Alpha E03 fault codewhat it means and how to fix it
Flue gas temperature has exceeded the safe limit. It usually means the heat exchanger is not transferring heat properly into the water.
Take care
Get a Gas Safe registered engineer to look at your Alpha E03
We do not pass enquiries to a repair firm, so there is no form here. Find a Gas Safe registered engineer through the Gas Safe Register, and ask to see their registration card when they arrive — it is the register, not the van, that tells you they are qualified.
What E03 means
A condensing boiler is supposed to send remarkably cool gases up the flue — that is the whole point of the design, because heat that goes up the flue is heat you paid for and did not use.
Alpha give E03 as high flue temperature, caused by "flue sensor intervention", and they publish the number and the sequence, which most manufacturers do not. Their flue thermostat acts at 110°C, and its first move is not to stop the boiler: it reduces the burner output to prevent damage and posts a different code, E47. Only if the temperature keeps climbing after that does the appliance lock out and show E03. That makes E03 the second stage of a fault rather than the start of one, and it has a practical consequence — an appliance that has been quietly running at reduced output for weeks was already telling you this, and the lockout is the point at which it stopped coping.
The reason underneath is nearly always that heat is not getting into the water where it belongs, so it leaves through the flue instead. Scale or sludge coating the inside of the heat exchanger is the classic cause, and it is progressive. The secondary concern is the flue itself: plastic flue systems are rated for the temperatures a condensing appliance normally produces, and sustained overtemperature is exactly what they are not designed for.
What causes it
| Cause | What is actually happening | How likely |
|---|---|---|
| Scaled or sludged heat exchanger | A deposit layer insulates the metal from the water. Common in hard water areas and on systems that have never had an inhibitor top-up. | Common |
| Poor circulation | Water passing too slowly picks up less heat per pass, so more of it goes out of the flue. Overlaps heavily with the causes of an overheat lockout. | Common |
| Flue gas sensor fault | The sensor is reporting a temperature that is not real. Worth eliminating, but it is not where an engineer will start. | Occasional |
| Appliance overfiring | Gas rate set too high after a previous repair, so the burner genuinely is producing more heat than the exchanger can absorb. Confirmed with a flue gas analyser. | Rare |
Can you fix it yourself?
Stop — this needs an engineer
What it costs to fix
Typical repair cost
£250–£1,100
A chemical clean with a magnetic filter is £400 to £750. A heat exchanger descale is £250 to £450. Replacing the exchanger runs £700 to £1,100 and is often the point at which a new appliance becomes the better value.
Typical UK prices including parts and labour, updated September 2026. Your quote may differ.
Parts you may need
We may earn a commission if you buy a part through one of these links. It does not change what you pay, and it does not affect which parts we list.
Magnetic system filter, 22mm
MAG-22£65–£120Plumbing merchant
Price checked 28 September 2026
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Related Alpha fault codes
- E02Get it checkedThe overheat thermostat has tripped. Almost always this means water is not moving through the appliance properly rather than that it is running too hot.Related: overheat and circulation
- E60Get it checkedThe appliance cannot confirm the pump is moving water. Left alone it becomes an overheat fault, so it is worth acting on promptly.Related: overheat and circulation
- E01Get it checkedThe appliance tried to light and could not confirm a flame, so it has locked out. In winter a frozen condensate pipe is the most likely reason.Related: same brand
- E10Get it checkedAlpha describe E10 as the primary pressure switch operating. Refilling to their green band usually clears it, but the switch itself is on their own list of causes.Related: same brand
- E16Get it checkedThe control board cannot confirm the fan is running at the speed it asked for, so it will not let the burner light. This one needs an engineer.Related: same brand
- E20Stop — call an engineerAlpha call this a flame sensing fault and define it as false flame detection: the appliance can see a flame while the gas valve is shut. Switch it off and call an engineer.Related: same brand
Own an Alpha boiler? Save it and its codes come first next time.
Related guides
Technical review
Not reviewed by an engineer
No engineer has read this page. Severity follows what the manufacturer publishes, and we would rather tell you that than let a byline imply a sign-off.
Sources and how we verified this
We establish the facts from the sources below and then write the explanation ourselves. We do not copy manufacturer wording and we do not host their manuals.
- Manufacturer technical and service documentation
This page has not been reviewed by an engineer, and we would rather tell you that than imply otherwise. The severity rating follows what the manufacturer publishes: where they say isolate the appliance and call someone, so do we.