Baxi E28 Fault Code: Causes, Fixes & Repair Costs
What does the Baxi E28 fault code mean?
The E28 fault code on a Baxi boiler signals a problem detected in the flue circuit — most commonly involving the flue NTC thermistor (the temperature sensor monitoring outgoing flue gases), the air pressure switch, or a physical blockage in the flue or condensate drain. When this fault triggers, the boiler locks out as a safety precaution and will not fire until the underlying issue is identified and resolved. The flue thermistor sits in the flue gas outlet and continuously feeds temperature data back to the PCB; if those readings fall outside expected parameters — or if flue gas flow is impeded — the boiler shuts down rather than risk an unsafe condition.
General guidance only — not a substitute for professional advice. 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
- Frozen condensate pipe Common
In cold snaps the external condensate pipe — typically a white or grey plastic pipe (21–32 mm diameter) running from the boiler to an outside drain — can freeze solid, backing up and triggering the E28 code. This is the single most frequent cause of Baxi lockouts during winter months and is one of the few E28 causes a homeowner can safely address themselves.
- Faulty flue NTC thermistor Common
The thermistor is a small heat-sensitive resistor clipped into the flue outlet. Over time it can degrade, develop a poor connection, or fail outright, causing it to report implausible temperatures to the PCB. A failed thermistor typically needs professional replacement.
- Blocked or obstructed flue terminal Common
The external flue terminal can accumulate debris, bird nests, ice, or compacted snow, restricting the exhaust of combustion gases. The resulting change in flue gas temperature or back-pressure is picked up by the thermistor or pressure switch, triggering the fault.
- Air pressure switch fault Sometimes
The air pressure switch confirms that the fan is creating adequate draught before ignition is allowed. A faulty diaphragm inside the switch, a cracked rubber hose connecting it to the fan assembly, or a blocked pressure sensing tube can all cause the switch to signal incorrectly and produce an E28 lockout.
- Wiring fault or poor connector Sometimes
Corrosion, chafing, or a loose plug on the thermistor wiring loom can interrupt the signal reaching the PCB, producing sporadic or persistent E28 faults even when the sensor itself is in good health.
- Low system pressure Sometimes
Pressure below approximately 0.8 bar can affect boiler operation and in some Baxi models contribute to sensor and airflow-related fault codes. While less directly linked to E28 than the causes above, it is worth checking as part of a systematic diagnosis.
- Faulty gas valve Rare
If the gas valve is under-delivering fuel, the burner flame may be weaker than expected, resulting in flue temperatures outside the range the thermistor anticipates. This is a less common route to E28 but can be the culprit when other causes have been ruled out.
- PCB failure Rare
If the main printed circuit board is misreading or ignoring a perfectly healthy thermistor signal, it can generate a spurious E28 code. PCB faults are relatively uncommon and are usually diagnosed only after all other causes are eliminated.
How to fix it
- Check whether the condensate pipe is frozen DIY safe
Look for the plastic condensate pipe (usually white or grey, 21–32 mm diameter) that exits the boiler and runs to an outside drain or soil stack. In freezing conditions, gently pour warm — not boiling — water along its length until it thaws and water flows freely. Once clear, attempt a boiler reset. If this resolves the fault, consider asking an engineer to lag the pipe to prevent a repeat.
- Inspect the external flue terminal for obstructions DIY safe
From outside your property, visually check the flue terminal (the round or rectangular grille where exhaust gases exit). Look for compacted snow, ice, bird nests, or debris blocking the opening. If you can safely clear a visible surface obstruction without climbing ladders, do so. Do not attempt to access or dismantle the flue itself.
- Check and top up system pressure if below 1 bar DIY safe
Look at the pressure gauge on the boiler — it should read between 1 and 1.5 bar when cold. If it is below 1 bar, use the filling loop (refer to your boiler manual for its location) to slowly top up to approximately 1.2 bar, then close the valve. Overfilling above 2 bar can cause separate problems, so stop well before that.
- Reset the boiler DIY safe
Once you have carried out any applicable DIY checks above, press and hold the reset button (usually marked with a flame or 'R' symbol) for approximately three seconds. Allow the boiler a full ignition cycle before deciding whether the fault has cleared. Do not reset more than two or three times in succession — repeated resets without clearing the root cause can mask a worsening fault.
- If the fault persists, call a Gas Safe registered engineer Gas Safe engineer
At this point the fault is likely to involve the flue thermistor, air pressure switch, wiring, gas valve, or PCB — all of which require a Gas Safe registered engineer to diagnose and repair safely. The engineer will use a flue gas analyser and electrical test equipment to pinpoint the defective component and replace it correctly. Do not attempt to open the boiler casing or handle any internal gas or electrical components yourself.
Parts you may need
- Flue NTC thermistor (Baxi compatible) · from £25
- Air pressure switch · from £45
- Pressure switch sensing hose / tube · from £12
- Condensate pipe insulation lagging · from £8
- Gas valve · from £120
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–£350, depending on the underlying cause.
Frequently asked questions
Can I fix the Baxi E28 fault myself without calling an engineer?
It depends on the cause. Thawing a frozen condensate pipe, visually clearing a blocked flue terminal, topping up system pressure, and resetting the boiler are all homeowner-safe actions. However, if those steps do not clear the fault, the problem almost certainly lies with an internal component — the thermistor, pressure switch, wiring, or gas valve — which must be handled by a Gas Safe registered engineer. Never open the boiler casing or touch gas components yourself.
How much does it cost to fix a Baxi E28 fault in the UK?
For most people the repair falls between £120 and £350, which covers an engineer's call-out, labour, and the cost of common parts such as a thermistor (around £25) or air pressure switch (around £45). A frozen condensate pipe costs very little if you thaw it yourself, or around £80–£120 if you call an engineer out in winter. Rare big-ticket repairs — such as a gas valve replacement (£200–£300 fitted) or PCB replacement (£300–£500 fitted) — sit above this typical range and are worth factoring in only if your engineer diagnoses one of those components.
Why does my Baxi boiler keep showing E28 and coming back after a reset?
A fault that returns shortly after resetting suggests the root cause has not been addressed. The most common repeat offenders are a condensate pipe that keeps re-freezing (because it is not insulated), a thermistor that is intermittently faulty, or a partial flue obstruction. Repeatedly resetting without fixing the underlying problem risks further wear on the boiler and can obscure a developing safety issue — so if E28 comes back more than a couple of times, call an engineer rather than continuing to reset.
Does an E28 fault mean my boiler needs replacing?
Not usually. E28 is most often caused by a straightforward component failure or a frozen condensate pipe, both of which can be repaired economically. A replacement might become the better financial decision if the boiler is over 12–15 years old, has a history of repeated breakdowns, or if an engineer diagnoses a failed heat exchanger — but for a boiler in otherwise reasonable condition, a repair is typically the more cost-effective route.