What Happens If a Gas Appliance Fails a CP12 (Gas Safety Record) Check?

A failed CP12 is a warning about the installation, not just the appliance

When a gas appliance does not pass a CP12 inspection, the important question is not simply whether the boiler, cooker or fire is old. The engineer is looking for evidence that gas is burning safely, that the gases produced when it burns are being removed correctly and that the installation is not creating a risk inside your property.

That means a failed check can involve the appliance itself, the flue (the pipe that carries gases outside), ventilation, gas pipes or the way the equipment has been installed. A fault may not be obvious from the outside. Gas appliances can continue to produce heat while still having a problem that requires the appliance to be taken out of use.

We carry out Gas Safety Check CP12 inspections across Harlow, Sawbridgeworth and Bishops Stortford, recording the condition of the relevant gas appliances and explaining what the findings mean in practical terms.

What can cause an appliance to fail?

Combustion is not taking place correctly

A gas appliance needs the correct balance of gas and air to burn properly. If that balance is affected by a blocked burner, damaged component or incorrect adjustment, combustion may be incomplete. The result can include unsafe levels of combustion products, including carbon monoxide.

This is one reason a visual check is not enough. A boiler or gas fire may appear to operate normally, but testing can show that it is not burning or operating as it should. Do not rely on the flame colour, lack of noise or the fact that the appliance still produces hot water as proof that it is safe.

The flue is not removing products safely

The flue carries combustion products away from the appliance and out of the building. Its condition, connections and route all matter. A loose connection, obstruction, damaged section or problem with the flue outlet can allow products to spill into a room instead of being discharged safely.

Flue-related faults deserve particular attention because the appliance may continue running while the problem remains hidden. Access also matters. A flue that passes through a cupboard, loft space or boxed-in area may require closer inspection than the visible part suggests.

Ventilation is inadequate

Some gas appliances need a dependable supply of air for safe operation. If ventilation has been blocked, covered or altered during building work, the appliance may not have the air it needs for combustion. Sealing a room, replacing doors or changing the layout around an appliance can affect this without any change being made to the appliance itself.

Never block a permanent air vent because it causes a draught. If you are unsure whether a vent is part of the safety arrangement, leave it clear and ask a Gas Safe Registered engineer to assess it.

The gas pipes or connections are not secure

The check can also identify concerns with gas pipes, connections or signs that gas could be escaping. A smell of gas is an urgent warning, but the absence of a smell does not prove that every connection is sound. Small defects, poor joints or deterioration may only become clear during proper testing.

What should you do after a failed check?

First, follow the engineer's safety advice. If an appliance is judged unsafe, do not continue using it until the fault has been dealt with and the engineer has confirmed what can safely happen next. Do not attempt to relight it, remove covers, adjust components or investigate the flue yourself.

Ask what failed and why. A useful explanation should identify the affected appliance or installation component, describe the safety concern in plain language and make clear whether the appliance should be disconnected from the gas supply. It should also distinguish between a fault that needs repair and a condition that prevents safe use until further work is completed.

The next step depends on the cause. A repair may involve replacing a defective component, correcting a connection, addressing ventilation or dealing with a flue problem. In other cases, the appliance may not be suitable for continued use. We assess the finding before recommending the appropriate repair work, rather than treating every failed check as the same problem.

Why ignoring the finding can make matters worse

Using an appliance after a failed safety check can allow a small defect to develop into a wider problem. A combustion fault can damage internal components. A flue issue can expose occupants to combustion products. A pipework defect can affect other appliances connected to the same gas supply. Delaying action can also leave you without clear evidence that the installation is safe to use.

For landlords, unfinished repair work can create difficulties when maintaining property records. For homeowners, a written safety finding gives you a clear starting point for deciding whether to repair, isolate or replace the affected appliance.

We are Gas Safe Registered and carry out gas safety checks for homes and rented properties in Harlow, Sawbridgeworth and Bishops Stortford. If your CP12 has recorded a defect, contact Border Heating Services Ltd to discuss the finding and arrange the appropriate next step. The work required depends on the appliance, access, the way the flue is installed and the condition of the installation, so request a clear assessment before authorising repairs.