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.
Correct meaning matters. Some older articles and forum posts wrongly described E28 as a "low water pressure" code. That is not correct for Baxi. Low pressure shows as a separate fault (and a low reading on the pressure gauge). E28 is specifically about flue gas temperature sensing — a part that sits in the sealed combustion and flue area of the appliance.
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:
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.
An E28 can be triggered by a faulty or aged thermistor, a damaged wiring connection to the sensor, moisture or corrosion at the connector, or a wider combustion/flue issue that pushes flue temperatures out of range. Diagnosing which one it is requires test equipment and access to the sealed area.
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.
Safety first. Never remove the boiler casing and never touch the flue, gas valve, gas pipework or any sealed combustion part. If you smell gas, leave the property and call the National Gas Emergency line on 0800 111 999. You can find or check an engineer at the Gas Safe Register (the body that replaced CORGI back in 2009).
You may attempt a single front-panel reset using the boiler's reset button — that is the only homeowner-safe step for E28. Press it once and wait to see if the boiler relights and holds. If E28 returns, stop there and book an engineer. 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 — 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.)
The figures below are indicative UK ranges for 2026 and will vary by region, engineer and how accessible the part is. Always get a written quote before any 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.
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.
No. That is a common myth from some older guides. E28 on a Baxi is a flue gas thermistor / flue temperature sensing fault. Low pressure is shown separately and confirmed by a low reading (below about 1 bar) on the boiler's pressure gauge.
You can press the front-panel reset button once. If the E28 code clears and stays away, great. If it comes straight back, leave it and book a Gas Safe registered engineer — repeated resets will not fix a sensor fault.
No. The sensor sits within the flue and sealed combustion area, so by law only a Gas Safe registered engineer may access and replace it, then re-check combustion afterwards.
The boiler usually locks itself out for safety, so it may not run anyway. Do not try to override or force it. Have it diagnosed before relying on it for heating or hot water.
Most breakdown-focused boiler cover plans are designed for faults like a failed sensor, subject to the policy's terms, excess and any parts limits. Check your specific plan, and remember new policies often have an initial waiting period.
Compare boiler cover plans from a selected panel of UK providers and check what is included for breakdowns like the E28 flue gas thermistor fault.
Compare boiler coverThis 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.