code | Meaning | DIY Fix? |
|---|---|---|---|
| Worcester Bosch | EA | Ignition lockout | 🔴 No — engineer required |
| Worcester Bosch | F1 | Low water pressure | 🟢 Yes — repressurise to 1–1.5 bar |
| Worcester Bosch | E9 | Safety thermostat tripped (overheat) | 🔴 No — engineer required |
| Worcester Bosch | E5 | Overheat cutout | 🔴 No — engineer required |
| Worcester Bosch | F7 | Flame circuit fault | 🔴 No — engineer required |
| Baxi | E119 | Low water pressure | 🟢 Yes — repressurise to 1.5 bar |
| Baxi | E1 28 | Ignition failure / lockout | 🔴 No — engineer required |
| Baxi | E1 168 | Gas valve / gas supply fault | 🔴 No — engineer required |
| Baxi | E160 | Flue / air pressure fault | 🔴 No — engineer required |
| Vaillant | F.22 | Low water pressure | 🟢 Yes — repressurise to 1–1.5 bar |
| Vaillant | F.28 | Ignition failure | 🟡 Check gas supply; reset once; engineer if persists |
| Vaillant | F.29 | Flame signal failure after ignition | 🔴 No — engineer required |
| Vaillant | F.75 | No pressure change detected at pump start | 🔴 No — engineer required |
| Vaillant | F.0 | Burner control fault | 🟡 Try reset once; engineer if it returns |
| Ideal | F1 | Ignition failure | 🟡 Check gas supply; reset once; engineer if persists |
| Ideal | F2 | Flame loss after ignition | 🔴 No — engineer required |
| Ideal | F9 | Overheat — safety thermostat triggered | 🔴 No — engineer required |
| Ideal | L1 | General lockout indicator | Depends on accompanying fault code |
Worcester Bosch Fault Codes
Worcester Bosch is the UK’s best-selling boiler brand, and their fault code system uses a combination of letters and numbers. The codes below are the most commonly reported across the Greenstar range.
EA — Ignition Lockout
EA is one of the most frequently seen Worcester Bosch fault codes, and also one of the most serious. It means the boiler attempted to ignite the burner but failed — and has shut itself down as a safety precaution.
Likely causes: faulty ignition electrode or lead, dirty or corroded igniter, gas supply interruption, blocked gas valve, or low gas pressure at the meter.
What to do: Before calling an engineer, check that your gas supply is working (try a gas hob). If the hob lights normally, the issue is within the boiler itself. Press reset once — if EA returns within the hour, call a Gas Safe engineer. Do not continue resetting.
Estimated repair cost: £100 – £250 depending on the faulty component.
F1 — Low Water Pressure
F1 is the most DIY-friendly Worcester Bosch fault code. It indicates that the system pressure has dropped below 0.5 bar — the minimum safe threshold.
Likely causes: a slow leak in the system (radiators, pipework, or the boiler itself), recent radiator bleeding without repressurising, or natural pressure loss over time.
What to do: Locate the filling loop (usually a small flexible hose beneath the boiler), slowly open both valves until the pressure gauge reads 1–1.5 bar, close the valves, and press reset. If the pressure drops again within a few days, there is a leak somewhere in the system — call an engineer.
Never pressurise above 2 bar — you risk triggering the pressure relief valve.
E9 — Safety Thermostat Tripped
E9 means the boiler’s high-limit safety thermostat has detected an overheating condition and cut the boiler off. This is a serious fault that should not be ignored.
Likely causes: blocked heat exchanger (often due to scale or sludge build-up), a failing pump that isn’t circulating water properly, or a faulty thermistor giving a false reading.
What to do: Do not repeatedly reset an E9 fault. A single reset to confirm the fault is acceptable; if it returns, the boiler needs a proper diagnosis. Continuing to run a boiler that is overheating risks permanent damage to the heat exchanger.
Estimated repair cost: £150 – £400+ depending on root cause.
E5 — Overheat Cutout Tripped
Similar to E9, E5 indicates that the boiler’s secondary overheat protection has triggered. The difference is that E5 is typically triggered by a specific sensor or circuit failure rather than genuine overheating, though both scenarios require professional attention.
What to do: Press reset once. If E5 returns, call an engineer — do not keep resetting.
F7 — Flame Circuit Fault
F7 means the boiler’s flame detection circuit has either detected a flame when there should not be one, or failed to detect a flame when there should be one. Both scenarios require an engineer — this is not a fault that can be resolved by repressurising or resetting.
Likely cause: faulty flame sensing electrode, PCB fault, or wiring issue.
Baxi Fault Codes
Baxi uses a numerical system, often displaying fault codes as “E” followed by two or three digits. Codes below apply across the Baxi 100, 200, 600, and Platinum ranges.
E119 — Low Water Pressure
E119 is the Baxi equivalent of Worcester Bosch’s F1 — the system pressure is too low to operate safely.
What to do: Repressurise using the filling loop to approximately 1.5 bar, then press reset. If you’re unsure where the filling loop is, it’s usually a flexible braided hose with two small valves underneath the boiler casing. Open both valves slowly until the pressure gauge reads 1–1.5 bar, then close them both.
