E84 Boiler Fault Code: What It Means on Every Brand

The E84 fault code appears on 2 brands — Baxi and Potterton — and it does not mean the same thing on each. The same two or three characters can signal a pressure problem on one boiler and an ignition fault on another, so identify your brand first and use the table below to jump to the right page.

Up to: Lockout — engineer likely 2 brands use this code Meaning differs by brand

Which brand is your boiler?

The brand is printed on the front casing. Tap your brand for the full causes, fixes and repair costs.

Brand	What E84 means	Severity	DIY?
BBaxi	The E84 fault code indicates a loss of communication between the boiler's printed circuit board (PCB) and one or more internal components — most commonly the gas valve. Full guide →	Lockout — engineer likely	No
PPotterton	The E84 code signals that your Potterton boiler has lost communication with an external control device — most commonly a room thermostat or a programmer/timer. Full guide →	Engineer may be needed	Some checks

E84 brand by brand

Baxi E84

The E84 fault code indicates a loss of communication between the boiler's printed circuit board (PCB) and one or more internal components — most commonly the gas valve. When the PCB cannot send or receive signals from the components it controls, the boiler locks out as a safety precaution. E84 sits within the E83–E87 family of codes on Baxi boilers, all of which point to internal communication breakdowns. The most frequent culprit is a disrupted link between the PCB and the gas valve control circuit, though wiring faults, a failing PCB, or a disconnected external control can also trigger this code.

Causes, fixes & costs for Baxi E84 → · All Baxi codes

Potterton E84

The E84 code signals that your Potterton boiler has lost communication with an external control device — most commonly a room thermostat or a programmer/timer. The boiler's PCB cannot detect a valid signal from the controller, so it raises this fault and may stop operating normally. Potterton groups codes E83 through E87 under the same communication-error umbrella, so you may see related codes if the problem shifts slightly. The good news is that this fault is not typically a gas-safety issue — it is an electrical and signal problem between your controls and the boiler's main board.

Causes, fixes & costs for Potterton E84 → · All Potterton codes

Frequently asked questions

Is the E84 fault code the same on every boiler?

No. E84 means different things on different brands — see the table above. Always identify your boiler's brand (it is printed on the front casing) before acting on any advice about the code.

Can I fix a E84 fault myself?

On some brands there are safe homeowner checks first — typically system pressure, the condensate pipe in cold weather, and a single reset. Where the code is a simple pressure or reset condition, that may be all it needs. Never open the boiler casing yourself.

Who should I call about a E84 fault?

For gas boilers, a Gas Safe registered engineer — it is a legal requirement for any work on the appliance. If you smell gas at any point, leave the property and call the National Gas Emergency line on 0800 111 999.

General guidance only — not a substitute for professional advice, and not certified by a registered engineer. Any work on a gas appliance must be carried out by a Gas Safe registered engineer. If you smell gas, call 0800 111 999.