Baxi E.01-.12 Fault Code: Causes, Fixes & Repair Costs
What does the Baxi E.01-.12 fault code mean?
Baxi's E-series codes are permanent lockout faults — the boiler has shut itself down and will not restart until the underlying problem is resolved and the reset button is pressed manually. The two-part number after the dot narrows the fault right down: for example, E.01–.04 means repeated flame loss, E.01–.11 points to a fan speed problem, E.02–.07 flags low system pressure, and E.04–.08 indicates the boiler has reached an unsafe temperature. All codes in the E.01–E.12 family work the same way — the boiler locks out, displays the code, and waits. Common sub-codes you may see include: E.01–.02 (return sensor reading higher than flow sensor), E.01–.04 (flame lost five times in 24 hours), E.01–.05 (return temperature sensor open-circuit), E.01–.11 (incorrect fan speed or draught), E.01–.12 (return temp greater than flow temp), E.01–.17 (no circulation detected), E.01–.20 (flue gas temperature above 140°C), E.02–.07 (low or no system water pressure), E.02–.16 (onboard control unit time-out — possible PCB damage), E.02–.17 (communication lost between gas valve and PCB), E.04–.01 (flow sensor short-circuit), E.04–.02 (flow sensor open-circuit), E.04–.03 (critically high flow temperature due to poor circulation), E.04–.04 (flue temperature sensor short-circuit), E.04–.08 (maximum safe temperature reached — safety thermostat open), and E.04–.10 (failure to ignite — check gas supply).
General guidance only — not a substitute for professional advice. 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
- Low system water pressure (E.02–.07) Common
A pressure reading below 0.5 bar is enough to trigger an immediate lockout. This is the most straightforward fault in the E-series family and is the first thing to check on any lockout. Slow leaks at radiator valves, the pressure relief valve, or system joints gradually bleed pressure away over weeks or months.
- Faulty or disconnected NTC temperature sensor Common
Baxi boilers rely on NTC thermistor sensors at the flow, return, and flue positions. These fail in two ways: a complete open-circuit (instant lockout, codes such as E.01–.02, E.01–.05, E.04–.01, E.04–.02, E.04–.04) or a gradual drift in resistance that throws off temperature readings and can cause phantom overtemperature or reversed-sensor faults. A loose connector on the wiring loom can mimic a dead sensor entirely.
- Repeated flame loss or ignition failure (E.01–.04, E.04–.10) Common
If the boiler fails to establish or hold a flame five times within 24 hours it locks out permanently. Causes include a weak or contaminated flame sensing electrode, worn ignition electrodes, a disrupted gas supply, or a gas valve that is not opening fully. A partial blockage of the flue or air intake can also destabilise combustion enough to cause repeated nuisance flame-outs.
- Circulation failure (E.01–.17, E.04–.03, E.01–.20) Sometimes
Insufficient water movement through the heat exchanger causes the flow temperature to spike while the return stays cool — or the flue gas temperature exceeds 140°C as heat cannot be carried away. The root cause is usually a failing or seized pump, a stuck or partially closed zone valve, heavily sludged pipework, or a combination of low pressure and airlocks.
- Incorrect fan speed or draught problem (E.01–.11) Sometimes
The boiler's pre-purge and post-purge fan speeds are monitored electronically. Strong external wind across the flue terminal, a worn fan bearing, or a defective gas/air unit can all cause the fan to run outside its permitted speed band. The boiler locks out rather than attempt to fire with potentially unsafe combustion air flow.
- PCB or communication fault (E.02–.16, E.02–.17) Rare
E.02–.16 indicates the onboard control and safety unit has timed out — prolonged operation in this state risks damaging the PCB itself, so it should not simply be reset repeatedly. E.02–.17 means the PCB has lost communication with the gas valve entirely, which prevents any firing. Relay contacts on older boards pit and oxidise over time, causing intermittent control losses.
How to fix it
- Note the full fault code on the display DIY safe
Write down the complete code including the sub-number (e.g. E.01–.11, E.02–.07). The sub-code tells you — and any engineer you call — exactly which system triggered the lockout, saving diagnostic time and potentially money.
