Baxi E.04-.05 Fault Code: Causes, Fixes & Repair Costs
What does the Baxi E.04-.05 fault code mean?
The Baxi E.04-.04 and E.04-.05 fault codes both indicate a permanent lockout caused by a problem with the flue gas temperature sensor (an NTC thermistor). Specifically, the boiler's PCB has detected that this sensor has short-circuited, is not connected at all, or has a poor connection that is producing unreliable readings. Because the PCB continuously monitors the resistance signal from this thermistor to track flue gas temperature, a short-circuit causes wildly incorrect data to reach the control board — and as a safety precaution, the boiler shuts down completely and will not restart until the fault is resolved and a manual reset is performed. These are classed as permanent faults, meaning a reset alone will not clear them without the underlying issue being fixed first. Two related sub-codes worth knowing: E.04-.06 triggers when a critically high flue gas temperature is actually reached (often alongside a circulation or sensor fault), and E.04-.08 indicates the boiler has hit its maximum safe temperature limit, which can also involve the safety thermostat opening or 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
- Failed flue NTC thermistor Common
The sensor itself has developed an internal short-circuit, meaning its resistance output is no longer reliable. This is the most frequent root cause of both E.04-.04 and E.04-.05 and almost always requires the sensor to be replaced.
- Disconnected or poorly seated wiring connector Common
The plug connecting the flue sensor to the wiring harness can work loose over time due to vibration or handling during previous servicing. Even a slightly unseated connector can cause the PCB to read the circuit as shorted.
- Damaged or corroded wiring harness Sometimes
The cable running from the flue sensor back to the PCB can suffer from heat damage, moisture ingress, or physical pinching. Corrosion at the connector terminals is a particular issue in older boilers or those installed in damp locations such as garages or outbuildings.
- PCB fault Rare
If a replacement sensor and fresh wiring connections do not clear the fault, the PCB itself may be unable to correctly interpret the sensor signal, either due to a failed input circuit on the board or component-level damage. This is the least common cause but the most expensive to remedy.
How to fix it
- Attempt a single boiler reset DIY safe
Locate the reset button on the front panel of your Baxi boiler (it is usually marked with a flame or reset symbol). Press and hold it for approximately 3–5 seconds until the boiler attempts to restart. Because E.04-.04 and E.04-.05 are permanent faults, a reset alone is very unlikely to produce a lasting fix — but it confirms the lockout is active and rules out a one-off transient glitch. Do not reset more than two or three times, as repeated failed resets will not resolve the underlying problem.
- Check that all other services are functioning normally DIY safe
Confirm your gas supply is working by testing another gas appliance such as a hob or gas fire. Check that your system pressure is between 1 and 1.5 bar on the boiler's pressure gauge — if it has dropped below 1 bar, top it up via the filling loop before calling an engineer, as low pressure can sometimes contribute to secondary fault codes.
- Arrange for a Gas Safe engineer to inspect the flue sensor wiring Gas Safe engineer
A registered engineer will access the flue sensor wiring harness and check for any obvious disconnections, signs of heat damage, corrosion at the terminals, or a connector that has not been fully clicked home. This is often the quickest and cheapest fix if the sensor itself is still intact.
- Engineer to test and replace the flue NTC sensor if required Gas Safe engineer
Using a multimeter, the engineer will check the sensor's resistance value against the manufacturer's specification at a known temperature. A short-circuited sensor will show near-zero resistance regardless of temperature and will need to be swapped out. The replacement part must be compatible with your specific Baxi model — always use a genuine Baxi part or a verified equivalent to maintain warranty and safety standards.
- Engineer to inspect the PCB if the fault persists after sensor replacement Gas Safe engineer
Should a confirmed replacement sensor and sound wiring still not clear the E.04-.04 or E.04-.05 code, the PCB's sensor-input circuit may be damaged. The engineer will assess whether a PCB repair or full board replacement is cost-effective relative to the age and condition of the boiler.
- Call a Gas Safe registered engineer Gas Safe engineer
All work beyond a basic reset and pressure top-up must be carried out by a Gas Safe registered engineer. You can verify any engineer's registration at the Gas Safe Register website (gassaferegister.co.uk). Always ask for a written quote before work begins, and check whether your Baxi boiler is still within its manufacturer warranty — Baxi offers up to 10 years' cover via accredited installers, provided annual services have been kept up to date.
Parts you may need
- Baxi flue NTC thermistor sensor · from £35
- Sensor wiring harness / loom · from £45
- Baxi PCB (printed circuit board) · from £260
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–£280, depending on the underlying cause.
Frequently asked questions
Why won't my Baxi boiler stay on after I reset it?
E.04-.04 and E.04-.05 are classified as permanent faults, which means the boiler's safety system requires the root cause to be fixed before the reset will hold. Resetting simply clears the lockout flag temporarily — if the flue sensor is still short-circuited or disconnected, the boiler will detect the fault again almost immediately and lock out once more. You will need a Gas Safe engineer to replace the sensor or repair the wiring before normal operation resumes.
What is the difference between E.04-.04, E.04-.05, E.04-.06, and E.04-.08?
All four are permanent lockout codes under the E.04 family, but they point to slightly different conditions. E.04-.04 and E.04-.05 both relate to a short-circuited, disconnected, or faulty flue NTC sensor — .05 typically indicates a wiring or connection issue alongside or instead of a sensor failure. E.04-.06 fires when the flue gas temperature actually reaches a critically high value, often combined with a circulation problem or sensor fault. E.04-.08 means the boiler has hit its absolute maximum safe temperature limit, which can involve the safety thermostat opening, poor water circulation, or again a sensor issue. If you see any of these, a Gas Safe engineer is needed.
How much does it cost to fix a Baxi E.04-.04 or E.04-.05 fault in the UK?
For a straightforward flue NTC sensor replacement including parts and labour, most homeowners in the UK pay between £120 and £280. The sensor itself typically costs £15–£50, and a Gas Safe engineer's call-out plus labour accounts for the rest. If the PCB also turns out to be faulty, the board alone can cost £180–£350 in parts, pushing the total repair bill considerably higher — in that scenario it is worth comparing the repair cost against a new boiler, particularly if your boiler is over 10 years old. Prices in London and the South East tend to sit at the higher end of any quoted range.
Can I fix the Baxi E.04-.04 fault myself?
The only steps a homeowner can safely take are attempting a reset (no more than two or three times) and checking that system pressure is above 1 bar. All other work — inspecting wiring, testing sensor resistance, replacing the NTC thermistor, or touching the PCB — must be done by a Gas Safe registered engineer. Working on flue components without registration is illegal under the Gas Safety (Installation and Use) Regulations 1998 and carries serious risks including carbon monoxide poisoning.