Baxi E28 Fault Code: Flue Gas Thermistor

What the Baxi E28 fault code means

On a Baxi combi or system boiler, the E28 error code relates to the flue gas thermistor — a temperature sensor (an NTC, or negative temperature coefficient, sensor) that monitors the temperature of the gases leaving the boiler through the flue.

The boiler's control board reads this sensor constantly. If the signal falls outside the expected range — too high, too low, an open circuit or a short — the boiler logs E28 and locks out as a safety measure.

What it means	What to check	Call an engineer?
Flue gas thermistor (NTC) / flue-temperature fault — not low pressure; the signal is outside the expected range, so the boiler locks out. In cold weather, often caused by a frozen condensate pipe.	In cold weather, check for and thaw a frozen condensate pipe first; then try one front-panel reset to see if it relights and holds	Yes — if there's no freeze, or E28 returns after a reset, the sensor sits in the sealed flue/combustion area and needs a Gas Safe engineer

What the flue gas thermistor actually does

Modern condensing boilers are designed to extract as much heat as possible from the burnt gas before it leaves the appliance. The flue gas thermistor checks that the temperature of those exhaust gases stays within safe, expected limits. It helps the boiler:

• confirm combustion is behaving normally and the heat exchanger is transferring heat correctly;
• protect the appliance and flue components from overheating;
• support the boiler's condensing operation and overall efficiency.

Because this sensor lives in the flue path — part of the sealed combustion circuit — anything that affects it is firmly inside Gas Safe territory, not something to poke at yourself.

Common causes of an E28

An E28 is the boiler protecting itself because the flue temperature signal is outside the range it expects. That can stem from the sensor itself or from something upstream affecting combustion or flue temperatures. The usual culprits are:

Likely cause	What's going on	Who fixes it
Frozen or blocked condensate pipe	In cold weather the external waste (condensate) pipe ices up and backs up; the trap can no longer drain, which disturbs combustion and flue temperatures and trips the E28 lockout. The most common cold-snap trigger.	Homeowner can safely thaw the external pipe — see below
Faulty or aged flue gas thermistor	The NTC sensor sends an out-of-range, open-circuit or short-circuit reading.	Gas Safe engineer
Damaged wiring or a corroded connector	Moisture or a loose connection at the sensor gives a false signal.	Gas Safe engineer
Blocked or damaged flue	An obstruction or fault in the flue alters the temperature of the gases the sensor reads.	Gas Safe engineer
Gas valve fault	A gas valve not delivering the right amount of gas changes combustion and flue temperature.	Gas Safe engineer

Low system pressure should also be ruled out while you are at the boiler — a quick glance at the gauge does that — but to be clear, E28 itself is not a low-pressure code; low pressure is reported separately. The one cause on the list you can safely tackle yourself is a frozen condensate pipe.

Frozen condensate pipe — the common cold-weather trigger

Every condensing boiler — and modern Baxi boilers are condensing — produces a small amount of acidic wastewater (condensate) that drains away through a thin plastic pipe, usually run outside to a drain. In sub-zero weather that external pipe can freeze.

When it does, condensate backs up inside the boiler, the trap can no longer clear, and combustion and flue temperatures are disturbed — which is exactly what the flue gas thermistor logic watches for. The result is that the boiler trips out, frequently showing E28 on a Baxi.

The tell-tale signs are an E28 (or a lockout) appearing during or just after a cold spell, often first thing on a frosty morning, sometimes with a gurgling sound from the boiler as condensate struggles to drain.

The good news: a frozen condensate pipe is one of the few E28 causes you can usually clear yourself, safely, in a few minutes — because the part you are dealing with is an external plastic waste pipe, nothing to do with gas, the flue or the sealed combustion area.

How to thaw a frozen condensate pipe safely

If E28 appears in cold weather, this is the one root-cause fix a homeowner can safely attempt:

1. Find the external condensate pipe. It is usually a white or grey plastic pipe (around 22–32 mm) exiting an outside wall low down, often running to a drain or gully. It is the waste pipe — not the flue terminal (the flue is the larger metal/plastic outlet where you may see vapour when the boiler runs). The frozen section is normally the most exposed point: the open end, a bend, or where it dips.
2. Warm the pipe gently. Pour warm water along the frozen section — work from the top down. Aim for hand-hot water: boil a kettle and let it cool for around 10–15 minutes so it is roughly lukewarm (well below boiling). A hot-water bottle, a microwaveable heat pack or a warm cloth held against the pipe also work well.
3. Never use boiling water and never a naked flame. Boiling water can crack the plastic pipe, and a flame is an obvious hazard near pipework — both can turn a five-minute fix into an expensive one. Be careful with spilled water on the ground in freezing conditions, as it can refreeze and become slippery.
4. Reset the boiler. Once the ice has cleared and the pipe drains freely, follow the boiler's instructions to reset (a single front-panel reset). The E28 should clear and the boiler relight.