- Check the pressure gauge DIY safe
Look at the pressure gauge on the boiler fascia. It should read between 1.0 and 1.5 bar when the system is cold. A reading below 1 bar — or below 0.5 bar for E.02–.07 — means the system needs repressurising. If the gauge reads zero or the needle is not moving at all, there may be a significant leak that needs professional attention before you top up.
- Repressurise via the filling loop if pressure is low DIY safe
Locate the filling loop (usually a flexible braided hose with one or two valves beneath the boiler). Open the valve(s) slowly until the gauge climbs to around 1.2–1.5 bar, then close them fully. Never overfill above 2 bar. If pressure drops again within a few days, there is a leak somewhere in the system that an engineer needs to find.
- Bleed radiators if you suspect an airlock DIY safe
Trapped air can prevent proper water circulation and contribute to overtemperature faults. Use a radiator bleed key on the bleed valve at the top of each radiator, starting on the ground floor. Release air until water flows steadily, then check the system pressure again and top up if it has dropped.
- Reset the boiler (up to twice) DIY safe
Press and hold the reset button on the front panel for 3–5 seconds until the boiler attempts to restart. If it relights and runs normally, monitor it over the next 24–48 hours. If it locks out again with the same code, do not keep resetting — repeated resets without fixing the cause can mask a worsening problem or, in the case of E.02–.16, risk PCB damage.
- Check that your gas supply is working DIY safe
Turn on a gas hob or fire to confirm gas is reaching your property. If no gas appliances are working, contact your gas supplier — there may be a supply interruption or your meter may have tripped. Do not attempt to work on the gas meter or any pipework yourself.
- Call a Gas Safe registered engineer for all remaining faults Gas Safe engineer
If the boiler relocks after a reset, or the code points to a sensor fault, fan fault, circulation problem, overtemperature condition, PCB issue, or anything gas-related, a Gas Safe registered engineer must carry out the diagnosis and repair. This includes all E.01–.11, E.01–.04, E.01–.17, E.01–.20, E.02–.16, E.02–.17, E.04–.03, E.04–.08, and E.04–.10 faults. You can find a local engineer at the Gas Safe Register website (gassaferegister.co.uk).
Parts you may need
- NTC flow or return temperature sensor · from £25
- Flue/overheat thermostat sensor · from £30
- Circulation pump · from £85
- Fan assembly (gas/air unit) · from £120
- Ignition/flame sensing electrode · from £20
- PCB (main printed circuit board) · from £200
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 just keep resetting my Baxi boiler to clear the E fault code?
A single reset is reasonable — it confirms the lockout is real and gives the boiler one chance to recover from a transient fault. Two resets are the absolute maximum before you should stop and call an engineer. Repeated resets without identifying the cause can mask a worsening problem, and for code E.02–.16 in particular, continuing to reset risks causing permanent damage to the PCB. If the boiler locks out again with the same code, the underlying fault needs fixing first.
My Baxi is showing E.01–.11 — is it safe to use the boiler?
No. E.01–.11 means the fan is running at an incorrect speed, which could indicate a draught problem at the flue terminal or a failing fan. The fan controls the safe removal of combustion gases, so the boiler must not be used until a Gas Safe registered engineer has inspected and resolved the fault. Do not attempt to reset the boiler repeatedly while this code is showing.
How much does it typically cost to fix a Baxi E-series lockout fault?
Most repairs fall in the £100–£350 range. A sensor replacement (NTC thermistor) is usually at the lower end — around £100–£200 including labour — while a pump or fan replacement typically costs £150–£350. Straightforward repressurisation is free if you do it yourself, or £50–£150 with an engineer call-out. More complex repairs such as a full PCB replacement can run to £300–£500 including parts and labour, and a gas valve replacement is similar. If a heat exchanger is involved, costs can exceed £500 — at which point replacement of an older boiler is often worth considering.
What is the difference between an E code and an H code on a Baxi boiler?
On Baxi boilers, E codes are permanent lockouts — the boiler has shut down and will not restart without a manual reset once the fault is resolved. H codes (such as H.01.xx) are temporary or informational statuses that the boiler may clear by itself without any intervention. If your display shows an H code and the boiler is still running or trying to run, monitor it first before calling an engineer. If it changes to an E code, treat that as a full lockout requiring attention.