Biasi E21 Fault Code: Causes, Fixes & Repair Costs
What does the Biasi E21 fault code mean?
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.
General guidance only — not a substitute for professional advice, and not certified by a Gas Safe engineer. Always have your specific appliance assessed by a qualified professional. Any gas work must be carried out by a Gas Safe registered engineer. If you smell gas or suspect carbon monoxide, leave the property and call the National Gas Emergency line on 0800 111 999.
Common causes
- Failed relay on the PCB Common
The printed circuit board contains a dedicated relay that sends the electrical signal to open the gas valve. If this relay burns out, sticks, or becomes intermittently faulty, the boiler's safety logic will detect the anomaly and trigger an E21 lockout. This is the single most common root cause of this code.
- Faulty gas valve solenoid Common
The gas valve itself contains an electromagnetic solenoid coil. If this coil develops an internal electrical fault — such as a short circuit or open circuit — the relay controlling it will register an error, causing the E21 lockout even though the PCB relay itself may be functioning correctly.
- Damaged or loose wiring between PCB and gas valve Sometimes
The cables and connectors linking the control board to the gas valve can suffer from corrosion, vibration-related loosening, or physical damage over time. A poor connection in this circuit can mimic a relay failure and produce an E21 code intermittently or permanently.
- Broader PCB failure Sometimes
A more widespread failure of the main control board — rather than just the single relay — can cause multiple relay outputs to malfunction. In this case the E21 code is a symptom of a board that needs professional testing or replacement.
How to fix it
- Note the exact code displayed and attempt a boiler reset DIY safe
Press and hold the reset button on your Biasi boiler for approximately 3–5 seconds (refer to your model's manual for the exact method). If the boiler fires up successfully, monitor it closely over the next hour. If E21 returns, do not reset more than two or three times in total — repeated reset attempts on a gas valve fault can be counterproductive and will not address the underlying problem.
- Check that your gas supply is working DIY safe
Confirm that other gas appliances in your home (such as a gas hob or fire) are operating normally. If they are not, contact your gas supplier rather than a heating engineer, as the issue may be an interrupted supply rather than a boiler fault.
- Contact a Gas Safe registered engineer for diagnosis Gas Safe engineer
Because E21 is an electrical fault within the gas valve control circuit, all further investigation must be carried out by a qualified professional. The engineer will use a multimeter to test the relay circuit on the PCB, check the resistance and continuity of the gas valve solenoid coil, inspect the wiring harness and connectors between the board and valve, and determine whether the gas valve, the PCB relay, or the wiring is the root cause.
- Allow the engineer to replace the faulty component Gas Safe engineer
Depending on their findings, the engineer will replace the defective part — most commonly the gas valve or the PCB, or occasionally carry out a targeted wiring repair. Both the gas valve and the PCB are regulated components that must only be fitted and commissioned by a Gas Safe registered engineer.
Parts you may need
- Biasi gas valve (EVG solenoid assembly) · from £120
- Biasi main PCB (control board) · from £180
- PCB relay (component-level replacement, if repairable) · from £15
- Wiring harness / connector loom · from £35
The exact spare depends on your boiler's GC number (on the data badge). Check this against the part before buying.
Typical repair cost
Expect to pay roughly £180–£380, depending on the underlying cause.
Frequently asked questions
Can I fix a Biasi E21 fault myself?
The only safe DIY steps are resetting the boiler (no more than two or three times) and checking your wider gas supply is working. Everything beyond that — testing the relay, inspecting the gas valve, or touching any wiring — must be done by a Gas Safe registered engineer. Working on gas valve electrical circuits without the correct qualifications is unsafe and illegal in the UK.
How much will it cost to repair a Biasi E21 fault?
Most people with an E21 fault end up paying somewhere between £180 and £380 all in, covering the call-out, diagnosis, parts, and labour. A wiring repair tends to sit at the lower end of that range, while a gas valve replacement usually falls in the middle. A full PCB replacement can push costs higher — sometimes £300–£500 or more depending on the Biasi model — but this is less common. Get a written quote before authorising any work.
What is the difference between Biasi fault codes E21 and E22?
Both are EVG lockout codes related to the gas valve, but they point to different problems. E21 means the valve control relay has detected a fault in its own control circuit — essentially the boiler cannot confirm the valve is being commanded correctly. E22 means the boiler has detected a flame after the gas valve should have closed, which is a different and potentially more serious safety concern. If your display shows E22, you should turn the boiler off and call a Gas Safe engineer promptly.
My Biasi boiler is still under warranty — do I need to pay for this repair?
Biasi boilers typically come with a 5–7 year warranty depending on the model. If your boiler is still within its warranty period, contact Biasi directly before arranging or paying for any repairs. Arranging an independent engineer to work on the boiler before checking your warranty position could invalidate your cover.