Baxi E.01-.02 Fault Code: Causes, Fixes & Repair Costs
What does the Baxi E.01-.02 fault code mean?
Baxi uses a two-part fault code system where the number before the dash identifies the fault category and the sub-code after the dash pinpoints the specific problem. Both E.01 and E.02 trigger a permanent lockout — the boiler shuts down completely and will not restart until the underlying fault is resolved and a manual reset is performed. E.01 sub-codes relate primarily to temperature sensor readings, circulation failures, and flue gas overheating. E.02 sub-codes cover system-level issues including low water pressure, PCB faults, and gas valve communication errors. Key sub-codes to know: E.01–.02 (return temperature reading higher than flow temperature — sensor fault or bad connection); E.01–.12 (return sensor temperature exceeds flow temperature, requires reset); E.01–.17 (no circulation detected — sensor not connected or circulation fault); E.01–.20 (flue gas temperature exceeded 140°C — likely a circulation or sensor problem); E.02–.00 (reset in progress); E.02–.07 (system pressure below 0.5 bar); E.02–.16 (CSU timeout or PCB damage); E.02–.17 (communication failure between PCB and gas valve); E.02–.19/.20/.21 (jumper configuration changed on PCB); E.02–.47 (wiring fault to an external device).
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
- Faulty or poorly connected NTC temperature sensor Common
The flow or return NTC sensor is either disconnected, has a loose wiring connection, or has failed entirely. When the return sensor reads a higher temperature than the flow sensor, the boiler interprets this as a dangerous or impossible condition and locks out. This is the most common cause of E.01–.02 and E.01–.12.
- Low system water pressure (E.02–.07) Common
If the pressure in the central heating circuit drops below 0.5 bar — usually due to a slow leak, a recently bled radiator, or a failing expansion vessel — the boiler will refuse to fire and display E.02–.07. Normal operating pressure for a Baxi boiler should sit between 1.0 and 1.5 bar when the system is cold.
- Circulation failure or seized pump Common
A pump that has seized, is running too slowly, or is blocked by sludge will cause the system water to stagnate and overheat in localised areas. The boiler detects abnormal temperature differentials between flow and return and triggers E.01–.17 or E.01–.20 as a safety response.
- PCB fault or CSU timeout (E.02–.16) Sometimes
The main printed circuit board has either experienced a channel service unit timeout or suffered direct damage — possibly from a power surge, moisture ingress, or component failure. When the PCB cannot operate reliably, the boiler locks out permanently until the board is repaired or replaced.
- Gas valve communication error (E.02–.17) Sometimes
The PCB has lost reliable communication with the gas valve, or there is a fault within the gas valve assembly itself. Because the boiler cannot safely control the gas flow, it stays locked out until the electrical connection or component is fixed by a qualified engineer.
- Flue gas overheating (E.01–.20) Sometimes
The flue gas temperature sensor has recorded readings above 140°C. This can result from a partial flue blockage, a poorly designed or incorrectly installed flue, or a combination of inadequate circulation and combustion issues. The boiler locks out to prevent damage and potential danger.
- Jumper or wiring configuration error (E.02–.19/.20/.21 and E.02–.47) Rare
A jumper connector on the PCB has been moved or is missing, changing the boiler's configuration unexpectedly — sometimes after a power interruption or a previous repair. Similarly, E.02–.47 indicates a wiring fault to an external device such as a programmer or zone valve. Both require an engineer to inspect and correct.
How to fix it
- Check your gas supply is working DIY safe
Before anything else, confirm that other gas appliances in your home (hob, gas fire) are working normally. If there is no gas supply, contact your gas supplier or call the National Gas Emergency Service on 0800 111 999. Do not attempt to reset the boiler if you suspect a gas supply issue.
- Check and top up system pressure if you see E.02–.07 DIY safe
Look at the pressure gauge on your boiler — it should read between 1.0 and 1.5 bar when the system is cold. If it reads below 1.0 bar (and particularly below 0.5 bar for E.02–.07), locate your filling loop — usually a silver or braided hose beneath the boiler with one or two valves — and slowly open the valve(s) until the gauge climbs to around 1.2–1.5 bar, then close the valve(s) firmly. If the pressure keeps dropping over days or weeks, there is likely a leak somewhere in the system that an engineer needs to find.