If E119 returns repeatedly, a Gas Safe engineer should check for leaks in the system.
E1 28 — Ignition Failure / Lockout
E1 28 is Baxi’s ignition lockout code. The boiler has attempted to ignite and failed, triggering a safety shutdown.
Likely causes: ignition electrode failure, gas supply issue, or a faulty gas valve.
What to do: Check your gas supply (test the hob), then press reset once. If E1 28 returns, do not reset again — call an engineer. Repeated ignition failures can damage the boiler’s gas valve.
E1 168 — Gas Valve or Gas Supply Fault
E1 168 indicates a fault specifically with the gas valve or an issue with the gas supply at the boiler. This always requires a Gas Safe engineer — gas valve work is a notifiable gas fitting and cannot be carried out by an unregistered person.
E160 — Flue or Air Pressure Fault
E160 means the boiler’s air pressure switch has detected that the fan or flue is not performing as expected. Common causes include a blocked or damaged flue, a faulty fan, or a failed pressure switch.
What to do: Check that there is nothing obstructing the flue terminal on the outside wall. If the flue is clear, the fault is internal — call an engineer.
Vaillant Fault Codes
Vaillant uses a decimal-point format for their fault codes (F.22, F.28, etc.), displayed across the ecoTEC range and most other Vaillant models.
F.22 — Low Water Pressure
F.22 is the most common Vaillant fault code by a wide margin. It means the system pressure has dropped below 0.8 bar (Vaillant’s minimum threshold is slightly higher than some other brands).
What to do: Repressurise using the filling loop until the gauge reads 1–1.5 bar, then press the reset button (usually held for 3 seconds on Vaillant models). If F.22 returns within a few days, there is a leak in the system — arrange an inspection.
If you smell gas at any point, do not attempt to repressurise. Leave the property and call the National Gas Emergency Service on 0800 111 999.
F.28 — Ignition Failure
F.28 indicates that the boiler failed to ignite during a start-up attempt. On Vaillant boilers, this is sometimes caused by a temporary gas supply interruption rather than a component fault.
What to do: Check that your other gas appliances are working. If they are, press reset once. If F.28 returns, the fault is within the boiler — call an engineer.
F.29 — Flame Signal Failure After Ignition
Unlike F.28 (failure before ignition), F.29 means the boiler ignited but the flame was not detected or was lost shortly afterwards. This is nearly always a component fault — either the flame sensing electrode or the gas valve — and requires an engineer.
F.75 — No Pressure Change Detected at Pump Start
F.75 is a Vaillant-specific code that indicates the boiler’s pressure sensor detected no change in pressure when the pump started. This can indicate a faulty pump, a faulty pressure sensor, or a blockage in the system.
What to do: Do not attempt to resolve this yourself. Call an engineer — F.75 often signals that a component is about to fail and should be addressed promptly.
F.0 — Burner Control Fault
F.0 is a general burner control fault that can sometimes be resolved with a single reset. If it returns, it indicates a PCB or control unit issue that requires professional diagnosis.
Ideal Boiler Fault Codes
Ideal boilers (the Logic, Vogue, and Evomax ranges) use a simple letter-and-number system. The “L” prefix indicates a lockout, while “F” indicates a specific fault type.
F1 — Ignition Failure
F1 on an Ideal boiler means ignition failed — the burner did not light during start-up. This is the most common Ideal fault code.
What to do: Check your gas supply is live (test the hob or check with a neighbour). If the gas supply is fine, press reset once. If F1 returns, the ignition electrode, lead, or gas valve needs inspection by an engineer.
F2 — Flame Loss After Ignition
F2 means the boiler ignited successfully but the flame was lost shortly afterwards. This is more serious than F1 and is almost always caused by a faulty flame sensing electrode or a gas valve that is not holding open correctly. Call a Gas Safe engineer.
F9 — Overheat — Safety Thermostat Triggered
F9 means the boiler’s safety thermostat has detected an overheating condition and shut the boiler down. Like Worcester Bosch’s E9, this should be taken seriously.
Likely causes: pump failure, heat exchanger blockage from sludge or scale, or a faulty thermistor.
What to do: Press reset once. If F9 returns, do not reset again — call an engineer. Continued operation with an overheating fault risks permanent damage.
L1 — General Lockout Indicator
L1 is not a fault code in itself — it is a lockout indicator that appears alongside a specific fault code on some Ideal models. Look for a second code displayed simultaneously or alternating with L1 to identify the underlying fault.
Need an Engineer Now?
If your boiler is showing a fault code that’s beyond a simple repressurise, don’t wait. Engineers in Birmingham, Manchester, and Leeds are available for same-day callouts — request a quote here and a local Gas Safe engineer will be in touch.
When Resetting Your Boiler Is Safe (and When It Isn’t)
The reset button is one of the most misused features on a boiler. Here is the definitive guidance:
One reset is acceptable. After a lockout, pressing reset once is a reasonable first step. It clears the fault memory and instructs the boiler to attempt a fresh start. If the underlying issue was a temporary one — a brief gas supply interruption, a momentary sensor glitch — the boiler will fire up normally.
