Baxi E.04-.01 Fault Code: Causes, Fixes & Repair Costs
What does the Baxi E.04-.01 fault code mean?
The E.04-.01 code tells you that the boiler's flow temperature sensor — a small NTC thermistor that monitors how hot the water leaving the heat exchanger is — has developed a short circuit. In a short-circuit condition, the sensor's electrical resistance drops to near zero, which makes the PCB believe the water is dangerously overheated. As a safety response, the boiler shuts down completely and locks out. Because this is classed as a permanent locking fault, the boiler will not restart by itself; it needs the underlying problem fixed before a reset will hold. Note the two closely related codes: E.04-.02 indicates the same sensor has gone open-circuit (disconnected or broken wire), and E.04-.03 signals that the maximum flow temperature has actually been exceeded or the sensor has short-circuited under a high-temperature condition.
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 or degraded NTC thermistor Common
The flow temperature sensor itself is the most common culprit. Over time, scale build-up on the sensor tip, magnetite contamination in the system water, or simple age-related degradation can cause the thermistor's resistance to collapse, producing the short-circuit reading that triggers E.04-.01.
- Damaged or corroded sensor wiring Sometimes
The cable running between the NTC sensor and the PCB can chafe, corrode, or become pinched inside the boiler casing. Any point where two conductors touch will create a short circuit and generate the same fault code even if the sensor itself is perfectly healthy.
- Loose or corroded wiring connector Sometimes
The plug-in connectors at the sensor end or at the PCB can work loose or corrode over time, especially in boilers exposed to fluctuating humidity. A poor connection can present electrically as a short circuit without any visible wire damage.
- PCB fault Rare
If the circuit board has suffered an electrical surge, capacitor deterioration, or relay wear, it can misinterpret valid sensor signals and throw a false E.04-.01 code. This is generally only confirmed once a new sensor and fresh wiring connections have failed to clear the fault.
How to fix it
- Check your gas supply is working DIY safe
Before assuming the boiler is at fault, quickly confirm that other gas appliances in your home (hob, gas fire) are working normally. If there is no gas supply, contact your supplier rather than resetting the boiler.
- Attempt a single reset DIY safe
Press and hold the reset button on your Baxi boiler for around three seconds, then wait two minutes to see whether the boiler fires up. Because E.04-.01 is a permanent locking fault, a reset alone is unlikely to cure an underlying sensor failure — but it can clear a one-off transient glitch. Do not reset more than two or three times in quick succession; repeated lockouts without a successful start mean the fault is real and needs investigation.
- Note the exact code displayed and when it appeared DIY safe
Write down the full code (E.04-.01) along with any pattern you have noticed — for example, whether it locks out immediately on start-up or only after running for a while. This information is genuinely useful to the engineer and can speed up diagnosis.
- Switch the boiler off and call a Gas Safe registered engineer Gas Safe engineer
Because this fault involves internal electrical components and requires the boiler casing to be opened, all further diagnosis and repair must be carried out by a qualified Gas Safe registered engineer. Turn the boiler off at the programmer or thermostat while you wait. Do not attempt to open the boiler casing, touch wiring, or handle the sensor yourself.
- Engineer tests the NTC thermistor with a multimeter Gas Safe engineer
The engineer will disconnect the flow sensor and measure its resistance in ohms, comparing the reading against the manufacturer's temperature-resistance chart. A near-zero resistance reading confirms a short circuit in the sensor. If the resistance is correct, attention shifts to the wiring loom and connectors.
- Engineer inspects and reseats wiring connections Gas Safe engineer
All connectors between the sensor and the PCB are checked for corrosion, looseness, or chafing. Reseating or replacing a corroded connector is a quick, low-cost fix that sometimes resolves the fault entirely without needing a new sensor.
- Engineer replaces the flow temperature sensor if required Gas Safe engineer
If the thermistor has failed, the engineer will fit a new OEM-compatible NTC sensor. Baxi genuine sensors are readily available from UK heating merchants. After fitting, the boiler is reset and run through a heating cycle to confirm the fault has cleared.
- Engineer tests or replaces the PCB if the fault persists Gas Safe engineer
Should a new sensor and clean connections fail to resolve E.04-.01, the PCB is the next candidate. PCB replacement is significantly more expensive, but the engineer will normally test the board's sensor-reading circuits before committing to a replacement.
Parts you may need
- Baxi flow temperature NTC thermistor sensor · from £15
- Sensor wiring loom / harness · from £35
- Wiring connector repair kit · from £8
- Baxi PCB (model-specific) · from £280
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 £90–£220, depending on the underlying cause.
Frequently asked questions
Can I reset the boiler myself when E.04-.01 appears?
Yes, you can try one or two resets — press and hold the Baxi reset button for about three seconds and wait to see if the boiler fires normally. However, E.04-.01 is a permanent locking fault, which means it will keep coming back until the root cause is fixed. If the code returns after a reset, stop trying and call a Gas Safe engineer, as repeated lock-outs without a successful start confirm a genuine component fault.
How much does it typically cost to fix Baxi E.04-.01 in the UK?
For most households, the repair involves replacing the flow NTC thermistor and possibly cleaning up corroded connectors. With parts and labour combined you can expect to pay roughly £90–£220. The sensor itself is inexpensive (around £10–£20 for a genuine Baxi part), so the bulk of the bill is the engineer's call-out and labour time. If the PCB turns out to be faulty as well, costs can rise to £350–£500 or more — but PCB replacement is far less common with this particular fault code.
What is the difference between Baxi fault codes E.04-.01, E.04-.02, and E.04-.03?
All three relate to the flow temperature sensor but describe different failure modes. E.04-.01 means the sensor is short-circuited (resistance near zero, giving a falsely high temperature reading). E.04-.02 means the sensor circuit is open — either the sensor has broken internally or a wire has disconnected (resistance reads infinite). E.04-.03 means the boiler has actually exceeded its maximum allowable flow temperature, or the sensor has short-circuited under that high-temperature condition. An engineer will use a multimeter to distinguish between them and determine whether the sensor, wiring, or something causing genuine overheating is to blame.
Is Baxi E.04-.01 covered under warranty?
Baxi boilers come with a minimum two-year warranty, extendable up to ten years when installed by a Baxi-accredited installer and serviced annually. If your boiler is within its warranty period and has been serviced every year as required, the sensor replacement should be covered at no cost to you. Check your warranty documents and contact Baxi directly (or your installer) before paying for any repair, as an unauthorised repair by a non-accredited engineer can invalidate the warranty.