E21 Boiler Fault Code: What It Means on Every Brand

The E21 fault code appears on 3 boiler brands — Biasi, Johnson & Starley and Ravenheat — and on all of them it points to the same kind of problem. The details still differ by brand (thresholds, sub-codes, reset procedure), so use the table below to jump to the page for your boiler.

Lockout — engineer likely 3 brands use this code

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 E21 means	Severity	DIY?
BBiasi	The E21 fault code on a Biasi boiler indicates an EVG lockout caused by a valve control relay failure. Full guide →	Lockout — engineer likely	No
JSJohnson & Starley	E21 on a Johnson & Starley unit is an ADC lockout: the analogue-to-digital converter — the circuit on the control board that turns sensor voltages into the numbers the processor works with… Full guide →	Lockout — engineer likely	Some checks
RRavenheat	The E21 fault code on a Ravenheat boiler indicates that the central heating return sensor has detected an overheating condition. Full guide →	Lockout — engineer likely	No

E21 brand by brand

Biasi E21

The E21 fault code on a Biasi boiler indicates an EVG lockout caused by a valve control relay failure. In plain terms, the boiler's control electronics have detected a problem with the relay responsible for opening and closing the gas valve, and the boiler has shut itself down as a safety measure. This is sometimes displayed as 'E21 + b' on the boiler's screen. It sits within a family of gas valve safety lockouts: E20–E23 all relate to the electromagnetic gas valve solenoid, while E22 specifically flags a flame being detected after the gas valve should have closed — a distinct but closely related fault. Because E21 involves the gas valve control circuit, it requires a Gas Safe registered engineer to investigate and resolve.

Causes, fixes & costs for Biasi E21 → · All Biasi codes

Johnson & Starley E21

E21 on a Johnson & Starley unit is an ADC lockout: the analogue-to-digital converter — the circuit on the control board that turns sensor voltages into the numbers the processor works with — has failed its self-check. Every temperature and pressure reading passes through the ADC, so a controller that cannot trust its converter cannot trust any measurement it makes, and the only safe response is a full lockout. Unlike a single failed sensor, this is a board-level fault: the practical outcomes are a recoverable glitch (cleared by a full power-cycle), or a genuine converter failure requiring control board replacement.

Causes, fixes & costs for Johnson & Starley E21 → · All Johnson & Starley codes

Ravenheat E21

The E21 fault code on a Ravenheat boiler indicates that the central heating return sensor has detected an overheating condition. The return thermistor sits on the pipework carrying cooled water back into the boiler after it has circulated through your radiators. It works alongside the flow thermistor to keep tabs on the temperature differential across the heating circuit. When the return water temperature climbs beyond safe operating limits — or the sensor reports a reading that suggests it has — the boiler's safety controls intervene and lock the unit out, displaying E21 to let you know something needs attention.

Causes, fixes & costs for Ravenheat E21 → · All Ravenheat codes

Frequently asked questions

Is the E21 fault code the same on every boiler?

Broadly, yes — on Biasi, Johnson & Starley and Ravenheat the E21 code points to the same class of fault, and the recommended response is similar. Model-specific details such as sub-codes and reset procedures still vary, so follow the page for your own brand.

Can I fix a E21 fault myself?

On some brands there are safe homeowner checks first — typically system pressure, the condensate pipe in cold weather, and a single reset. The underlying repair usually needs a registered engineer. Never open the boiler casing yourself.

Who should I call about a E21 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.