Biasi E133 Fault Code: Causes, Fixes & Repair Costs
What does the Biasi E133 fault code mean?
The E133 code (sometimes shown alongside a red lockout light as ER01 on older Biasi models) means your boiler made several attempts to ignite and failed to establish a flame it could detect — so it has shut itself down as a safety precaution. Essentially, the boiler tried to light, something prevented it from doing so successfully, and it has locked out to stop any unsafe condition developing. You will have no heating or hot water until the underlying cause is resolved. Note on Biasi model variants: older models such as the Riva Plus and Garda display this type of ignition lockout as ER01 or ER02, while newer models including the Inovia and Advance Plus use the E-code format (E133, E110, E118). The fault and its causes are the same across all these variants — only the display wording differs.
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
- Gas supply interrupted or unavailable Common
If the boiler is not receiving gas it simply cannot ignite. Check whether other gas appliances in your home (such as a gas hob) are working. A prepayment meter that has run out of credit is a surprisingly frequent culprit, as is a gas isolation valve that has been accidentally turned off. A wider supply interruption from your network operator is also possible.
- Frozen or blocked condensate pipe Common
During cold snaps, the plastic condensate pipe that exits the boiler through an external wall can freeze solid. When it cannot drain, the boiler locks out. This is a very common cause of E133 appearing overnight or first thing on a frosty morning, and it is one of the few faults a homeowner can resolve without an engineer.
- Faulty or dirty ignition electrode or flame sensor Common
The ignition electrode creates the spark that lights the gas, and the flame sensor (sometimes part of the same component) checks that a flame has actually formed. If either is worn, cracked, or coated in residue, the boiler may spark without the control board ever receiving confirmation that a flame is present — triggering the lockout. This is one of the most common workshop findings when an engineer investigates a persistent E133.
- Low or incorrect gas pressure at the boiler Sometimes
Even when the gas supply is technically on, the pressure reaching the boiler may be too low to support reliable ignition. This can be a supply-side issue or an internal fault such as a partially closed or failing gas valve. Low gas pressure requires an engineer to diagnose and set correctly.
- Air trapped in the gas line Sometimes
If the boiler has been sitting unused for an extended period — such as at the end of summer — air can work its way into the gas pipework. The first ignition attempt draws air rather than gas, causing a failed light. In many cases the boiler will clear this itself after a reset or two, but if air ingress is significant an engineer may need to purge the line.
- Faulty gas valve Sometimes
The gas valve controls the flow of gas to the burner. If it sticks closed, fails electrically, or develops a fault, gas will not reach the burner regardless of whether supply pressure is adequate. Diagnosis and replacement must be carried out by a Gas Safe registered engineer.
- Blocked or restricted flue Sometimes
The flue removes combustion gases and draws in fresh air for ignition. A blockage — caused by debris, a bird nest, or ice forming at the terminal — can upset the air-to-gas ratio enough to prevent a stable flame. The boiler detects this and locks out.
- Faulty PCB (printed circuit board) Rare
The PCB orchestrates the entire ignition sequence, including sending the signal to spark and reading the flame sensor response. A damaged or failing PCB can misfire this sequence or fail to recognise a flame even when one has formed. PCB faults tend to present after other causes have been ruled out, and replacement is an engineer-only job.
How to fix it
- Check that other gas appliances are working DIY safe
Before touching the boiler, go to your gas hob or another gas appliance and check it lights normally. If nothing gas-powered is working, the problem is with your gas supply rather than the boiler itself. Contact your gas supplier or check your prepayment meter credit. If you smell gas at any point, do not use any switches — leave the property, turn off the gas at the meter if safe to do so, and call the National Gas Emergency Service on 0800 111 999.
- Check for a frozen condensate pipe DIY safe
If the E133 has appeared during or after a cold snap, locate the white plastic pipe (typically 22 mm or 32 mm diameter) that exits your boiler and runs to an outside drain or wall. If any section feels solid or ice is visible, this is the likely culprit. Pour lukewarm — not boiling — water gently over the frozen section until it thaws and water flows freely. Never use a naked flame to thaw the pipe. Once thawed, reset the boiler. To prevent recurrence, lag the pipe with foam pipe insulation or self-regulating heat tape.
- Check and restore the boiler's system pressure DIY safe
Look at the pressure gauge on the boiler's front panel. When the system is cold, the needle should sit between 1 and 1.5 bar. If it reads below 1 bar, repressurise the system using the filling loop (typically a braided hose or built-in lever beneath the boiler). Open the valve(s) slowly and watch the gauge, closing them once the pressure reaches around 1.2 to 1.5 bar. Never overpressurise above 2 bar. Consult your boiler's user guide if you are unsure where the filling loop is on your specific Biasi model.
- Reset the boiler DIY safe
Once you have completed the checks above, attempt a reset. On most Biasi models, press and hold the reset button on the front panel for around 5 to 10 seconds until the display changes and the boiler begins its start-up sequence. Wait up to 10 minutes to see whether it ignites successfully. If it locks out again immediately, try once more — but do not reset more than two or three times in succession. Repeated failed ignition attempts can cause gas to accumulate in the combustion chamber, which is unsafe.
- If the fault persists, contact a Gas Safe registered engineer Gas Safe engineer
If the boiler continues to display E133 after the checks above, the cause is almost certainly something that requires professional diagnosis — most likely a worn ignition electrode, a dirty or failing flame sensor, a gas pressure issue, or a faulty gas valve. Only a Gas Safe registered engineer is legally permitted to work on gas components, ignition systems, and the boiler's internal controls. Ask the engineer to carry out a full combustion check and service at the same time if one is overdue.
Parts you may need
- Ignition electrode · from £55
- Flame sensor / ionisation probe · from £45
- Gas valve · from £180
- PCB (printed circuit board) · from £280
- Condensate pipe insulation / lagging kit · from £12
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 £100–£350, depending on the underlying cause.
Frequently asked questions
Can I fix the Biasi E133 fault myself?
Some of the most common causes are safe for a homeowner to deal with: checking your gas supply is on, thawing a frozen condensate pipe, topping up boiler pressure, and resetting the boiler. If the fault clears after one of these steps, you may not need an engineer at all. However, anything involving the ignition electrode, flame sensor, gas valve, or internal wiring must be handled by a Gas Safe registered engineer — it is both unsafe and illegal to attempt gas component repairs without the correct registration.
Why does E133 keep coming back after I reset the boiler?
A lockout that keeps returning after every reset is telling you the root cause has not been addressed. A one-off interruption — a momentary gas supply blip or a temporary freeze — will usually clear with a single reset. Persistent E133 faults point to a worn ignition electrode, a dirty flame sensor, low gas pressure, or a failing gas valve. At that point, you need an engineer to investigate rather than continuing to reset the boiler.
What is the difference between E133 and ER01 on a Biasi boiler?
They describe exactly the same fault — an ignition lockout — but appear on different generations of Biasi boilers. Older models such as the Riva Plus and Garda use the ER prefix (ER01, ER02), while newer models including the Inovia and Advance Plus use the E-code system (E133, E110). If you see ER01 on your display alongside a red lockout light, treat it identically to E133 and follow the same diagnostic steps.
Could a PCB replacement cost more than a new boiler?
On an older Biasi boiler, it can be close. PCB replacement typically costs £350 to £550 including parts and labour, but on a boiler that is more than 10 years old and may have other components nearing the end of their life, that spend is hard to justify. A Gas Safe engineer should be able to advise you on whether repair or replacement makes more financial sense once they have diagnosed the exact fault. The headline repair cost range on this page reflects the most common repairs (electrode, sensor, or gas valve work) rather than a full PCB replacement.