Baxi E.00-.04 Fault Code: Causes, Fixes & Repair Costs
What does the Baxi E.00-.04 fault code mean?
Baxi boilers display fault codes in two parts — the prefix and a suffix — and **E.00–.04** and **E.04–.00** are two entirely different faults that happen to share similar-looking digits. Read both parts of the code on your display carefully before attempting any diagnosis. **E.00–.04** signals a permanent lockout caused by an open-circuit on the return temperature sensor. The boiler's control board is receiving no signal from the NTC thermistor that monitors the temperature of water returning from your heating system — either because the sensor has failed, its wiring has broken, or a connector has come loose. The boiler shuts down completely and will not restart until the fault is resolved and the boiler is manually reset. **E.04–.00** signals a permanent lockout caused by the boiler failing to detect its gas valve. The PCB has lost communication with the gas valve — this could be a wiring issue, a problem with the air/gas (venturi) assembly, or a failed valve. Because gas is involved, this fault is treated as a safety-critical lockout. Both codes are classed as **Permanent Faults** on Baxi boilers, meaning the appliance will remain off until the underlying problem is fixed — do not attempt to keep resetting the boiler as a workaround. Other faults in the E.04 series worth knowing: **E.04–.01** and **E.04–.02** relate to the flow temperature sensor (short-circuit and open-circuit respectively); **E.04–.03** indicates critical flow temperature, often due to poor circulation; **E.04–.04** and **E.04–.05** point to the flue temperature sensor; and **E.04–.08** means the safety thermostat has opened, usually because of a circulation failure.
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
- Return NTC sensor failure (E.00–.04) Common
The NTC thermistor that reads the return water temperature has developed an internal fault and is no longer producing a valid resistance signal. This is the most frequent root cause of E.00–.04 and usually requires the sensor to be replaced.
- Loose or disconnected sensor wiring (E.00–.04) Common
The connector or wiring between the return temperature sensor and the PCB has worked loose or become detached — possibly due to vibration over time or a disturbed connection during a previous service. The board sees an open circuit and locks out.
- Corroded or damaged sensor terminals (E.00–.04) Sometimes
Moisture ingress or age can corrode the sensor's electrical terminals or the wiring loom connector, increasing resistance to the point where the PCB can no longer read the sensor correctly.
- Gas valve wiring loose or broken (E.04–.00) Common
The wiring harness connecting the gas valve to the PCB has a poor connection, a broken wire, or a damaged connector. The board cannot detect the valve and triggers a lockout as a safety measure.
- Failed or seized gas valve (E.04–.00) Common
The gas valve itself has failed — either stuck, seized, or electrically open-circuit on one of its coils. Gas valves can wear out over years of cycling and may need replacement.
- Faulty air/gas venturi assembly (E.04–.00) Sometimes
The gas-air unit (venturi) that the valve is integrated with may be defective or blocked, preventing the PCB from correctly detecting or communicating with the gas valve circuit.
- PCB failure (either code) Rare
If wiring and components test correctly but the fault persists, the PCB itself may have failed — either losing the ability to read the sensor signal (E.00–.04) or losing communication with the gas valve (E.04–.00).
How to fix it
- Do not keep resetting the boiler repeatedly DIY safe
Both E.00–.04 and E.04–.00 are permanent lockout codes. A single reset attempt is reasonable to see if the fault clears, but if it returns immediately, further resets will not help and may mask the problem. Stop after one or two attempts.
- Check that your gas supply is working DIY safe
For E.04–.00 especially, confirm that other gas appliances in your home (hob, gas fire) are working normally. If they are not, contact your gas supplier — the issue may be with the incoming supply rather than the boiler.
- Check system pressure DIY safe
Low water pressure (below around 0.8 bar) can contribute to sensor-related faults. Check the pressure gauge on the boiler front — it should typically read between 1.0 and 1.5 bar when cold. If it is low, top up using the filling loop and reset the boiler. Instructions for this are in your user manual.
- Have a Gas Safe engineer inspect the return temperature sensor wiring (E.00–.04) Gas Safe engineer
An engineer will trace the sensor wiring from the return pipework connection to the PCB, checking for any broken wires, corroded terminals, or dislodged connectors. A simple reconnection can resolve this fault if a connector has simply worked loose.