- Bleed any radiators that feel cold at the top DIY safe
Air trapped in radiators can contribute to poor circulation and temperature imbalance faults. Use a radiator bleed key to release any trapped air from the highest and coldest radiators first. Remember to check and top up the system pressure again afterwards, as bleeding radiators will reduce it slightly.
- Attempt a boiler reset DIY safe
Once you have addressed any pressure issue, press and hold the reset button on the boiler's front panel for approximately 2–3 seconds, or until the boiler begins its restart sequence. Allow it a minute or two to complete its ignition cycle. Do not reset the boiler more than two or three times — repeated unsuccessful resets can mask a worsening fault and may cause additional damage.
- Check visible sensor wiring for obvious damage DIY safe
If you are comfortable doing so, visually inspect the wiring around the boiler casing for any cables that appear burnt, pinched, or disconnected at their plug connectors. Do not touch internal components or attempt to reconnect anything — this check is limited to what is visible without removing panels. If something looks obviously wrong, report it to your engineer.
- Call a Gas Safe registered engineer for persistent or safety-related faults Gas Safe engineer
If the fault returns after a reset, or if the sub-code points to a sensor fault (E.01–.02, E.01–.12, E.01–.17), flue overheating (E.01–.20), PCB damage (E.02–.16), a gas valve communication error (E.02–.17), or a wiring/jumper issue (E.02–.19 to .21, E.02–.47), do not attempt further DIY work. A Gas Safe registered engineer will carry out a full diagnostic, test the sensors, check pump operation, inspect the flue, and replace any faulty components safely and legally.
Parts you may need
- NTC flow or return temperature sensor · from £25
- Central heating pump (e.g. Grundfos UPS2 15-50) · from £65
- Baxi replacement PCB · from £180
- Gas valve assembly · from £120
- Flue gas temperature sensor · from £30
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 a Baxi E.01 or E.02 fault myself?
It depends on the sub-code. If you see E.02–.07, you can often resolve it yourself by topping up the system pressure to around 1.2–1.5 bar using the filling loop — no tools or qualifications required. A basic boiler reset is also safe to try up to two or three times. However, faults involving temperature sensors, the pump, the PCB, the gas valve, or the flue must be diagnosed and repaired by a Gas Safe registered engineer. Attempting internal boiler repairs without Gas Safe registration is illegal and potentially dangerous.
Why does my Baxi keep showing E.01–.02 even after I reset it?
If the E.01–.02 code reappears every time you reset the boiler, the underlying cause has not been fixed. The most likely culprits are a faulty or disconnected return NTC temperature sensor, or a sensor that is correctly connected but has drifted out of calibration and is giving inaccurate readings. In some cases, a seized pump causing poor circulation can also produce a temperature inversion between flow and return. An engineer will test the sensor resistance with a multimeter and replace it if needed — typically a straightforward and relatively affordable repair.
How much does it cost to repair a Baxi E.01 or E.02 fault?
Most people pay somewhere between £100 and £350 all-in. A sensor reconnection or replacement at the lower end, a pump replacement in the middle, and a pump plus system power-flush at the higher end covers the majority of cases. PCB replacement is less common but more expensive — typically £300 to £500 including parts and labour — and if your boiler is still under its Baxi warranty (2–10 years depending on the installer), that cost may be covered, so check before you pay.
Is it safe to keep using my boiler if it shows E.01 or E.02?
No. Both E.01 and E.02 are permanent lockout codes, meaning the boiler has already shut itself down and will not fire until the fault is cleared. This is a deliberate safety measure — the boiler has detected a condition it considers unsafe to continue operating in. Do not attempt to bypass the lockout or repeatedly reset the boiler hoping it will clear. Arrange for a Gas Safe engineer to inspect it promptly, particularly if the sub-code suggests overheating, gas valve issues, or PCB failure.