E5 Boiler Fault Code: What It Means on Every Brand

E5 is a sensor fault on most of the brands that use it — Alpha, Worcester Bosch and the Grant heat pump all point at a temperature thermistor — but Firebird uses it for burner ignition failure, Remeha for an overheat-related lockout, and Viessmann's meaning depends on the model. Sensor faults are cheap repairs; ignition and overheat faults need proper diagnosis. Check your brand first.

Up to: Lockout — engineer likely 6 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 E5 means	Severity	DIY?
AAlpha	The E5 code on an Alpha boiler signals that the primary flow NTC thermistor — the sensor responsible for monitoring the temperature of water leaving the heat exchanger — has detected an… Full guide →	Lockout — engineer likely	No
FFirebird	The E5 code on a Firebird oil boiler signals that the burner has attempted to fire but failed to establish or confirm a flame, triggering an automatic safety shutdown known as a lockout. Full guide →	Lockout — engineer likely	No
GGrant	E5 on a Grant Aerona³ heat pump reports a return water temperature sensor error: the thermistor measuring the water coming back from your radiators or underfloor circuit has failed or is… Full guide →	Engineer may be needed	No
RRemeha	The E5 fault code on Remeha boilers — most commonly seen on the Avanta and Avanta Plus — indicates that the boiler has attempted to ignite and establish a stable flame but has failed to… Full guide →	Lockout — engineer likely	No
VViessmann	The E5 fault code on Viessmann boilers has two distinct meanings depending on your specific model. Full guide →	Engineer may be needed	No
WBWorcester Bosch	The E5 fault code on a Worcester Bosch boiler points to a problem with the flow temperature sensor — the component that monitors how hot the water is as it circulates through your heating… Full guide →	Lockout — engineer likely	No

E5 brand by brand

Alpha E5

The E5 code on an Alpha boiler signals that the primary flow NTC thermistor — the sensor responsible for monitoring the temperature of water leaving the heat exchanger — has detected an abnormally high temperature. As a safety measure, the boiler locks out to prevent damage to itself and to your home. The root cause may be a genuine overheating event (for example, poor water circulation) or a fault with the sensor or its wiring that causes it to report a falsely high reading. Either way, the boiler will not restart until the underlying problem is resolved.

Causes, fixes & costs for Alpha E5 → · All Alpha codes

Firebird E5

The E5 code on a Firebird oil boiler signals that the burner has attempted to fire but failed to establish or confirm a flame, triggering an automatic safety shutdown known as a lockout. This is the boiler's built-in protection mechanism — it halts all combustion activity rather than allow unsafe operating conditions to continue. The fault appears across Firebird's Enviromax and Envirogreen condensing ranges and must be cleared before the boiler will attempt to restart.

Causes, fixes & costs for Firebird E5 → · All Firebird codes

Grant E5

E5 on a Grant Aerona³ heat pump reports a return water temperature sensor error: the thermistor measuring the water coming back from your radiators or underfloor circuit has failed or is reading implausibly. Together with the flow sensor (code E4), the return reading gives the controller the temperature difference across your heating system — its measure of how much heat the house is actually absorbing — and it feeds frost protection and heat-metering logic besides. Grant's documented remedy is the familiar sensor routine: if the sensor is faulty, it should be replaced, with a resistance check against the table making the diagnosis certain before parts are fitted.

Causes, fixes & costs for Grant E5 → · All Grant codes

Remeha E5

The E5 fault code on Remeha boilers — most commonly seen on the Avanta and Avanta Plus — indicates that the boiler has attempted to ignite and establish a stable flame but has failed to confirm one through its ionisation sensing system. Specifically, the boiler made five or more unsuccessful attempts to detect a valid ionisation signal during a single heat demand before locking out for safety. The ionisation current needs to sit between 3 and 9 microAmperes for the boiler to accept the flame as stable; if that threshold is not consistently met, the boiler shuts down. This is a lockout fault, meaning the boiler will not restart automatically and must be reset — though a reset alone will rarely cure the underlying problem.

Causes, fixes & costs for Remeha E5 → · All Remeha codes

Viessmann E5

The E5 fault code on Viessmann boilers has two distinct meanings depending on your specific model. On the Vitodens 200, it signals that the boiler has entered emergency mode or is cooling down because of an internal BUS fault — a breakdown in the communication network that links the boiler's control components. This can also cause the circulation pump to run at maximum speed due to an open circuit. On the Vitodens 222-F, E5 means the burner is blocked: the boiler water temperature is too high and needs to reduce, with air being vented automatically. It can also point to a fault within the burner control unit. Because the meaning differs between models, it's important to identify your boiler before proceeding with any diagnosis.

Causes, fixes & costs for Viessmann E5 → · All Viessmann codes

Worcester Bosch E5

The E5 fault code on a Worcester Bosch boiler points to a problem with the flow temperature sensor — the component that monitors how hot the water is as it circulates through your heating system. When this sensor detects that temperatures are drifting outside safe operating limits, or when it sends readings the PCB can't make sense of, the boiler locks out to protect itself from overheating damage. The most common sub-code you'll see alongside E5 is E5 218, which specifically indicates the flow temperature has climbed too high — often because of poor water circulation, a struggling pump, or air trapped in the system. You may also encounter E5 332, which typically points more directly to a sensor signal fault or wiring issue rather than a genuine over-temperature condition. In either case, the boiler won't restart until the underlying fault has been resolved.

Causes, fixes & costs for Worcester Bosch E5 → · All Worcester Bosch codes

Frequently asked questions

Is the E5 fault code the same on every boiler?

No. E5 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 E5 fault myself?

Not usually. On every brand listed, E5 points to a fault that needs a suitably registered engineer to diagnose and repair. A single reset to rule out a transient glitch is reasonable; repeated resets are not.

Who should I call about a E5 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.