Potterton E28 Fault Code: Causes, Fixes & Repair Costs
What does the Potterton E28 fault code mean?
The E28 code on a Potterton boiler indicates a fault with the flue thermistor — the sensor responsible for monitoring the temperature of exhaust gases leaving the boiler. When this sensor stops working correctly, the boiler's control board can no longer confirm that combustion gases are being expelled safely, so it shuts down as a precaution. You may notice the boiler firing up and beginning to warm the system before cutting out before radiators reach temperature. The E28 appears across a number of Potterton models including the Combi Gold HE A, Combi Promax HE Plus, and Promax System HE Plus. Because the flue system is directly linked to safe boiler operation, this fault should be investigated promptly rather than repeatedly reset and ignored.
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 failed flue thermistor Common
The NTC thermistor sensor itself degrades over time and can give inaccurate readings or stop responding entirely. This is the single most common root cause of an E28. The boiler detects an out-of-range or implausible temperature signal from the flue and locks out for safety.
- Loose or damaged wiring to the thermistor Common
The wiring loom connecting the flue thermistor to the PCB can work loose, corrode, or suffer insulation damage. Even a partial break in the circuit produces the same symptom as a dead sensor, because the control board simply receives no valid signal.
- Frozen or blocked condensate pipe Sometimes
During cold snaps, the condensate drain pipe can freeze solid, causing a back-up that disrupts normal flue operation. This can trigger flue-related fault codes including E28. In milder weather a partial blockage from debris can cause a similar effect.
- Thermistor incorrectly positioned or dislodged Sometimes
If the sensor has moved out of its designated pocket — due to vibration, a previous repair, or a poor original installation — it will measure ambient rather than flue temperatures, producing erratic readings that trigger the fault.
- Gas valve fault affecting combustion Rare
If the gas valve is not supplying the correct fuel flow, combustion temperatures fall outside normal parameters. The flue thermistor then reports unusual readings even though the sensor itself is fine, causing the boiler to flag an E28.
How to fix it
- Check your gas supply is on DIY safe
Confirm that other gas appliances in the property — cooker, gas fire — are working normally. If there is no gas supply across the property, contact your gas supplier rather than an engineer.
- Reset the boiler once or twice DIY safe
Press and hold the reset button for 3–5 seconds as described in your Potterton manual. Allow the boiler a full ignition attempt before judging success. Do not reset more than two or three times — repeated resets can mask the underlying fault and cause additional wear.
- Check and restore system pressure if low 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 to top it up slowly. Flue-related faults can sometimes coincide with low pressure causing multiple lockout signals at once.
- Inspect the condensate pipe for freezing (cold weather only) DIY safe
If outdoor temperatures are below freezing, the white plastic condensate pipe running outside the property may be frozen solid. Wrap a warm (not boiling) wet cloth around the pipe or pour lukewarm water over it to thaw the blockage. Never use a naked flame. Once thawed, reset the boiler.
- Check the flue terminal outside is clear DIY safe
Have a look at where the flue exits the building. Ensure the terminal grille is not blocked by a bird nest, leaves, or snow. Do not attempt to open or dismantle any flue components — simply clear any visible external obstruction if it is safely accessible.
- Call a Gas Safe registered engineer to diagnose and repair the fault Gas Safe engineer
If the fault persists after the steps above, a Gas Safe engineer is required. They will test the flue thermistor with a multimeter and compare resistance readings against expected values, inspect and re-seat the sensor if it has moved, check and repair the wiring loom, and test the gas valve and combustion parameters. They will replace the thermistor or any other failed components as needed.
Parts you may need
- Flue NTC thermistor sensor · from £35
- Thermistor wiring harness / loom · from £25
- Gas valve · from £120
- PCB (printed circuit board) · from £180
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 £90–£280, depending on the underlying cause.
Frequently asked questions
Is the E28 fault dangerous — do I need to stop using my boiler immediately?
The boiler has already shut itself down as a safety precaution, so in most cases the immediate risk is limited. However, because the E28 relates to the flue system and exhaust gas monitoring, you should not attempt to force the boiler to keep running. Arrange for a Gas Safe engineer to inspect it promptly, and do not attempt more than two or three resets while you wait.
How much does it cost to fix a Potterton E28 fault in the UK?
For the most common repair — replacing the flue thermistor sensor and any associated wiring — most homeowners pay between £90 and £180 including call-out and labour. If the gas valve needs attention the cost typically rises to around £200–£280. A PCB replacement is a less common outcome but can cost considerably more, often £350–£500 or above, which may prompt a conversation about whether replacing an older boiler is the better value.
Can I replace the flue thermistor myself to save money?
No. Although the thermistor itself is an inexpensive part, it sits within the flue system and working on it requires the boiler to be properly isolated and inspected as part of a broader safety check. Only a Gas Safe registered engineer should access internal boiler components or the flue assembly. Attempting it yourself could void any warranty, breach building regulations, and — more importantly — create a safety risk.
Why does my Potterton boiler fire up and then cut out before the heating gets hot — is that E28?
Yes, this is a classic symptom associated with E28. The boiler starts the ignition sequence and begins heating, but the flue thermistor either sends an implausible temperature reading or no reading at all, causing the control board to lock out before the radiators have had time to warm up properly. A reset may briefly clear it, but the lockout will recur until the underlying sensor or wiring fault is repaired.