- Have an engineer test the return NTC sensor resistance (E.00–.04) Gas Safe engineer
Using a multimeter, the engineer will check the resistance of the NTC sensor against the manufacturer's reference chart. A reading outside ±10% of the expected value at a known temperature confirms sensor failure and the need for replacement.
- Have an engineer inspect the gas valve wiring and connectors (E.04–.00) Gas Safe engineer
The engineer will check the wiring harness at the gas valve and at the PCB for broken wires, loose connectors, or signs of heat damage. This is a gas-adjacent task and must only be carried out by a Gas Safe registered engineer.
- Have an engineer test and if necessary replace the gas valve or air/gas unit (E.04–.00) Gas Safe engineer
The engineer will check for correct voltage reaching the gas valve coils. If voltage is present but the valve is not responding, or if the venturi assembly is found to be defective, replacement of the gas valve or the combined gas-air unit will be required.
- If sensor and wiring are sound, have the PCB assessed Gas Safe engineer
Should all sensor and wiring tests pass but the fault code persists, the PCB may be failing to correctly process the signal. A Gas Safe engineer with Baxi experience can advise whether PCB replacement or an upgrade is the most cost-effective route given the boiler's age.
- Call a Gas Safe registered engineer Gas Safe engineer
Both of these codes require professional diagnosis and repair. If you have not already done so, contact a Gas Safe registered engineer. You can verify any engineer's registration at gassaferegister.co.uk. Attempting to work on gas valves, sensors wired into the gas circuit, or PCBs without the correct qualifications is unsafe and illegal.
Parts you may need
- Return NTC temperature sensor · from £25
- Flow NTC temperature sensor (related E.04 series faults) · from £25
- Gas valve (Baxi compatible) · from £110
- Air/gas venturi assembly · from £85
- Wiring harness/loom · from £40
- PCB (printed circuit board) · from £180
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 £120–£350, depending on the underlying cause.
Frequently asked questions
What is the difference between E.00–.04 and E.04–.00 on a Baxi boiler?
They look similar but are completely unrelated faults. E.00–.04 is an open-circuit fault on the return temperature sensor — a relatively straightforward sensor or wiring repair. E.04–.00 is a gas valve detection failure, which is more serious and always requires a Gas Safe engineer since it directly involves gas components. Always read both halves of the code on your Baxi display before drawing any conclusions.
Can I fix either of these fault codes myself?
There are a couple of checks you can safely do yourself: confirm your gas supply is on, check and top up boiler pressure if it is below 1 bar, and try a single reset. Beyond that, both codes involve either sensor wiring inside the boiler or gas valve components, so further investigation and repair must be carried out by a Gas Safe registered engineer. Attempting DIY work on gas components is unsafe and could invalidate your boiler warranty.
How much does it typically cost to repair an E.00–.04 or E.04–.00 fault?
Most homeowners pay somewhere between £120 and £350 for these repairs, depending on whether the issue turns out to be a loose connector (cheaper) or a full sensor or gas valve replacement (more involved). A return NTC sensor typically costs around £100–£180 including labour; a gas valve replacement usually runs £180–£280 all-in. If the PCB is the root cause, that is a more expensive repair — often £300–£500 — and may prompt a conversation with your engineer about whether replacing an older boiler is a better investment.
Why won't my Baxi boiler reset after I clear the E.00–.04 or E.04–.00 code?
Both are permanent lockout faults, which means the boiler's safety system will re-lock every time you reset it until the underlying fault is actually repaired. Repeatedly resetting without fixing the cause will not help and may indicate that the sensor, wiring, or gas valve component genuinely needs attention. If the boiler locks out again immediately after a reset, stop resetting and arrange for an engineer visit.
Does this fault affect my Baxi warranty?
Baxi offers warranties of up to 10 years on new boilers installed by an accredited Baxi installer, but annual servicing by a Gas Safe engineer is required to keep the warranty valid. If your boiler is within the warranty period and has been serviced annually, contact Baxi directly — the repair may be covered. Using a non-registered person to attempt repairs will void the warranty.