E4 spans seven brands and at least five meanings — from Baxi's ignition failure to Protherm's heat-exchanger overheat, sensor faults on Alpha, Firebird and the Grant heat pump, and Remeha's repeated-ignition lockout. The only thing they have in common is that the boiler has stopped. Use the brand table below rather than any generic advice.
The brand is printed on the front casing. Tap your brand for the full causes, fixes and repair costs.
E4 brand by brand
The E4 fault code on Alpha boilers signals that the boiler's control system has detected a problem with the gas valve — the critical component responsible for regulating the flow of gas to the burner. Because the boiler cannot verify that the gas valve is behaving safely, it locks itself out immediately as a precaution. You may also see this fault referenced alongside the code 0A9 on some Alpha models; both point to the same gas valve fault condition. The boiler will not fire until the underlying cause has been investigated and resolved by a qualified professional.
Causes, fixes & costs for Alpha E4 → · All Alpha codes
The Baxi E4 code means the boiler tried to light the burner but could not confirm a stable flame within its permitted attempts. The control board detects no flame signal during the ignition sequence and locks the boiler out as a safety measure. E4 appears across several Baxi ranges — including the Baxi 200, 400, and older Platinum/Solo models — and always relates to this same ignition or flame-establishment failure. Until the underlying cause is fixed, the boiler will continue to lock out after each reset attempt.
Causes, fixes & costs for Baxi E4 → · All Baxi codes
The E4 code on a Firebird oil boiler indicates a problem with the boiler's control thermistor — the temperature sensor that works alongside the temperature potentiometer and PCB to regulate how much heat the boiler produces. When this sensor gives readings outside the expected range, or stops responding altogether, the boiler logs E4 and may shut down to protect itself. Note that on some Firebird model variants, E4 can also be associated with an overheat or high-limit condition, particularly if poor circulation has caused the boiler to run too hot. Firebird's fault code display typically shows the code for around 30 seconds before clearing, unless the boiler is in diagnostic mode.
Causes, fixes & costs for Firebird E4 → · All Firebird codes
E4 on a Grant Aerona³ heat pump reports an outgoing (flow) water temperature sensor error: the thermistor measuring the temperature of heated water leaving the heat pump for your radiators or underfloor heating has failed or reads implausibly. This is the reading the whole system steers by — flow temperature is what the controller modulates the compressor against, what weather compensation adjusts, and what protects the plate heat exchanger — so the unit stops rather than heat blind. Grant's documented remedy is the standard one: if the sensor is faulty, it should be replaced, with the diagnosis made by resistance check. Its sibling E5 covers the return water sensor.
Causes, fixes & costs for Grant E4 → · All Grant codes
The E4 fault code on a Protherm boiler indicates that the heat exchanger has reached an unsafe temperature — specifically, the water temperature inside the heat exchanger has exceeded 92°C. When this threshold is crossed, the boiler triggers a safety lockout and shuts itself down to prevent damage to internal components. The fault is displayed as a blinking light pattern (one green flash followed by four red flashes on some models). The two broad explanations are: either the heat exchanger genuinely overheated due to restricted water flow, or the overheating sensor circuit has developed an open-circuit fault and is falsely reporting high temperatures. Either way, the boiler will not restart until the underlying issue is resolved.
Causes, fixes & costs for Protherm E4 → · All Protherm codes
The E4 fault code on Remeha boilers indicates that the boiler has failed to establish a flame after repeated start attempts — typically more than five consecutive tries without any flame being detected. This can be caused by the absence of an ignition spark, a spark that fails to produce a stable flame, or a flame that lights briefly but is not properly detected by the ionisation sensor. The boiler locks out to prevent unburned gas from accumulating, which is why you'll see E4 displayed rather than the boiler simply retrying indefinitely.
Causes, fixes & costs for Remeha E4 → · All Remeha codes
The Viessmann F4 fault code — sometimes searched for as E4, which is how certain older Vitodens displays render it — signals that the boiler's control board has been unable to confirm a stable flame after an ignition attempt. In plain terms, the burner either failed to light or lit briefly before the flame sensor reported that no flame was present, prompting the boiler to lock out as a safety precaution. Until the underlying cause is resolved, you will have no central heating or hot water. The fault is sometimes also written as 'F:04' on certain Vitodens touchscreen interfaces, but the diagnosis and remedies are identical.
Causes, fixes & costs for Viessmann E4 → · All Viessmann codes
Frequently asked questions
Is the E4 fault code the same on every boiler?
No. E4 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 E4 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 E4 fault?
For gas boilers, a Gas Safe registered engineer — it is a legal requirement for any work on the appliance. For oil boilers, an OFTEC-registered technician. For heat pumps, an MCS or F-Gas qualified heat pump engineer. 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.
