Chaffoteaux 101 Fault Code: Causes, Fixes & Repair Costs
What does the Chaffoteaux 101 fault code mean?
Fault code 101 on a Chaffoteaux boiler signals that the water temperature inside the primary circuit has climbed to an unsafe level, triggering the boiler's built-in safety lockout. In plain terms, the heat exchanger has got too hot — either because heat is not being carried away fast enough by circulating water, or because a sensor or component has reported a dangerously high reading. The boiler shuts itself down automatically to prevent damage. You may be able to reset it once or twice, but if the code returns, an underlying fault needs professional attention.
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
- Circulation pump failure or seizure Common
The pump is responsible for keeping hot water moving around the primary circuit. If it has seized, lost power, or is running too slowly, heat builds up rapidly in the heat exchanger and the 101 overheat lockout is triggered. This is one of the most frequent causes of this code, particularly on older Chaffoteaux appliances.
- Blocked or scaled plate heat exchanger Common
Limescale and debris can accumulate inside the heat exchanger over time, especially in hard-water areas. This restricts water flow and traps heat in one place. If the 101 error appears predominantly when hot water is being drawn off rather than during central heating operation, a clogged heat exchanger is a strong candidate.
- Low system pressure or closed isolating valves Common
If system pressure has dropped below around 1 bar, or an isolating valve somewhere in the circuit has been accidentally closed, there is not enough water moving through the boiler to dissipate heat properly. This causes temperatures to spike. Checking the pressure gauge is a safe first step for any homeowner.
- Faulty NTC temperature sensor Sometimes
The NTC thermistor monitors primary circuit temperature and feeds readings to the PCB. A sensor that has drifted out of calibration or developed an intermittent fault can report false high-temperature readings, causing the boiler to lock out even when actual temperatures are within normal range.
- Failed or waterlogged expansion vessel Sometimes
The expansion vessel absorbs pressure fluctuations as water heats and expands. When it fails — typically because the internal membrane has perished — pressure swings become erratic, which can disrupt circulation and contribute to localised overheating events.
- PCB or electrical fault Rare
A surge or component failure on the main control board can cause it to misinterpret sensor signals or fail to control the pump correctly, resulting in a spurious or persistent 101 error. This tends to be a diagnosis of exclusion once other components have been ruled out.
How to fix it
- Reset the boiler once DIY safe
Switch the boiler off using its power button or controls, wait at least two to three minutes for internal components to fully reset, then switch it back on. A single isolated overheat event can sometimes be cleared this way. If the 101 code returns within a short time, do not keep resetting — repeated resets without fixing the root cause can worsen damage to the pump or heat exchanger.
- Check and restore system pressure DIY safe
Look at the pressure gauge on the boiler fascia. It should read between 1.0 and 1.5 bar when the system is cold. If it is below 1 bar, top it up carefully using the filling loop — typically a braided flexible hose with one or two valves beneath the boiler. Open the valve(s) slowly, watch the gauge, and close them once the needle reaches around 1.2 to 1.5 bar. Do not overpressurise beyond 2 bar.
- Check that all valves in the system are open DIY safe
Walk around your radiators and check that the lockshield and thermostatic radiator valves (TRVs) are not fully closed on every radiator simultaneously. Also confirm that any service valves near the boiler (often blue or red handled) are in the open position — handle in line with the pipe means open. Closed valves starve the circuit of flow and can cause overheating.
- Bleed your radiators if they feel cold at the top DIY safe
Air trapped in radiators reduces circulation efficiency. Using a radiator bleed key, open the bleed valve at the top corner of each radiator until water (not air) comes out, then close it. Re-check system pressure afterwards and top up if needed. This will not fix a pump fault but removes a contributing factor.
- Have a Gas Safe engineer inspect the pump and circulation Gas Safe engineer
A registered engineer will check whether the circulation pump is running, measure its output, and determine whether it has seized or is undersized for the system. Pump replacement is one of the most common fixes for a persistent 101 error. This is not a DIY task — the pump connects directly to the primary water circuit and gas-side components.
- Have the heat exchanger inspected and descaled or replaced if necessary Gas Safe engineer
An engineer can assess flow rates through the plate heat exchanger and check for scale restriction. In hard-water areas, a chemical descale may restore performance. If the exchanger is too far gone, replacement will be required. This is a significant but sometimes unavoidable repair on older appliances.
- Have the NTC sensor and PCB tested Gas Safe engineer
If the pump and heat exchanger check out, the engineer should test the NTC thermistor with a multimeter for resistance values matching the manufacturer's spec. A drifting or failed sensor is a relatively inexpensive fix. PCB faults are more costly and the engineer may also assess whether repair or boiler replacement is the better financial decision at this stage.
- Call a Gas Safe registered engineer if the code persists or keeps returning Gas Safe engineer
If the 101 fault code returns after a reset, or you have any doubt about the system's safety, stop using the boiler and contact a Gas Safe registered engineer. You can verify registration at gassaferegister.co.uk. Repeated overheating puts the pump, heat exchanger, and PCB at risk of permanent damage.
Parts you may need
- Circulator pump (compatible replacement) · from £80
- NTC temperature sensor / thermistor · from £20
- Expansion vessel (8–12 litre) · from £45
- Plate heat exchanger · from £150
- 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–£450, depending on the underlying cause.
Frequently asked questions
Can I just keep resetting the boiler when code 101 appears?
You can try a single reset — occasionally a momentary event such as a brief drop in pressure can trigger a one-off overheat lockout that clears itself. However, if the 101 code returns within the same session or comes back over the following days, do not keep resetting. Each overheat cycle puts additional stress on the pump and heat exchanger. Repeated lockouts mean something specific has gone wrong and needs a Gas Safe engineer to diagnose it properly.
How much does it cost to fix a Chaffoteaux 101 fault in the UK?
The most common repair — pump replacement — typically costs between £200 and £400 including parts and labour. An NTC sensor swap is cheaper, usually £100 to £200 all in. If the plate heat exchanger needs replacing, expect to pay £350 to £600 or more. On a boiler that is already 10 or more years old, a heat exchanger or PCB repair can push costs close to or above the price of a new boiler, so it is worth asking your engineer for an honest assessment of the appliance's overall condition before committing to an expensive component replacement.
Why does the 101 error appear mainly when I run hot water, not heating?
The plate heat exchanger in a combi boiler handles the transfer of heat to domestic hot water as a secondary duty. Scale deposits tend to restrict flow through this component first, and the higher demand placed on it during hot water production can push temperatures over the safety threshold even when the heating circuit is coping fine. If this pattern matches what you are seeing, a blocked or scaled heat exchanger is the most likely culprit and your engineer should inspect it as a priority.
Will annual servicing prevent a Chaffoteaux 101 fault?
Servicing significantly reduces the risk. During a service, an engineer checks pump operation, system pressure, the expansion vessel pre-charge, and cleans key components. Catching early signs of pump wear or scale buildup before they cause an overheat lockout is far less disruptive and expensive than an emergency repair in the middle of winter. Chaffoteaux themselves recommend annual servicing, and most boiler warranties require it as a condition of cover.