Baxi E.04-.02 Fault Code: Causes, Fixes & Repair Costs
What does the Baxi E.04-.02 fault code mean?
The E.04-.02 code is a permanent lockout fault on Baxi boilers, indicating that the flow temperature sensor (an NTC thermistor) is showing an open-circuit condition. In practical terms, the boiler's PCB is receiving no valid signal from the sensor that monitors the temperature of water leaving the heat exchanger. Because the boiler cannot safely regulate its output without this data, it shuts down completely and will not restart until the underlying fault is corrected and a reset is performed. The NTC thermistor works by changing its electrical resistance in proportion to water temperature — as the water gets hotter, resistance drops — and this continuous signal tells the PCB how hard the burner needs to work. An open circuit means that electrical path is broken entirely, either at the sensor itself, somewhere in the wiring, or at the PCB connector.
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
- Sensor connector loose or completely unplugged Common
The most frequent cause of an open-circuit reading is a poor or broken connection at the NTC thermistor plug itself. Vibration over time, or a previous service visit where the connector was not fully re-seated, can leave the sensor electrically disconnected from the PCB even though it looks physically in place.
- Damaged or corroded wiring harness Common
The cable running from the flow NTC sensor back to the PCB can suffer from corrosion at the connector pins, chafing against pipework, or heat damage. Any break or high-resistance joint in this harness will present as an open circuit to the PCB, triggering E.04-.02.
- Failed NTC thermistor Sometimes
The thermistor element itself can fail internally, producing an open-circuit resistance rather than the expected value for its temperature. This can happen through normal age-related degradation, scale or magnetite deposits affecting heat transfer to the sensor tip, or water ingress into the sensor body if there is any leakage around the sensor pocket.
- PCB input circuit fault Rare
If the sensor and the entire wiring harness both test within specification but the fault code persists, the input circuitry on the PCB that reads the NTC signal may have failed. This is the least common explanation and should only be considered once all other causes have been ruled out by testing.
How to fix it
- Check the boiler's gas supply and power DIY safe
Before anything else, confirm the boiler has mains power and that the gas supply to the property is on — check other gas appliances such as a hob. A wider power or gas interruption can occasionally trigger sensor-related lockouts. If gas and power are both confirmed, proceed.
- Attempt a single boiler reset DIY safe
Locate the reset button on the boiler's front panel — on most Baxi models it is clearly labelled. Press and hold it for approximately 3–5 seconds until the boiler attempts to restart. If E.04-.02 clears and the boiler runs normally, monitor it over the next hour. If the code returns immediately or after a short time, do not keep resetting — repeated resets will not fix a hardware fault and you should move on to calling an engineer.
- Gas Safe engineer: visually inspect the flow NTC sensor and wiring Gas Safe engineer
The engineer will locate the flow temperature sensor on the outlet pipe from the heat exchanger (position varies by model — refer to the Baxi service manual). They will check that the sensor connector is fully and securely plugged in, inspect the wiring harness along its entire run for signs of chafing, heat damage, or corrosion, and look for any evidence of water leakage around the sensor pocket that may have damaged the connector or sensor body.
- Gas Safe engineer: test sensor resistance with a multimeter Gas Safe engineer
With the boiler isolated from the mains, the engineer will disconnect the thermistor wiring at the PCB end to eliminate stray parallel resistances, then use a multimeter on the ohms range to measure the sensor's resistance. This reading is compared against Baxi's specification for the current water temperature. A correct NTC sensor shows a finite, temperature-dependent resistance (typically several kilohms at room temperature); a truly open circuit will read infinite or over-range, confirming sensor failure.
- Gas Safe engineer: replace the flow NTC thermistor if faulty Gas Safe engineer
If the sensor fails the resistance test, the engineer will fit a replacement Baxi-compatible NTC thermistor. The old sensor is removed from its pocket or mounting point, the new one installed and secured, and the wiring connector firmly re-attached. The engineer will then power the boiler up, reset the lockout, and run the boiler through a heating cycle to confirm the fault has cleared and that there are no leaks around the sensor pocket.
- Gas Safe engineer: inspect and test the PCB if sensor and wiring are both sound Gas Safe engineer
If the NTC sensor and its wiring harness both test correctly but E.04-.02 persists, the PCB's NTC input circuit is the likely culprit. The engineer will carry out further electrical tests on the board. If the PCB is confirmed faulty it will need to be replaced — this is a more expensive repair, so it is worth getting a quote and comparing it against the cost of a new boiler if the appliance is already old.
- Call a Gas Safe registered engineer Gas Safe engineer
Because E.04-.02 is a permanent lockout involving internal electrical components, all diagnostic and repair work beyond the basic reset must be carried out by a Gas Safe registered engineer. You can verify a heating engineer's registration at gassaferegister.co.uk. Always ask for a fixed-price quote covering both parts and labour before authorising any work.
Parts you may need
- Baxi flow NTC thermistor sensor · from £12
- NTC sensor wiring harness / loom · from £35
- Baxi PCB (model-specific) · 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 £100–£220, depending on the underlying cause.
Frequently asked questions
Can I fix Baxi E.04-.02 myself by replacing the sensor?
No — and for two reasons. First, accessing the flow NTC sensor requires removing the boiler casing and working on internal components, which legally must only be done by a Gas Safe registered engineer in the UK. Second, the sensor itself is cheap (around £10–£15 for the part), so the labour cost of a professional visit is the main expense regardless. Attempting the work yourself could invalidate your boiler warranty, your home insurance, and could put you at legal risk. The only safe DIY step is a single reset attempt.
How much does it typically cost to fix E.04-.02 on a Baxi boiler?
For most UK homeowners, a straightforward flow NTC sensor replacement — covering the call-out, engineer's time, and the part — comes to roughly £100–£220. The sensor itself is inexpensive (under £30), so the bulk of the bill is labour. If the fault turns out to be in the wiring harness the cost is similar. A PCB replacement is significantly more expensive and can push the total to £300–£450 or beyond depending on the model, so if the PCB is the diagnosis on an older boiler it is worth comparing that quote against the cost of a new appliance.
Why has my Baxi boiler locked out and won't reset after E.04-.02?
E.04-.02 is classed as a permanent fault, meaning the boiler is designed to stay locked out until the root cause is resolved — a reset alone will not clear it if the sensor or wiring remains broken. The PCB needs to see a valid, in-range resistance signal from the flow NTC before it will allow normal operation. If your reset attempt is immediately followed by the same code reappearing, the sensor, connector, or harness is almost certainly the issue and an engineer visit is required.
Will an annual boiler service help prevent E.04-.02 from occurring?
Yes, it can. During a service a Gas Safe engineer will inspect sensor connectors and wiring for early signs of corrosion or loose connections, and will check for any water leaks near the sensor pocket — all of which are precursors to the open-circuit fault that triggers E.04-.02. A service typically costs £80–£120 and is also a condition of most Baxi extended warranties. Catching a corroding connector during a routine visit is far cheaper than an emergency callout in winter.