Baxi E50 Fault Code: Causes, Fixes & Repair Costs
What does the Baxi E50 fault code mean?
The E50 fault code on Baxi boilers indicates a problem with the domestic hot water (DHW) NTC (Negative Temperature Coefficient) temperature sensor. The boiler's control board continuously monitors resistance readings from this thermistor to track how hot the water is — when that resistance falls outside the expected range, the board concludes the sensor is unreliable and locks the boiler out as a safety precaution. On certain models such as the Baxi Platinum, E50 can also be triggered when the maximum permissible flow temperature is exceeded and the PCB steps in to halt operation. In either case, the boiler will not fire until the fault is diagnosed and cleared.
General guidance only — not a substitute for professional advice. 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 failed DHW NTC thermistor Common
The hot water NTC sensor has drifted out of calibration or failed entirely, causing its resistance reading to fall outside the range the PCB considers valid. This is by far the most frequent root cause of E50 and typically requires the sensor to be replaced.
- Overheating due to poor circulation Sometimes
If water is not circulating properly — often because of a seized or sluggish pump — heat builds up in the heat exchanger. The resulting temperature spike can trigger the E50 safety shut-down even when the sensor itself is undamaged.
- Low system pressure Sometimes
When system pressure drops below approximately 1.0 bar, water flow through the boiler becomes restricted. Inadequate circulation can lead to localised overheating, which in turn causes erratic sensor readings and may trigger E50.
- Loose or damaged sensor wiring Sometimes
Vibration, age, or a previous repair can leave the wiring connector to the NTC sensor loose, corroded, or intermittently open-circuit. The PCB receives no credible signal and raises E50 even though the sensor element itself may still be functional.
- PCB fault Rare
In rarer cases, particularly where maximum flow temperature has been exceeded repeatedly, the PCB itself can be damaged. This can manifest as E50 even after a healthy new sensor has been fitted.
How to fix it
- Reset the boiler once DIY safe
Press and hold the reset button (usually marked with a flame symbol) for around 3 seconds. If the fault is transient — caused by a momentary temperature spike rather than a failed component — a single reset may restore normal operation. Do not repeat this more than two or three times in total; repeated resets without a fix will not resolve an underlying fault and may mask a more serious issue.
- Check system pressure DIY safe
Look at the pressure gauge on the boiler fascia. For most Baxi models it should read between 1.0 and 1.5 bar when the system is cold. If it has dropped below 1.0 bar, top it up using the filling loop beneath the boiler — turn the valves slowly until the gauge reaches about 1.2 bar, then close them. A step-by-step guide for your specific model will be in your user manual.
- Check for obvious flue obstructions DIY safe
Go outside and have a look at the flue terminal on the exterior wall. Make sure it is not blocked by debris, a bird nest, or overgrown vegetation. Never probe inside the flue — just clear anything obvious from the external terminal. Poor ventilation can contribute to overheating faults.
- Have a Gas Safe engineer test the NTC sensor resistance Gas Safe engineer
An engineer will disconnect the sensor wiring and use a multimeter to measure its resistance at a known temperature, comparing the reading against the manufacturer's specification curve. A reading that is wildly out of range or open-circuit confirms the sensor has failed and needs replacing.
- Have the NTC sensor and seal replaced if faulty Gas Safe engineer
With the boiler isolated electrically and the domestic water supply turned off, the engineer will drain the relevant section, remove the old sensor, fit a new genuine or approved-equivalent NTC thermistor, and replace the sealing washer. The part itself is relatively inexpensive (around £7–£20); the majority of the cost is labour.
- Have the circulation pump inspected Gas Safe engineer
If the sensor checks out as healthy, the engineer will assess whether the pump is running freely and producing adequate flow. A seized pump can be freed or replaced as necessary to restore proper circulation and prevent repeat overheating faults.
- Have the PCB inspected if the fault persists Gas Safe engineer
If a new sensor and a working pump do not clear the fault, the engineer will investigate the PCB — checking for signs of heat damage, verifying that the sensor input circuit is intact, and determining whether the board needs replacing.
- Call a Gas Safe registered engineer Gas Safe engineer
If the boiler has not returned to normal operation after checking pressure and attempting a reset, contact a Gas Safe registered engineer. You can verify registration at gassaferegister.co.uk. E50 is a sensor or overheating fault that requires diagnostic tools and component access beyond what a homeowner should attempt.
Parts you may need
- Baxi DHW NTC thermistor sensor (e.g. part no. 720789201) · from £13
- Sensor sealing washer / O-ring · from £3
- Baxi circulation pump · from £85
- Baxi PCB (main control board) · from £220
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 £150–£350, depending on the underlying cause.
Frequently asked questions
Can I fix a Baxi E50 fault myself?
The safe DIY steps are limited to a single boiler reset, checking that system pressure is between 1.0 and 1.5 bar (topping up via the filling loop if needed), and making sure the flue terminal outside is not visibly blocked. Everything beyond that — testing and replacing the NTC sensor, inspecting the pump, or touching any wiring — must be done by a Gas Safe registered engineer. Sensor replacement involves isolating electrics and handling pressurised water components, which is not appropriate for a DIY repair.
How much does it cost to fix a Baxi E50 fault in the UK?
For the most common repair — replacing the DHW NTC thermistor — most homeowners pay in the region of £150 to £350 including parts, labour, and the engineer's callout. The sensor component itself costs only around £7–£20, so the bulk of the bill is the engineer's time. If the root cause turns out to be a seized pump (roughly £85–£150 for the part) the total can reach the higher end of that range. A PCB replacement is significantly more expensive and would be quoted separately by your engineer.
Why does my Baxi boiler show E50 even after I've reset it?
If E50 returns shortly after a reset, the underlying fault — most likely a failed NTC sensor or a circulation problem causing genuine overheating — has not gone away. Repeated resets without a repair will not fix the boiler and could potentially mask a worsening issue. Stop resetting and call a Gas Safe engineer to diagnose the root cause properly.
What is an NTC sensor and why does it matter?
NTC stands for Negative Temperature Coefficient. It is a type of thermistor whose electrical resistance decreases predictably as temperature rises. Baxi boilers use these sensors to tell the PCB exactly how hot the domestic hot water is at any given moment. If the resistance reading the PCB receives falls outside the expected range — because the sensor has aged, corroded, or failed — the boiler cannot safely regulate water temperature and shuts down, displaying E50. Replacing the sensor restores the PCB's ability to monitor and control the hot water circuit correctly.