Main E28 Fault Code: Causes, Fixes & Repair Costs
What does the Main E28 fault code mean?
The E28 fault code on Main boilers indicates a problem with the flue thermistor — a small NTC (Negative Temperature Coefficient) temperature sensor that monitors the temperature of exhaust gases leaving the boiler through the flue. When this sensor sends a reading outside the expected range, or stops communicating with the control board altogether, the boiler locks out as a safety measure. The fault can also be triggered by a blocked or frozen condensate pipe, a flue obstruction restricting gas flow, wiring faults between the sensor and the PCB, or — less commonly — a failing gas valve or PCB itself. Because Main boilers share their fault code architecture with Baxi and Potterton (all part of the BDR Thermea Group), the E28 code carries the same meaning across all three brands.
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
- Faulty or failing flue thermistor (NTC sensor) Common
The flue thermistor is the most frequently blamed component when E28 appears. Over time the sensor can drift out of calibration, develop an open or short circuit, or simply fail with age. When it does, the control board receives readings that are implausible or absent entirely and locks the boiler out. Replacing the sensor is a straightforward job for a Gas Safe engineer but is not a DIY task.
- Frozen or blocked condensate pipe Common
In cold weather the plastic condensate pipe — which carries acidic condensate water away from the boiler to an external drain — can freeze solid, causing a blockage. The resulting back-pressure can disrupt the flue gas path and trigger sensor errors including E28. This is the first thing to investigate in temperatures below about 2 °C and is one of the few checks a homeowner can safely carry out themselves.
- Loose, corroded or damaged wiring connections Sometimes
The wiring loom between the flue thermistor and the PCB can suffer from loose push-fit connectors, corrosion at terminals, or chafed insulation — particularly on older boilers or those installed in damp utility rooms. A poor connection produces intermittent or incorrect signals that the control board interprets as a sensor fault. An engineer can check continuity and reseat or replace the relevant wiring.
- Flue blockage or damage Sometimes
A bird's nest, debris, or a damaged flue terminal can restrict the flow of exhaust gases, causing flue temperatures to behave abnormally and triggering E28. A visible external check of the flue terminal is safe for a homeowner, but clearing an internal blockage or repairing flue sections must be done by an engineer.
- Faulty gas valve Sometimes
If the gas valve is sticking or failing to open correctly, the burner may fire weakly or erratically, causing unusual flue temperature readings that confuse the thermistor circuit and produce an E28 lockout. Gas valve faults are less common than sensor faults but should be considered if the thermistor and wiring both test healthy.
- PCB fault or sensor communication failure Rare
In a small number of cases the printed circuit board itself develops a fault that prevents it from correctly interpreting signals from the flue thermistor, even when the sensor is working normally. PCB failure is typically the last thing an engineer will investigate once all other components have been ruled out.
How to fix it
- Reset the boiler — once or twice only DIY safe
Press and hold the reset button (or switch the boiler off at the mains for 30 seconds, then back on) to clear the lockout. If E28 disappears and the boiler runs normally, monitor it closely over the next few hours. Do not reset more than two or three times — repeated lockouts without a fix indicate a real underlying fault that needs a professional diagnosis.
- Check whether the condensate pipe is frozen (cold-weather fault) DIY safe
In freezing weather, locate the condensate pipe — usually a white or grey plastic pipe (22 mm or 32 mm diameter) exiting the boiler through an external wall. Feel along its length for a solid, icy section or look for visible frost. If frozen, thaw it gently by applying a warm (not boiling) wet cloth or a heat pack to the affected section, working from the boiler end outward. Once thawed, reset the boiler. To prevent a repeat, lag the pipe with foam pipe insulation rated for outdoor use.
- Carry out a visual check of the external flue terminal DIY safe
From the outside of the property, look at the flue outlet (the round plastic terminal on the wall or roof). Check that it is not blocked by a bird's nest, leaves, ice, or debris, and that the terminal guard is intact. Do not climb onto the roof or use a ladder unsafely. If you can see an obvious obstruction that is clearly accessible at ground level, you may remove it carefully. Otherwise, report it to an engineer.
- Check your system pressure is within range DIY safe
Look at the boiler's pressure gauge. It should read between 1.0 and 1.5 bar when the system is cold. If it is below 0.8 bar, top it up via the filling loop following the instructions in your boiler manual. Very low pressure can contribute to a range of faults — though it is not a primary cause of E28, it is worth ruling out.
- Call a Gas Safe registered engineer to inspect and test the flue thermistor and wiring Gas Safe engineer
An engineer will use a multimeter to measure the resistance of the NTC thermistor across the expected temperature range and compare it against the manufacturer's specification. They will also inspect the wiring connectors for corrosion or looseness and test continuity back to the PCB. If the thermistor is out of range or has failed, they will replace it with a compatible part and retest. This is not a DIY task — accessing the sensor involves working inside the boiler casing.
- Have an engineer inspect the flue internally and check the air pressure switch Gas Safe engineer
If the thermistor and wiring are healthy, the engineer will inspect the flue internally for partial blockages or condensate pooling, and check that the flue is correctly sealed and properly sloped to drain. They will also test the air pressure switch, which works alongside the thermistor as part of the combustion safety circuit. A faulty or waterlogged pressure switch can contribute to E28 on some Main boiler variants.
- Have the gas valve and PCB assessed if all other components are healthy Gas Safe engineer
If the thermistor, wiring, flue, and pressure switch are all found to be in good condition, the engineer will assess the gas valve for correct operation and gas rate, and — as a last resort — test the PCB. Both components require Gas Safe competency to work on safely. Your engineer will advise whether repair or replacement is the more cost-effective route, taking the boiler's age and overall condition into account.
Parts you may need
- Flue thermistor (NTC temperature sensor) · from £25
- Wiring loom / thermistor harness · from £30
- Air pressure switch · from £45
- Gas valve (compatible replacement) · from £95
- PCB (printed circuit board) · from £180
- Condensate pipe insulation kit · from £12
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–£320, depending on the underlying cause.
Frequently asked questions
Can I fix the Main E28 fault code myself?
The only homeowner-safe steps are thawing a frozen condensate pipe, resetting the boiler once or twice, and doing a visual check of the external flue terminal. Everything else — testing and replacing the flue thermistor, checking the wiring, inspecting the gas valve, or working on the PCB — must be carried out by a Gas Safe registered engineer. Attempting gas or electrical component work without the correct qualifications is illegal and dangerous.
Why does my Main boiler show E28 every winter but not in summer?
If E28 appears only in cold weather, a freezing condensate pipe is almost certainly the cause. The condensate pipe carries water produced by the boiler's condensing process to an external drain, and in temperatures around or below freezing it can ice up and block. Insulating the external section of the pipe with foam lagging rated for outdoor use is the most effective long-term fix and should prevent the problem recurring.
How much does it cost to fix a Main E28 fault?
Most homeowners with an E28 fault caused by a faulty flue thermistor or wiring issue pay in the region of £120–£320 including parts and labour. A straightforward thermistor swap at the lower end of that range is the most common repair. If the fault turns out to involve the gas valve, costs are typically around £180–£250 all-in. A PCB replacement is less common and can push costs higher — your engineer can advise once a diagnosis has been made.
Is E28 on a Main boiler the same as on a Baxi or Potterton?
Yes — Main, Baxi, and Potterton boilers all belong to the BDR Thermea Group and share the same fault code architecture. E28 means the same thing across all three brands: a flue thermistor or related sensor/communication fault. The diagnostic and repair process is essentially identical, and many spare parts are interchangeable between models in the same product family.