Baxi E433 Fault Code: Causes, Fixes & Repair Costs
What does the Baxi E433 fault code mean?
The E433 fault code on Baxi EcoBlue Combi and System boilers indicates that the boiler's control board has detected a problem with one of its temperature sensors (thermistors) or an outdoor weather compensation sensor. The boiler locks out as a precaution until the fault is resolved. Depending on which sensor has triggered the code, it could be the central heating flow thermistor, the flue thermistor, the hot water flow sensor, or an outdoor sensor wired into the system. In each case, the PCB is receiving a signal that falls outside the expected range — either no signal at all (open circuit) or a reading that doesn't match real-world conditions.
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 open-circuit NTC thermistor Common
NTC thermistors are small resistive sensors that change resistance with temperature. At 25°C the correct reading is around 12kΩ. When a thermistor fails completely (open circuit), resistance goes infinite and the PCB immediately detects a fault and locks out. This is the single most frequent cause of E433 on Baxi EcoBlue boilers.
- Loose or corroded wiring connections Common
Boiler vibration over time can work plug-in connectors loose on the sensor harness. Corrosion at connector pins — especially on older boilers or those in damp plant rooms — produces an intermittent or incorrect signal that mimics a failed sensor. An engineer can confirm this with a multimeter before replacing any parts.
- Age-related sensor drift Sometimes
Rather than failing outright, thermistors can gradually drift off their calibration curve as they age. The resistance-temperature relationship shifts enough to cause the boiler to misread water or flue temperature, eventually pushing a reading far enough out of range to trigger E433. This is particularly common on boilers that are eight or more years old.
- Limescale build-up around the sensor pocket Sometimes
In hard-water areas, scale accumulates inside the sensor pocket in the heat exchanger or flow pipe. The scale layer insulates the thermistor from the actual water temperature, causing the sensor to under-read. Over time the discrepancy becomes large enough to trigger a fault. A system power flush or descale may be required alongside sensor replacement.
- Outdoor weather compensation sensor disconnected or absent Sometimes
If the boiler's IP menu has been configured to expect an outdoor sensor but the sensor is missing, unplugged, or wiring has been disturbed, the E433 can appear. An engineer checks the outdoor sensor value in the IP menu and verifies the wiring back to the sensor head.
- PCB fault Rare
The PCB interprets sensor signals and raises the fault code. In rare cases the board itself is faulty and misreports a sensor error even when all sensors test correctly. Engineers should only consider this after sensors and wiring have been thoroughly checked, as PCB replacement is expensive.
How to fix it
- Check the boiler display and note any additional digits or letters shown alongside E433 DIY safe
Some Baxi EcoBlue models display a sub-code or sensor identifier alongside the main fault. Note down exactly what is shown on the display and have it ready when you speak to an engineer — it helps them identify which specific sensor (CH flow, flue, HW flow, or outdoor) has triggered the lockout.
- Attempt a single boiler reset DIY safe
Press and hold the reset button (usually marked with a flame or 'R' symbol) for about 3–5 seconds and wait for the boiler to attempt a restart. If the boiler fires up and runs normally, monitor it over the next hour. If E433 returns, do not keep resetting — repeated resets without fixing the underlying fault can cause further damage and will not resolve a genuine sensor failure.
- Check that your gas supply is working DIY safe
Confirm other gas appliances in your home (hob, gas fire) are working normally. If you have no gas supply, contact your gas supplier rather than continuing to reset the boiler.
- Have a Gas Safe engineer test sensor resistance and wiring Gas Safe engineer
The engineer will disconnect each suspect sensor and measure resistance across its terminals with a calibrated multimeter. At 25°C the reading should be approximately 12kΩ. An open circuit or a significantly different reading confirms a faulty thermistor. They will also check the integrity of the wiring loom and connector pins for signs of corrosion or loose contacts.
- Engineer to inspect the flue thermistor and check for flue blockage Gas Safe engineer
If the flue sensor is implicated, the engineer checks the sensor value in the boiler's diagnostic menu and physically inspects the flue terminal for blockages such as bird nests, debris, or ice. A blocked flue can cause abnormal temperature readings at the sensor even when the sensor itself is healthy.
- Engineer to check the outdoor sensor configuration in the IP menu Gas Safe engineer
If an outdoor weather compensation sensor is involved, the engineer accesses the installer (IP) menu and confirms the displayed outdoor sensor value is realistic. They then trace the sensor wiring from the PCB terminal back to the sensor head to identify any breaks or loose connections.
- Replace the faulty thermistor or sensor Gas Safe engineer
Once the defective sensor is confirmed, the engineer will fit the correct replacement part. Baxi-specific thermistors are widely available from heating merchants. The job typically takes under an hour on a first visit and the boiler can usually be returned to service the same day.
- If all sensors test healthy, have the PCB assessed Gas Safe engineer
When sensors and wiring check out correctly but E433 persists, the PCB becomes the prime suspect. A Gas Safe engineer may be able to test PCB output signals or may recommend a like-for-like board replacement. Given the cost of a new PCB (£300–£500 fitted), it is worth discussing whether boiler replacement makes more sense on older units.
- Call a Gas Safe registered engineer if you have not already done so Gas Safe engineer
Beyond the basic reset and supply checks above, all diagnostic and repair work on a boiler must be carried out by a Gas Safe registered engineer. You can verify any engineer's registration at GasSafeRegister.co.uk before they visit. Do not use the boiler if it continues to lock out repeatedly.
Parts you may need
- Baxi EcoBlue NTC thermistor (CH flow sensor) · from £18
- Baxi EcoBlue flue thermistor · from £22
- Baxi EcoBlue hot water flow sensor · from £20
- Baxi outdoor weather compensation sensor · from £45
- Sensor wiring harness / connector pins · from £12
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–£280, depending on the underlying cause.
Frequently asked questions
Can I fix the Baxi E433 fault myself?
The safe DIY steps are limited to attempting one reset and confirming your gas supply is on. All sensor testing, wiring checks, and part replacement must be carried out by a Gas Safe registered engineer. Thermistors sit inside the pressurised water circuit and on components connected to the gas path, so working on them without the correct qualifications is both illegal and dangerous.
How much does it cost to repair a Baxi E433 fault in the UK?
For most people the repair involves a sensor or thermistor replacement, which typically costs £100–£280 including parts and labour when carried out by an independent Gas Safe engineer. Callout and diagnostic fees alone are usually £60–£100. If the PCB turns out to be the root cause, replacement costs rise to £300–£500 fitted — at that point, especially on a boiler over ten years old, it is worth comparing the repair cost against a new boiler installation.
Why does my Baxi EcoBlue keep showing E433 after I reset it?
A reset only clears the lockout temporarily — it does not fix the underlying fault. If E433 keeps returning after one or two resets, there is almost certainly a genuine sensor or wiring issue that needs professional diagnosis. Continuing to reset the boiler repeatedly is not advisable and can mask a developing fault, so book a Gas Safe engineer to investigate properly.
Could limescale cause the Baxi E433 code?
Yes, particularly in hard-water areas such as the South East, East Anglia, and the Midlands. Scale builds up inside the sensor pocket and acts as an insulating layer between the thermistor and the water it is supposed to measure. This causes the sensor to misread temperatures by an increasing margin until the boiler detects the discrepancy and faults. An engineer may recommend a system power flush or chemical descale alongside fitting a new sensor to prevent a recurrence.