Boiler Not Working? Common Problems, Fixes & Costs

A broken-down boiler is disruptive for any home, especially during winter.

Before you book an emergency call-out, there are several simple checks you can carry out yourself.

Some boiler issues are surprisingly easy to fix yourself.

Important note: Boiler repairs can be highly hazardous. While this guide helps you identify common faults, you should always consult a Gas Safe registered engineer to perform any internal repairs. It will ensure your safety and keep your manufacturer's warranty valid. Never ignore a boiler that is behaving unusually - faulty appliances can leak carbon monoxide, an extremely dangerous, odourless, and tasteless gas.

6 steps for checking your boiler yourself

If your heating or hot water has stopped working, run through this step-by-step diagnostic checklist before calling for help.

1. Run through the absolute basics

You should always rule out external utility issues first:

• Prepayment meters: If you use a prepayment gas or electricity meter, check that you haven't run out of credit or pushed it into negative emergency credit
• Gas supply: Turn on another gas appliance in your home (like a gas hob) to verify if gas is actively flowing to your property
• Fuse box: Check your consumer unit to see if a fuse has tripped.

2. Check for recent power cuts

A temporary power cut or a tripped fuse can desynchronise your boiler’s timer settings. If this has reset, your boiler won't know when it's supposed to turn on.

What to do 

Inspect the display clock on your boiler controls. If it is displaying the wrong time, reset your timer controls with the correct date and time.

3. Inspect the boiler pressure gauge

Your system needs the correct pressure to run efficiently. If there’s too little, the boiler will lock out for safety. And if there’s too much, it can damage your pipes and seals, and potentially cause leaks.

What to do 

 Locate your pressure gauge. Modern boilers display this digitally, while older systems have a hydraulic dial with a needle, often found underneath the unit.

What's normal

A healthy cold boiler should read between 1 and 1.5 bar.

If the needle has fallen below 1 bar, you will need to repressurise the system yourself using your filling loop or filling key. And if it’s below 0.5, the issue is much more urgent.

4. Turn up your thermostat

Boilers only fire up when there is an active "call for heat" from your wall thermostat. If your home is warmer than the thermostat's target temperature, the boiler will remain idle.

What to do

Turn your thermostat up to 21°C. This forces a call for heat and lets you verify if the boiler responds.

5. Reset your boiler (and respect the sequence of operations)

Modern boilers follow a strict, step-by-step electronic sequence of operations to start up safely.

If a built-in safety sensor detects a missing step (like no ignition or restricted airflow), the boiler will lock out.

What to do 

Locate the reset button on your boiler's control panel and hold it down.

Safety first!

If your boiler locks out repeatedly, never attempt to bypass or "jump out" safety controls to force it to run.

In the boiler trade, engineers warn: "Don't be the hero". A repeated lockout is telling you that a real mechanical issue exists and needs professional diagnostics.

Understanding your boiler's error code

When modern boilers lock out, a fault code appears on their control panel. It should be a letter and number combination that tells you which startup sequence step failed.

Different manufacturers have different code formats. Check the back of your boiler's manual, or search "[your boiler brand] fault codes" on the manufacturer's website. 

Below are the most common fault codes for the main UK boiler brands.

Low water pressure

This means your boiler can't circulate water safely.

• Worcester Bosch: D5 or EA 218
• Vaillant: F22 or F75
• Ideal: F1 or L1 (on some models)
• Baxi/Potterton: E119 or E1

What to do: Follow the re-pressurising guide above to bring the system back to 1–1.5 bar, then press reset.

Ignition failure

This means your boiler attempted to light but couldn't detect a flame.

• Worcester Bosch: EA 338 or E9
• Vaillant: F28 or F29
• Ideal: F2 or F3
• Baxi/Potterton: E28 or E131

What to do: First, check your gas is on (try turning on a gas hob). If it is, reset the boiler. If it locks out again immediately, you will need to contact a Gas Safe engineer.

Condensate blockage 

When the external pipe carrying wastewater from your boiler is frozen or blocked.

• Worcester Bosch: EA 229
• Vaillant: F34
• Ideal: F4 or L2

What to do: See the frozen condensate pipe guide above. Once it’s thawed and pipe is flowing, reset your boiler.

Fan or flue fault 

When the fan that expels combustion gases isn't running correctly (so the boiler won't ignite for safety reasons).

• Worcester Bosch: E5 or EA 217
• Vaillant: F49 or F64
• Ideal: F5 or F6
• Baxi/Potterton: E3 or E117

What to do: This one requires a Gas Safe engineer, so do not attempt to reset repeatedly.

6. Check the pilot light or burner scanner

For older boilers (pre-2005) Check if the permanent pilot flame has gone out. If it has, check your instruction manual to see if you can safely relight it yourself.