Repeated resets are not. If the boiler locks out again within an hour, the fault is persistent. Continuing to reset forces the boiler to attempt ignition repeatedly, which:
- Floods the heat exchanger with unburnt gas (creating a dangerous accumulation)
- Puts unnecessary strain on the ignition electrode and lead
- Can damage the gas valve through repeated cycling
- Masks the symptoms of an overheating fault, potentially causing heat exchanger failure
Never reset if:
- You can smell gas anywhere near the boiler
- The fault code relates to overheating (E9, F9, E5)
- The boiler has locked out more than twice in 24 hours without a clear explanation
- The fault code relates to the flue or air pressure system
If in doubt, leave the boiler off and call a Gas Safe engineer. The cost of a repair is always less than the cost of a replacement.
Which Fault Codes Require an Engineer?
Call immediately (do not reset)
- Any fault code accompanied by a gas smell
- Worcester Bosch E9 or E5 (overheat) recurring after first reset
- Vaillant F.29 (flame signal failure after ignition)
- Ideal F9 (overheat) recurring after first reset
- Baxi E1 168 (gas valve fault)
Book same day
- Worcester Bosch EA (ignition lockout) returning after one reset
- Baxi E1 28 (ignition lockout) returning after one reset
- Vaillant F.28 (ignition failure) returning after one reset
- Ideal F1 (ignition failure) returning after one reset
- Worcester Bosch F7 (flame circuit fault)
- Vaillant F.75 (pump pressure fault)
- Baxi E160 (flue fault)
- Ideal F2 (flame loss after ignition)
Can attempt DIY first, then call if it recurs
- Worcester Bosch F1 (low pressure) — repressurise first
- Baxi E119 (low pressure) — repressurise first
- Vaillant F.22 (low pressure) — repressurise first
- Vaillant F.0 (burner control) — single reset acceptable
How to Find Your Boiler’s Manual
If your boiler shows a fault code not listed in this guide, the full error code list is in your boiler’s installation and service manual. Every manufacturer publishes these as free PDFs:
- Worcester Bosch: worcester-bosch.co.uk — search by model number
- Baxi: baxi.co.uk/support
- Vaillant: vaillant.co.uk/homeowners/support
- Ideal: idealheating.com/support
If you’ve lost the model number, it’s printed on a label inside the boiler’s front casing — usually on the right-hand side. Most Greenstar, ecoTEC, and Logic models also have a condensed code list printed on the inside of the front panel, so you can check basic codes without going online.
Frequently Asked Questions
What does a boiler fault code mean?
A fault code is a two- or three-character code displayed on your boiler’s screen when it detects a problem and shuts down (locks out). The code identifies the type of fault so an engineer can diagnose the issue quickly. Most codes require a professional repair; a small number can be resolved by repressurising the boiler or resetting it.
Can I reset my boiler when it shows a fault code?
You can press the reset button once to attempt a restart. If the boiler locks out again within a short time, do not keep resetting it — repeated resets without fixing the underlying fault can damage the heat exchanger or gas valve. Call a Gas Safe engineer instead.
What is the most common Worcester Bosch fault code?
EA (ignition lockout) and F1 (low water pressure) are the most frequently reported Worcester Bosch fault codes. EA usually indicates a gas supply or ignition component issue and requires an engineer. F1 can often be resolved by repressurising the system to between 1 and 1.5 bar.
What does Vaillant F.22 mean?
F.22 on a Vaillant boiler means the water pressure is too low (typically below 0.5 bar). You can usually fix this yourself by repressurising the system using the filling loop until the gauge reads 1–1.5 bar, then pressing reset. If the pressure drops again within a few days, there is likely a leak in the system.
What does Baxi E119 mean?
E119 is Baxi’s low water pressure fault code. It means the system pressure has fallen below the safe operating threshold. Repressurise the boiler via the filling loop to approximately 1.5 bar and press reset. If the fault returns repeatedly, a Gas Safe engineer should inspect for leaks.
What does Ideal boiler fault code F1 mean?
On Ideal boilers, F1 indicates an ignition failure — the boiler attempted to light but the burner did not ignite. This is commonly caused by a gas supply interruption, a faulty ignition lead, or a worn igniter. Check that your gas supply is working (try the hob), then press reset once. If the fault persists, call a Gas Safe engineer.
When should I call an engineer for a boiler fault code?
Call a Gas Safe engineer if: the fault code returns after a single reset; the code relates to ignition, gas valve, flue, or overheating; you smell gas at any point; or the boiler has locked out more than twice in 24 hours. Never attempt to repair gas components yourself — it is illegal without Gas Safe registration.
Ready to Get an Engineer Out?
If your boiler is showing a fault code that won’t clear, or keeps coming back, it’s time to call a professional. Our network of vetted, Gas Safe registered engineers covers the whole of the UK — with same-day availability in most areas.
Whether you’re in London, Birmingham, Manchester, Sheffield, or Leeds — fill in the form below and we’ll connect you with a local engineer who can diagnose and fix the fault, usually on the same day.