Chaffoteaux 433 Fault Code: Causes, Fixes & Repair Costs
What does the Chaffoteaux 433 fault code mean?
Fault code 433 on a Chaffoteaux boiler belongs to the '4xx' code series, which covers faults in the external peripheral board and zone sensor group. Code 433 specifically points to a problem with a zone temperature sensor — most likely a short circuit or open circuit on a zone 3 flow or return NTC thermistor — or a communication failure between the main PCB and an external zone control module. Depending on your exact Chaffoteaux model (for example the Talia Green System 35 is one confirmed model where this code appears), the boiler may continue to run in a degraded state or shut down entirely, displaying 'Err' alongside the code in full lockout situations.
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 failed zone NTC thermistor Common
The zone 3 temperature sensor (an NTC thermistor) has developed an open circuit or short circuit. This is the most common trigger for code 433 — the sensor's resistance reading moves outside the expected range, prompting the boiler's electronics to flag a fault. Sensors degrade over time due to heat cycling and general wear.
- Loose or corroded wiring to the sensor Common
The wiring harness connecting the zone sensor to the main PCB or external board can develop loose pins, corroded terminals, or damaged insulation. Even a small increase in contact resistance at a connector can be enough to register as a sensor fault.
- Moisture or water ingress on connectors or PCB Sometimes
Condensation or a minor internal leak can introduce moisture onto sensor connectors or the external board. This can cause erratic resistance readings or intermittent communication errors between the zone board and the main PCB, triggering code 433.
- Failed external zone control board Sometimes
The peripheral zone board itself may have failed — either through age-related component wear, damaged solder joints, or water ingress — resulting in a loss of communication with the main PCB. If the sensor and wiring test fine, attention turns to this board.
- Main PCB communication failure Rare
In rarer cases the main PCB is unable to communicate correctly with the external zone module even when both the sensor and zone board are intact. This is typically a consequence of a failing PCB rather than the root cause of the 433 code.
How to fix it
- Attempt a single boiler reset DIY safe
Locate the reset button on your Chaffoteaux boiler's control panel and press it once. Allow the boiler a full minute to restart and attempt ignition. If the fault code 433 reappears promptly, do not keep resetting — repeated resets will not cure an underlying sensor or board fault and may mask important diagnostic information for an engineer.
- Check your system pressure DIY safe
Look at the pressure gauge on the boiler. A healthy cold system should read between 1.0 and 1.5 bar. If the needle is below 1 bar, repressurise the system by briefly opening the filling loop valves until the gauge reaches around 1.2 bar, then close them. Low pressure alone is unlikely to cause code 433, but it is always worth ruling out.
- Visually inspect sensor connectors for moisture DIY safe
With the boiler switched off at the programmer and the electrical supply isolated at the fused spur, you can carefully open the boiler casing to visually check whether there is obvious moisture or corrosion around any visible sensor connector. Do not touch any wiring or components — this is a visual check only. If you see signs of a water leak, call an engineer promptly.
- Contact a Gas Safe registered engineer to test the zone sensor Gas Safe engineer
An engineer will use a multimeter to measure the resistance of the zone 3 NTC thermistor across its temperature range. A healthy sensor produces a predictable, smooth resistance curve; an open or short circuit reading confirms the sensor has failed and needs replacing. This is not DIY-safe work.
- Have the engineer inspect all wiring to the sensor and external board Gas Safe engineer
The engineer will trace the wiring from the zone sensor back to the external board and main PCB, checking each connector for loose pins, corrosion, chafing, or heat damage. A wiring fault is often cheaper to fix than a component replacement and is always worth eliminating first.
- Replace the faulty zone NTC sensor if confirmed defective Gas Safe engineer
If the sensor tests as open or short circuit, the engineer will fit a replacement NTC thermistor. This is straightforward work for a competent engineer and is usually the most cost-effective resolution for code 433.
- Replace the external zone board if sensor and wiring are sound Gas Safe engineer
Should the sensor and wiring prove healthy but the fault persists, the engineer will test and if necessary replace the peripheral zone board. This board handles communication between zone controls and the main PCB, and its failure can mimic a sensor fault.
- Call a Gas Safe registered engineer if you have not already done so Gas Safe engineer
If the reset has not cleared the fault, or if the fault returns after clearing, book a Gas Safe registered engineer. Fault code 433 involves electronic components and sensor circuits that must be diagnosed and repaired by a qualified professional. You can find Gas Safe registered engineers at gassaferegister.co.uk.
Parts you may need
- Zone NTC thermistor sensor · from £25
- Sensor wiring loom or harness · from £35
- External zone control board · from £120
- Main PCB · 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 £120–£400, depending on the underlying cause.
Frequently asked questions
Will my Chaffoteaux boiler keep working with fault code 433 showing?
It depends on your model. Chaffoteaux uses two types of fault indication: some faults are non-locking warnings that allow the boiler to keep running in a reduced capacity, while others trigger a full shutdown displayed with 'Err' alongside the code. Code 433 can behave either way. If your heating or hot water has stopped, the boiler has locked out and will need a reset followed by an engineer visit. Even if the boiler is still running, you should not ignore a 433 fault — a failing zone sensor can lead to poor temperature control and eventual lockout.
Can I replace the zone sensor myself to fix code 433?
We strongly advise against it. While replacing a sensor might appear straightforward, the work involves accessing internal wiring, connectors, and in some cases the external zone board — all of which sit alongside gas and electrical components inside the boiler. Any work beyond a basic reset must be carried out by a Gas Safe registered engineer. Attempting repairs yourself could invalidate your boiler warranty, breach your home insurance policy, and create a safety risk.
How much does it typically cost to fix a Chaffoteaux 433 fault?
Most people with this fault end up paying between £120 and £400 all in, covering the engineer's diagnosis time plus parts and labour to replace either the zone NTC sensor or the external zone board. A simple sensor swap tends to sit at the lower end of that range. If the fault turns out to be a main PCB failure — which is less common but possible — costs can rise to £450–£700. On a boiler that is more than ten years old, it is worth asking the engineer whether a full replacement would be better value than a major component repair.
What is the 4xx code series on Chaffoteaux boilers?
Chaffoteaux groups its fault codes by the first digit, with each group representing a different functional area of the boiler. The '4' prefix covers external board and zone sensor faults. Codes in the 411–413 range relate to temperature sensor problems in specific heating zones, and 433 falls within this same group — pointing to a zone 3 sensor or external board communication issue. If you see a code starting with '4' on a Chaffoteaux boiler, the starting point is always to investigate zone sensors, their wiring, and the external control board before assuming a main PCB fault.