E1 is a good example of why the brand matters: the same two characters mean five different things. On Baxi, Potterton and Main it is low system pressure — the cheapest fix on this site, usually a filling-loop top-up. On Remeha it is a flow-temperature (overheat) fault; on Protherm a flue or air-pressure error; on Firebird oil boilers a thermistor fault; and on Alpha it is easily confused with the separate E01 code. Find your brand in the table before acting on anything.
The brand is printed on the front casing. Tap your brand for the full causes, fixes and repair costs.
E1 brand by brand
Alpha boilers display two closely related fault codes that are often confused: E1 and E01. On older or certain Alpha models, E1 signals low water pressure — the system pressure has dropped below the safe operating threshold, causing the boiler to shut down to protect itself. On modern Alpha combi and system boilers (including the E-Tec, InTec, and Evoke ranges), E01 indicates an ignition or flame failure — the boiler attempted to light the burner but either failed to ignite at all, or the flame was not detected or maintained after ignition. Both codes result in a safety lockout, meaning the boiler will not fire until the underlying problem is resolved and the fault is cleared. Check your boiler display carefully: a pressure reading below 1 bar alongside E1 points to a pressure issue, while E01 with normal pressure points to an ignition problem.
Causes, fixes & costs for Alpha E1 → · All Alpha codes
The E1 code on Baxi, Potterton, and Main boilers signals that the sealed central heating circuit has dropped below its safe operating pressure, causing the boiler to lock out and suspend both heating and hot water. On older or simpler models, E1 appears on its own as a straightforward low-pressure alert. However, on the HE A Range (including the Combi Platinum, Combi Duo-tec, System Megaflow, and Heat Only Sola), the display alternates between "E" or "E1" and a two- or three-digit number — so the full code you see might be E119, E133, E168, E110, or E118. Each of these points to a distinct underlying fault: E119 and E118 both confirm water pressure has fallen (E118 specifically below 0.5 bar); E133 indicates a gas supply or ignition problem, sometimes caused by a frozen condensate pipe; E110 means the boiler has shut down due to overheating; and E168 is a general internal safety shutdown. Identifying which sub-code is showing will guide the correct fix.
Causes, fixes & costs for Baxi E1 → · All Baxi codes
The E1 code on a Firebird oil boiler indicates a fault with the control thermistor — the temperature-sensing component that works alongside the PCB and the boiler's temperature potentiometer to regulate how much heat the burner produces. When the boiler's control board cannot get a reliable signal from this sensor, it displays E1 and typically shuts the boiler down to prevent it running out of control. Note that on many other UK boiler brands E1 often signals low water pressure, but on a Firebird it specifically means a thermistor fault — so always treat the code in its Firebird context.
Causes, fixes & costs for Firebird E1 → · All Firebird codes
On Protherm boilers, the E1 fault code signals a flue gas extraction or air pressure error. This is not the same as the E1 code on Baxi, Potterton, or other common UK brands, where E1 typically indicates low system pressure. On a Protherm, the boiler has detected that it cannot safely confirm proper ventilation before firing — so it locks out as a precaution. The air pressure switch is the key safety device at the centre of this fault: it verifies that the flue fan is running and creating enough draught to expel combustion gases before the burner is allowed to ignite. If that confirmation doesn't arrive — whether because the flue is obstructed, the fan has slowed or stopped, or the switch itself has failed — the boiler shuts down and displays E1. A frozen condensate pipe during cold snaps can also trigger this lockout. Because this fault is directly related to safe removal of harmful flue gases, it should be taken seriously and not repeatedly reset without investigation.
Causes, fixes & costs for Protherm E1 → · All Protherm codes
The E1 code appears when your Remeha boiler detects that the water temperature leaving the heat exchanger (the flow temperature) has climbed above the maximum permitted threshold. As a safety response, the boiler shuts itself down to prevent damage or a dangerous overheat situation. In most cases the boiler will attempt to restart but will lock out again quickly if the underlying cause is not addressed. This fault is most commonly seen on the Remeha Avanta range and is nearly always linked to a problem with water circulation — either not enough water in the system, air trapped in the circuit, or something physically preventing water from moving freely around the heating loop.
Causes, fixes & costs for Remeha E1 → · All Remeha codes
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.
