Baxi E.04-.03 Fault Code: Causes, Fixes & Repair Costs
What does the Baxi E.04-.03 fault code mean?
The E.04-.03 code appears on Baxi boilers when either the flow temperature has climbed above the permitted maximum, causing the boiler to lock out for safety, or the flow temperature sensor (NTC thermistor) has short-circuited and is reporting dangerously high temperatures even when none exist. Because the boiler cannot distinguish between a genuine overheat and a sensor failure, it shuts down in both cases. This is classed as a permanent fault — it will not self-clear once triggered and requires the underlying problem to be resolved before the boiler can be reset and returned to normal operation.
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
- Faulty or short-circuited NTC flow temperature sensor Common
The NTC thermistor that monitors the flow temperature can develop an internal short-circuit, causing it to report a dangerously high reading to the PCB regardless of the actual water temperature. This triggers the lockout without the boiler having genuinely overheated. It is a very common cause of this specific code and is usually a straightforward replacement for a Gas Safe engineer.
- Failed or seized circulation pump Common
If the pump is not moving heating water around the system effectively, heat builds up rapidly inside the heat exchanger. The flow temperature rises quickly to the safety cut-off threshold, triggering the lockout. A pump that is humming but not turning, or completely silent when it should be running, is a strong indicator of this cause.
- Airlocks reducing water circulation Sometimes
Trapped air pockets in the system reduce the flow rate of heating water through the boiler. With less water passing over the heat exchanger, the temperature of that water rises sharply. Bleeding radiators and checking system pressure (which should sit between 1 and 1.5 bar when cold) can resolve airlocks, though the lockout still needs to be reset afterwards.
- Sludge or debris blocking the heat exchanger Sometimes
A build-up of magnetite sludge or scale within the heat exchanger or pipework restricts water flow, creating hot spots and causing the flow temperature to exceed safe limits. Boilers in older systems without a magnetic filter are particularly susceptible. Repeated lockouts of this type are a strong sign the system needs a power flush.
- Faulty overheat thermostat Rare
The boiler's high-limit thermostat acts as a secondary safety device. If it has developed a fault — either failing to reset after a genuine overheat or triggering spuriously — it can contribute to a persistent lockout condition that continues to generate the E.04-.03 code even after the primary cause has been addressed.
How to fix it
- Allow the boiler to cool down fully DIY safe
The boiler locks out because it believes temperatures are dangerously high. Leave it switched off at the programmer or room thermostat for at least 15–20 minutes so that any genuine heat build-up can dissipate before you attempt anything else.
- Check system pressure and bleed radiators if needed DIY safe
Look at the pressure gauge on the boiler — it should read between 1 and 1.5 bar when the system is cold. If it is below 1 bar, top it up carefully using the filling loop until it reaches around 1.2 bar, then close the filling loop valves. If radiators feel cold at the top, bleeding them to release trapped air may restore circulation and prevent the temperature from spiking again.
- Reset the boiler — but only once or twice DIY safe
Press and hold the reset button (usually on the front panel) for 3–5 seconds until the boiler attempts to restart. If it fires up and runs without returning the fault code, monitor it over the next hour. If the fault returns immediately or after a short run, do not keep resetting — repeated overheating cycles can crack the heat exchanger, which is a very expensive repair.
- Have a Gas Safe engineer test the NTC flow temperature sensor Gas Safe engineer
An engineer will disconnect the sensor and check its resistance with a multimeter at a known water temperature. A short-circuited sensor will give a reading far outside the expected range and must be replaced. This is often the quickest and most affordable fix for this fault code.
- Have a Gas Safe engineer inspect the circulation pump Gas Safe engineer
The engineer will check whether the pump is running at the correct speed and moving adequate water volume. A pump that is mechanically seized, electrically failed, or set to too low a speed setting is a primary cause of genuine overheating. Replacement is straightforward but must be carried out by a qualified engineer.
- Consider a power flush if sludge is suspected Gas Safe engineer
If the system has no magnetic filter, radiators are cold at the bottom, or this fault has recurred multiple times, a Gas Safe engineer or heating engineer should carry out a power flush to clear sludge and debris from the pipework and heat exchanger. A magnetic filter should then be fitted to prevent recurrence.
- Call a Gas Safe registered engineer if the fault persists Gas Safe engineer
If the boiler continues to lock out after your initial checks, do not attempt further resets or any work on internal components. Contact a Gas Safe registered engineer to carry out a full diagnosis. You can verify any engineer's registration at gassaferegister.co.uk.
Parts you may need
- NTC flow temperature sensor (thermistor) · from £25
- Circulation pump (Baxi compatible) · from £85
- Overheat thermostat / high-limit stat · from £20
- Magnetic system filter (e.g. Fernox TF1 or Adey MagnaClean) · from £55
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–£380, depending on the underlying cause.
Frequently asked questions
Can I fix the Baxi E.04-.03 fault code myself?
The only homeowner-safe actions are checking system pressure, bleeding radiators, allowing the boiler to cool, and attempting a reset once or twice. Everything beyond that — testing or replacing the flow sensor, inspecting the pump, or investigating overheating — must be carried out by a Gas Safe registered engineer. This is a permanent lockout fault, meaning it will not clear on its own, and repeated resets without fixing the root cause risk cracking the heat exchanger.
How much does it cost to fix a Baxi E.04-.03 fault in the UK?
For most households, the repair falls between £120 and £380 including parts and labour. Replacing the NTC flow sensor is typically at the lower end (around £100–£180 all-in), while a circulation pump replacement averages around £200–£280. A power flush, if sludge is the root cause, usually costs £300–£500 depending on the size of the system. In rare cases where repeated overheating has damaged the heat exchanger, costs can rise to £500 or more — at that point, on an older boiler, replacement may be worth considering.
Why does my Baxi boiler keep coming back with E.04-.03 after I reset it?
If the fault returns within minutes or after a short run, the underlying problem has not been resolved. The two most likely culprits are a short-circuited flow temperature sensor — which will continue to report false high temperatures on every start-up — or a circulation pump that is not moving water effectively, causing genuine overheating each time the burner fires. A Gas Safe engineer needs to diagnose which of these is responsible before any further resets are attempted.
Could a low system pressure cause the E.04-.03 fault on a Baxi boiler?
Low pressure (below 1 bar) reduces the volume of water circulating through the heat exchanger, which can contribute to localised overheating and trigger this lockout. It is worth checking the pressure gauge and topping up to around 1.2 bar via the filling loop before calling an engineer. However, if the pressure is fine and the fault persists, the cause is more likely a faulty sensor or failing pump rather than pressure alone.