Boiler Not Working? A Step-by-Step Troubleshooting Guide

When your boiler stops working, you don't always need to call an engineer. Most lockouts and faults fall into a handful of common categories that can be diagnosed in ten minutes and often fixed in under an hour. This guide walks through the diagnosis in the order an engineer would do it — so you can either resolve the issue yourself or give a clear description when you call for help.

First: rule out the basics

Before assuming a serious fault, confirm:

• The boiler has power. Display lit? Fused spur switched on? Tripped breaker at the consumer unit?
• The thermostat is calling for heat. Battery dead? Schedule active? Temperature set above current room temperature?
• The gas supply is on. Have you used the gas hob recently? Is the gas meter clearly active? In rare cases, the supply has been cut off temporarily.
• The system pressure is in range. Should read 1.0–1.5 bar cold. Below 0.5 bar, the boiler locks out automatically.

These four account for a significant share of "broken boiler" callouts that turn out to be no-fault. Always check first.

Read the fault code

Modern boilers show a fault code or letter when they lock out. Write it down before doing anything else — it's the single most useful piece of information when calling an engineer. Common codes by category:

• F22 / E22 / F1 / F4: Low system pressure
• F75: Pressure sensor failure
• F28 / E133 / F11: Ignition failure
• F29 / F12: Flame loss after ignition
• F61–F64: Gas valve or electronic control failure
• E133 / F71: Frozen condensate pipe (in winter)
• F0 / F1 / F2: NTC sensor faults (temperature sensors)

Exact code meanings vary by brand. Your boiler manual (or a search for "your boiler model + the code number") will give the specific diagnosis.

Low pressure faults — the easiest fix

If the pressure gauge reads below 1 bar, the boiler refuses to fire as a safety measure. Repressurising takes about three minutes via the filling loop. See our step-by-step pressure guide.

If pressure returns to normal and the boiler fires, monitor for 24 hours. If pressure drops again within days, there's a leak somewhere — call a plumber to find it before continuing to top up.

Frozen condensate pipe — common winter problem

If the boiler locks out during cold weather and shows a fault code like F1, F4, or E133, the condensate pipe (white plastic pipe exiting the boiler through an external wall) has frozen. To thaw:

1. Pour warm (not boiling) water along the external section of the condensate pipe
2. Repeat for 10–15 minutes until water flows freely from the open end
3. Reset the boiler — it should fire normally

To prevent future freezing, lag the external pipe with foam pipe insulation — a £5 fix that saves a winter callout. See also our guide on preventing frozen pipes.

Ignition failures (F28, F29, E133)

If the boiler tries to fire but fails — you hear a clicking sound and the burner doesn't catch, or it fires briefly then cuts out — the cause is usually one of:

• Gas supply issue: Has the gas meter run out of credit (prepayment meters)? Has the gas supply been interrupted? Are other gas appliances working normally?
• Worn ignition electrodes: The small spark electrodes near the burner degrade over time. Replacement is a Gas Safe engineer job.
• Failed gas valve: Internal valve component fault. Engineer required.
• Air in the gas line after a supply interruption. Often clears after a few reset attempts.

You can safely try to reset the boiler twice. If it locks out again immediately, stop and call a Gas Safe engineer — repeated reset attempts on a failed ignition can damage components and waste gas.

Sensor faults (F0, F1, F2, F75)

Modern boilers have multiple temperature and pressure sensors monitoring operation. If one fails, the boiler locks out to prevent unsafe operation. Sensor replacement is typically a 30–60 minute engineer job costing £100–£200 including the part.

No display at all

If the boiler is completely dark with no display lit:

• Check the fused spur switch on the wall next to the boiler is switched on
• Check the consumer unit (fuse box) — has a circuit tripped?
• Try replacing the 3A fuse in the fused spur (turn the spur off first)
• If still no power after these checks, the internal PCB or transformer has likely failed — engineer required

Boiler fires but no heat reaches radiators

The boiler runs, you can hear it firing, but radiators stay cold. Common causes:

• Stuck diverter valve (combi boilers) — sending all output to the hot water side
• Failed pump — water heated but not circulating
• Closed thermostatic valves on all radiators (check)
• Air locked system — bleed all radiators in order from boiler outwards

See our diagnostic guide on central heating not working for the full sequence.

Boiler fires but no hot water

Heating works fine, but no hot water from the taps. For combi boilers, this is almost always the diverter valve — stuck in heating position and not switching over to hot water. Engineer required for replacement. Typical cost £180–£280 including the part.

For system boilers, check the cylinder thermostat first — there's often a reset button on the side of the cylinder that may have tripped.

When to stop trying and call

• You've reset the boiler twice and it locks out immediately each time
• You smell gas anywhere near the boiler — call 0800 111 999 immediately
• Visible water leak from the boiler or pipework
• Burning smell or unusual sounds from the boiler
• The boiler is over 12 years old and faults are recurring
• You're unsure what a fault code means and the manual doesn't help

Our central heating engineers attend boiler faults across all PE postcodes — typically within 24 hours of booking. Common spare parts (pumps, diverter valves, pressure sensors, fans, PCBs for major brands) are carried on every van so most repairs are completed in a single visit.