Common symptoms of an E28 fault

• The boiler displays E28 and goes to lockout — no heating and, on a combi, no hot water.
• The fault may appear intermittently at first, then become permanent.
• The boiler may fire briefly and then shut down shortly after ignition.
• The display can show the code immediately on a reset attempt if the sensor fault is constant.

As covered above, an E28 can be triggered by a frozen condensate pipe, a faulty or aged thermistor, a damaged wiring connection to the sensor, moisture or corrosion at the connector, a blocked or damaged flue, a gas valve fault, or another combustion issue that pushes flue temperatures out of range.

Apart from the frozen condensate pipe, diagnosing which one it is requires test equipment and access to the sealed area.

Why E28 is usually a Gas Safe engineer job — not a DIY fix

The flue gas thermistor is mounted within the flue and combustion section of the boiler. Getting to it means working behind the casing, around the sealed combustion circuit and the flue — exactly the components only a Gas Safe registered engineer is legally allowed to work on.

Checking, testing or replacing this sensor also means re-verifying combustion afterwards, which needs a flue gas analyser.

Fix the root cause first, then reset

Resetting an E28 without addressing what caused it just relocks the boiler. Follow this order:

1. In cold weather, check for a frozen condensate pipe first — and thaw it using the safe method above if it is iced up. This is the most common cold-snap cause and the only one you can safely fix yourself.
2. Try a single front-panel reset. Once any freeze is cleared, press the boiler's reset button once and wait to see if it relights and holds. That single reset is the only homeowner-safe step for E28.
3. If E28 returns, call a Gas Safe registered engineer. A code that comes straight back points to the thermistor, wiring, flue or combustion — sealed-area work. Repeatedly hammering the reset button will not fix a sensor fault and can mask a genuine combustion problem.

For context, while you are at the boiler it is worth glancing at the pressure gauge to rule pressure out — a healthy system reads roughly 1–1.5 bar when cold and around 2 bar when hot. Below about 1 bar is low.

That is unrelated to E28, but it is reassurance that the code is not a pressure issue. (If you ever do need to top up, that is done via the filling loop and is a separate, homeowner-safe task.)

Stopping it happening again in winter

If a frozen condensate pipe was behind your E28, a few steps reduce the chance of it recurring in the next cold snap:

• Lag the external pipe. Wrap the exposed plastic pipe in weatherproof foam pipe lagging (Baxi recommends waterproof, weatherproof lagging). This is a simple, homeowner-safe DIY job and the most effective first line of defence.
• Keep the heat ticking over in very cold weather. Running the heating at a low, steady temperature overnight during a hard frost helps keep condensate moving rather than letting it sit and freeze.
• Leave re-routing and pipe changes to an engineer. Re-routing the pipe internally, fitting a larger-diameter pipe or adding a trace heater can all help — but these are jobs for a Gas Safe registered installer, not DIY.
• Book an annual boiler service. A yearly service by a Gas Safe registered engineer keeps the flue, sensors and combustion in good order and is the best way to catch problems — like a tired thermistor or a poorly sited condensate run — before they cause a lockout.

Likely costs in 2026

The figures below are indicative UK ranges (last checked 2026) and will vary by region, engineer and how accessible the part is. A frozen condensate pipe you thaw yourself costs nothing. Always get a written quote before any chargeable work.

Job	Indicative cost (2026)
Diagnostic call-out / inspection	£70–£120
Flue gas thermistor (NTC) replacement	£130–£260
Wiring / connector repair	£90–£180
Wider combustion or flue fault (varies widely)	£200+

If your boiler is still under manufacturer warranty, contact Baxi (or your installer) before paying for independent repairs — unauthorised work can affect cover. If you hold a boiler cover or service plan, a sensor fault like this is usually exactly the sort of repair a policy is meant to handle.

Where boiler cover fits in

An E28 is a good illustration of why many households take out a policy: it is an unpredictable breakdown that needs a registered engineer and a chargeable part. A boiler cover plan can turn that one-off bill into a fixed monthly cost, with call-outs and many parts included.

Whether that maths works for you depends on your boiler's age and your appetite for surprise repairs — our guide on whether boiler cover is worth it walks through the trade-offs, and what boiler cover actually includes explains the typical exclusions to watch for.

If you are weighing up plans, it is worth comparing what is covered, the excess, and any parts limits side by side. You can compare boiler cover from a selected panel of providers, or browse our pick of the better-value plans and cheaper options if budget is the priority.

This article is general information, not advice. We are an independent comparison site and may earn commission from providers on our panel, which does not cover the whole market. Always use a Gas Safe registered engineer for any work inside the boiler.