BoilerFaultCodesYour boiler has detected that the electrical wiring coming into the unit is swapped or incorrectly connected, preventing it from starting safely.
Book a Gas Safe engineer within 24–48 hours. Your boiler may be unsafe or could break down completely if left.
Technical description: PHASE REVERSAL
What causes this fault?
This fault occurs when the live and neutral electrical wires leading into the boiler have been accidentally swapped, usually following electrical work in the home or a smart thermostat installation. The boiler's safety sensors detect that electricity is flowing in the wrong direction and prevent the unit from firing to avoid damage or electric shocks. In rarer cases, a fault within the internal control board can cause it to misinterpret correct wiring as a reversal.
Engineer required
This fault requires a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.
Find a Gas Safe engineer
Search the official register for a qualified engineer in your area.
Gas Safe Register →Protect yourself from future repair bills
A boiler cover plan would cover faults like H n — no call-out fees, no surprises.
Affiliate links — we may earn a commission at no extra cost to you.
Ideal Classic M3050 M5080 is discontinued — time for a new one?
Parts for discontinued models can be hard to find and expensive. A new A-rated boiler could save you money on both repairs and energy bills.
Affiliate links — we may earn a commission at no extra cost to you.