Gas Safety Certificate (CP12): Exactly What a Gas Safe Engineer Checks

What Is a CP12 Gas Safety Certificate?

A CP12 is the formal record of a landlord gas safety check, carried out by a Gas Safe registered engineer. The term CP12 comes from the old CORGI Pro-forma 12 — CORGI was the predecessor body to Gas Safe Register. The certificate documents the inspection of every gas appliance and flue in a rented property and confirms whether each appliance is safe for continued use.

Under the Gas Safety (Installation and Use) Regulations 1998, landlords must have all gas appliances checked annually by a Gas Safe registered engineer and provide tenants with a copy of the certificate within 28 days of the check, or before a new tenancy begins.

What the Engineer Checks

A Gas Safe engineer conducting a CP12 inspection will check the following for each gas appliance (boiler, gas fire, cooker, or gas hob):

Appliance Operation and Safety Devices

• Burner operation and ignition performance.
• Thermostat and time control function.
• Flame failure device (FFD) — this cuts the gas supply if the flame goes out, preventing unburned gas accumulating. The engineer will manually test it.
• Overheat protection thermostat.
• Pressure relief valve (where applicable).

Combustion and Flue Performance

• Flue flow test — a smoke match or equivalent is used to confirm combustion gases are drawing away from the appliance into the flue and not spilling into the room.
• Combustion analysis on boilers — CO and CO2 levels are measured at the flue terminal. High CO readings indicate incomplete combustion, which may result in a dangerous (at risk) notice.
• Flue integrity — visual inspection for corrosion, damage, or disconnection at accessible joints.

Ventilation

• Adequate permanent ventilation is verified for open-flued appliances. Sealed (room-sealed) appliances, like modern combi boilers, draw combustion air from outside and do not require permanent room ventilation, but the air intake and flue terminal must be unobstructed.

Gas Tightness

• The gas installation is pressure-tested to check for leaks. The engineer isolates the supply, records the pressure, and checks for any drop over a set period.

Pass Versus Fail: What the Outcomes Mean

Appliances are assessed under three categories:

• Safe to use: The appliance passes all checks and is issued with a satisfactory result on the certificate.
• At risk (AR): The appliance has a fault that is not immediately dangerous but poses a risk if used. The engineer will advise the landlord or tenant not to use it and will attempt to contact the gas transporter if occupants cannot be reached.
• Immediately dangerous (ID): The appliance presents an immediate risk to life. The engineer is obliged to disconnect it (with the occupier's consent) or to label it and notify the gas transporter. It cannot legally be used until repaired and re-inspected.

A CP12 is issued even if an appliance is condemned — the certificate records the outcome of the check, not a blanket pass. A landlord receiving a certificate with AR or ID appliances must arrange repair or replacement before the property is occupied or before use continues.

The 12-Month Validity

The certificate is valid for 12 months from the date of inspection. To maintain continuous compliance, the annual check should be booked before the existing certificate expires. There is no grace period — a landlord whose certificate lapses for even one day is technically in breach of the regulations.

Homeowners (as opposed to landlords) are not legally required to hold a CP12, but an annual service by a Gas Safe engineer gives equivalent safety assurance. Most boiler manufacturers also require annual servicing as a condition of warranty.

Finding a Legitimate Gas Safe Engineer in London

Always verify an engineer's Gas Safe registration at gassaferegister.co.uk before booking. The register is free to search and shows the engineer's licence number, the appliance types they are qualified to work on, and whether their registration is current. An unregistered person carrying out gas work is committing a criminal offence.