Chaffoteaux 303 Fault Code: Causes, Fixes & Repair Costs
What does the Chaffoteaux 303 fault code mean?
Fault code 303 on a Chaffoteaux boiler indicates one of two closely related problems depending on the model generation and firmware version. On many units it signals a detected anomaly on the main PCB (printed circuit board), causing the boiler to lock out and refuse to restart. On other models — particularly older ones — code 303 acts as an overheating or poor-circulation safety lockout, triggered when the primary circuit water temperature climbs above safe thresholds (around 95°C) or when flow through the heat exchanger becomes insufficient. Both scenarios end the same way: heating and hot water cut out and the boiler displays 303. Because the two root causes share several upstream triggers — a seized pump, trapped air, or a failing NTC sensor can stress the PCB and cause overheating simultaneously — the diagnostic steps overlap significantly. Treat this as a serious fault requiring prompt 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
- PCB fault following electrical surge or moisture ingress Common
The most frequently cited cause across Chaffoteaux service records. A voltage spike — from a power cut or lightning strike — can damage sensitive components on the control board. Moisture from a slow internal leak sitting near the board over time produces the same result. Engineers report that 70–80% of persistent 303 codes on Chaffoteaux boilers trace back to direct PCB component failure.
- Trapped air causing poor circulation Common
Air pockets in the primary circuit reduce water flow, causing localised overheating that trips the 303 safety lockout. This is particularly common after any recent plumbing work, a recent pressure drop, or if the boiler has not been bled for several years. On boilers over eight years old, a partially fouled heat exchanger amplifies the effect.
- Seized or failing circulation pump Common
If the pump impeller has jammed or the motor is failing, water cannot circulate properly through the heat exchanger. The boiler detects insufficient flow or rising temperature and shuts down with code 303. A seized pump is often preceded by reduced heating performance and intermittent noise from the boiler.
- Low system pressure Common
When system pressure drops below around 0.5–0.8 bar, flow through the circuit becomes inadequate. The boiler's safety logic interprets this as a circulation fault and triggers a lockout. Pressure loss is commonly caused by a small leak somewhere on the system or by air escaping through a radiator bleed valve that was not fully closed afterwards.
- Faulty NTC flow temperature sensor Sometimes
The NTC thermistor monitors the flow temperature in real time. If it has drifted out of calibration or failed, it can report false high-temperature readings to the PCB, causing the board to trigger the 303 safety lockout even when actual water temperature is normal. At 25°C the sensor should read close to 10 kΩ; a significant deviation points to a faulty sensor.
- Oxidised or loose PCB connectors Sometimes
Even without a dramatic electrical event, vibration and thermal cycling over years can loosen plug-in connectors on the board or allow corrosion to build up on the contacts. This can cause intermittent signal errors that the PCB logs as a fault 303.
How to fix it
- Reset the boiler DIY safe
Press and hold the reset button on the boiler's front panel as described in your user manual. Wait at least two minutes after the boiler attempts to restart before checking whether the fault has cleared. If code 303 returns within a short time, attempt a reset no more than two or three times in total — repeated resets without resolving the underlying cause will not help and may mask the problem. If the code persists, move to the next step.
- Check the system pressure gauge DIY safe
Look at the pressure gauge on the front of the boiler. It should read between 1 and 1.5 bar when the system is cold. If the needle is below 1 bar, the system needs topping up. Locate the filling loop — usually a flexible braided hose with one or two valves beneath the boiler — and open it slowly, watching the gauge closely. Stop at 1.5 bar and close the valve firmly. Restart the boiler and see if the fault clears. Never exceed 2 bar.
- Bleed the radiators to remove trapped air DIY safe
Switch the heating off and allow the system to cool. Starting with the radiator furthest from the boiler, use a bleed key to open the bleed valve a quarter-turn until water dribbles out steadily without air spluttering, then close it. Work back towards the boiler, bleeding each radiator in turn. After bleeding, re-check the system pressure and top up again if it has dropped below 1 bar. Restart the boiler to see if circulation has improved.
- Check whether the condensate pipe is frozen (in cold weather) DIY safe
During freezing temperatures, a blocked condensate pipe can sometimes trigger unusual fault codes. Locate the white plastic condensate pipe that exits the boiler and runs outside or to a drain. If it feels solid or you can see ice, thaw it gently with a hot water bottle or warm (not boiling) water poured along its length. Once clear, reset the boiler. If the weather is mild or the pipe is not frozen, skip this step.
- Call a Gas Safe registered engineer for internal diagnosis Gas Safe engineer
If code 303 persists after the checks above, the fault requires professional investigation inside the boiler casing. A Gas Safe engineer will inspect the circulation pump — checking whether it is running, whether it hums without turning, and whether it delivers correct flow — and test it with a multimeter. They will also measure the NTC sensor resistance and compare it against specification. If the pump and sensor are both satisfactory, the engineer will examine the PCB for scorch marks, melted components, water residue, and damaged connectors, using Chaffoteaux diagnostic tooling where available to read fault memory from the board. Do not open the boiler casing or attempt any of these checks yourself.
Parts you may need
- Circulation pump · from £90
- NTC flow temperature sensor · from £25
- Main PCB (control board) · from £280
- Pressure relief valve · from £20
- Filling loop assembly · from £18
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–£420, depending on the underlying cause.
Frequently asked questions
Can I fix Chaffoteaux fault code 303 myself?
Some preliminary checks are safe for a homeowner: resetting the boiler, topping up system pressure via the filling loop, and bleeding radiators to clear trapped air. These steps are worth trying first and cost nothing. However, if the fault persists, all further diagnosis involves opening the boiler casing and working on electrical or gas-side components, which legally requires a Gas Safe registered engineer in the UK. Do not attempt to remove the pump, test the PCB, or inspect internal wiring yourself.
How much does it cost to fix fault code 303 on a Chaffoteaux boiler?
For the most common repairs, expect to pay roughly £150–£250 for a circulation pump replacement (parts and labour) and around £80–£180 for an NTC temperature sensor swap. These figures cover most cases. If the PCB itself needs replacing, costs rise considerably — typically £450–£700 in 2025 depending on the model and your region — because the board is a specialist part that engineers often need to order directly from the manufacturer. If your boiler is over 10–12 years old and the PCB has failed, it is worth getting a replacement boiler quote alongside the repair quote, as the economics can favour a new installation.
Why does my Chaffoteaux boiler keep showing 303 after I reset it?
A fault that returns quickly after a reset means the underlying cause has not been resolved. The most likely explanations are that system pressure has dropped again (pointing to a leak), air keeps finding its way into the circuit, the circulation pump is failing and cannot maintain adequate flow, or the PCB has a component fault that a reset cannot clear. If two or three resets do not keep the boiler running, stop resetting and call a Gas Safe engineer — continuing to reset a boiler with an active fault can occasionally cause further damage.
Is fault code 303 a PCB fault or an overheating fault on Chaffoteaux boilers?
Both interpretations are correct, depending on the model and firmware version. Chaffoteaux is a French brand and documentation across different model generations is not always consistent in English sources. On some units code 303 is logged directly as a PCB or control board anomaly. On others it is a high-temperature or low-flow safety lockout that shuts the boiler down before damage occurs. In practice, a Gas Safe engineer will work through the circulation components first — pump, pressure, air — because those are cheaper to fix and are statistically more common causes, before concluding that the board itself has failed.