For modern condensing boilers If your boiler is starting up but immediately shutting down intermittently, the issue could be a dusty burner scanner (flame sensor). 

If this tiny optical sensor gets covered in soot or dust, it can't "see" the burner flame. This causes the circuit board to shut the gas valve down as a safety precaution. An engineer can quickly clean this during a routine boiler service.

6 common boiler problems (& what they cost to fix)

If you have completed the basic checks and your boiler is still refusing to work, an internal component has likely failed. The below lists common failures and how much they are likely to cost.

In the heating industry, small details create big problems. Neglecting minor faults can lead to serious damage down the line.

(There are steps you can take for boiler maintenance, but sometimes it’s too late for that!)

Here are the six most common component failures, why they happen, and what you can expect to pay a Gas Safe engineer to fix them.

1. Central heating pump failure

The pump acts as the heart of your heating system, pushing hot water out of the boiler and around your radiators.

• Why it fails: Pumps often become inactive during autumn after being unused during the summer. This can lead to stagnant water and sludge cause the internal propeller or bearings to seize up
• Signs: Noisy operation, radiators failing to heat up, or a leak dripping directly from the pump casing
• Cost to fix: Expect to pay around £200 for a high-quality replacement pump, plus the engineer's labour to fit it.

2. Faulty printed circuit board (PCB)

The PCB is your boiler's "brain." It coordinates every part of the startup sequence.

• Why it fails: Typically caused by age, electrical surges, or water leaking internally onto the board from a cracked component
• Signs: Complete loss of power, a flickering display screen, or highly sporadic operation where the boiler works one minute and shuts off the next. Check if the boiler’s fuse has simply blown before replacing the entire board
• Cost to fix: A replacement PCB is a specialist part and usually costs up to £250, plus labour.

3. Broken thermocouple

The thermocouple is a simple safety device positioned next to the pilot flame (found in older boilers with a standing pilot light).

• Why it fails: Exposure to extreme temperature fluctuations over many years causes metal fatigue. The constant expansion and contraction eventually crack the sensor, preventing it from verifying if the flame is lit. For safety, the boiler will refuse to ignite to prevent unburned gas from escaping.
• Signs: The pilot light won't stay lit.
• Cost to fix: While the sensor part is incredibly cheap, the standard cost for a Gas Safe engineer to supply and fit a new thermocouple is approximately £150.

4. Leaking pump seal or pressure relief valve

• Why it fails: Rubber seals and gaskets naturally dry out, perish, and wear down over time.
• Signs: Continuous dripping or pooling water underneath your boiler casing.
• Cost to fix: If your pump is older, an engineer will usually advise replacing the entire pump rather than just the seal, as an old pump is likely to fail soon anyway. Expect to pay approximately £300 for the replacement pump and labour.

5. Faulty extractor fan

The fan is a critical safety component. It extracts dangerous flue gases out of your home through the exhaust pipe.

• Why it fails: The bearings and electrical motor windings build up wear and tear over time. If the fan fails to run, the PCB will halt the startup sequence and refuse to ignite the burner
• Signs: An "air fault" code flashing on your display panel
• Cost to fix: Depending on your boiler type and model, replacement fans cost between £100 and £200. Add to this approximately the same in labour, and the total to around £200 to £400.

6. Motorised valve failure

Motorised zone valves open and close to direct hot water to either your radiators or your hot water cylinder.

• Why they fail: Like pumps, motorized valves are highly susceptible to seizing up during summer inactivity
• Signs: You have hot water but completely cold radiators (or vice-versa)
• Cost to fix: Replacing a motorised valve is a major, time-consuming job, as your entire heating system must be drained down before the old part can be removed. You can expect to pay approximately £300 for the part and professional installation.

System pipe and radiator issues

Sometimes, your boiler is functioning perfectly, but issues elsewhere in your central heating system prevent heat from circulating.

Airlocks and cold radiators 

If your radiators are hot at the bottom but cold at the top, air is trapped in your pipes. You can easily resolve this yourself in minutes by bleeding your radiators.

Limescale and sludge 

Over time, rust and minerals build up inside your heating system. Limescale and sludge are the "silent killers" of heating efficiency. They block water flow and coat the heat exchanger, forcing your boiler to use more gas. This directly increases your energy bills. 

If your water pressure drops repeatedly, or radiators are cold at the bottom, your system may require a power flush to clear the debris. This is something you cannot do yourself - you will need to call a professional.

Frozen condensate pipes 

During freezing weather conditions, the white plastic condensate pipe running outside your house can freeze solid.

This ice blockage causes acidic water to back up, triggering an automatic safety shutdown. 

You can resolve this yourself by gently pouring warm (not boiling) water over the outdoor pipe to thaw it. Afterwards, you just need to reset your boiler.

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