Overview
E28 on a Potterton Assure combi or system boiler means the boiler has detected a fault with the flue temperature sensor (an NTC thermistor) or its circuit. The flue sensor monitors the temperature in the flue or heat exchanger area; if the sensor reading is out of expected range, missing, or shorted, the boiler will show E28 and may lock out to prevent unsafe operation. The fault can be caused by a failed sensor, damaged wiring or connector, corrosion, or occasionally an internal PCB issue reporting a false fault. Severity: this is a safety-related sensor fault. The boiler may refuse to run until the issue is resolved, because the flue sensor helps detect over-temperature or flue problems. Some simple causes (loose connector, temporary glitch) can be cleared by a reset, but the underlying problem often requires diagnosis and replacement of the sensor or repair of wiring. Replacing or testing the flue thermistor and any internal wiring is work for a qualified Gas Safe engineer; do not attempt to bypass safety interlocks or run the boiler with suspects sensors. DIY vs professional: homeowners can perform basic safe checks (power-cycle/reset, visual checks of external flue for blockage, and inspect visible wiring/connectors for obvious damage) but detailed electrical testing, dismantling the boiler casing, sensor replacement or PCB work must be carried out by a Gas Safe registered engineer. If the error persists after basic checks, call a professional promptly.
Possible Cause: Flue negative temperature coefficient (NTC) fault
Troubleshooting Steps
Safety precautions:
1. If you suspect a gas or combustion smell, evacuate the property and call your gas emergency number immediately. Do not attempt checks if you smell gas.
2. Only perform non-invasive checks that do not involve opening sealed boiler covers that expose gas or live electrical components unless you are a qualified engineer. Isolate mains power to the boiler before touching any cables or connectors.
3. Never bypass safety devices or interlocks. Do not attempt to force the boiler to run with error codes present.
Initial homeowner checks (safe, low-skill):
1. Note the exact error code and any other codes or symptoms (e.g., boiler locked out, lights flashing). Take a photo of the display for the engineer.
2. Power-cycle the boiler: switch the boiler off at the programmer or isolator, wait 30 seconds, then switch back on. Some sensor errors clear if they were transient.
3. Check the external flue terminal on the outside of the building for visible blockage (birds’ nests, leaves, snow, debris). If the terminal is blocked, carefully clear the obstruction from outside but do not probe into the flue.
4. Check for recent works, damp, or rodent activity around the boiler which might have disturbed cables or damaged the sensor cable.
5. Check boiler pressure and thermostats if you notice other faults, but low pressure is not the primary cause of E28.
Further diagnostic steps (only if comfortable and safe; otherwise call an engineer):
1. With the boiler powered down at the isolator, locate any accessible sensor connectors or wiring looms visible without removing sealed panels. Look for loose connectors, visibly damaged cables, signs of corrosion or water ingress. If you find a loose connector, you may reseat it firmly and then restore power to check for change.
2. If the connector or cable is wet or corroded, do not continue operating the boiler — isolate power and call an engineer. Moisture on low-voltage sensor wiring can cause intermittent faults.
3. If you have electrical competence and the correct multimeter and are confident, you can measure the NTC sensor resistance at the connector with the sensor at room temperature. Only do this with power isolated and if the connector is accessible without dismantling sealed components. Do not attempt internal measurements beyond visible connectors. (Note: expected resistance values vary by model; interpretation is best left to an engineer.)
4. Record whether the E28 is persistent or intermittent, and whether it follows any events (power cut, freezing weather, recent service work). Intermittent faults often point to wiring/connector issues; persistent faults commonly mean sensor failure.
What a professional will do:
1. A Gas Safe engineer will isolate and open the boiler, visually inspect and test the flue NTC sensor, wiring and the associated PCB input. They will measure the sensor resistance across temperatures to confirm if the NTC is within specification.
2. If the sensor is faulty or wiring is damaged, the engineer will replace the flue thermistor and/or repair wiring. If wiring and sensor are good, they will check the PCB and flue thermostat/overheat devices; a PCB fault or flue thermostat lockout (E130/E131) may require further repair or replacement.
3. The engineer will also check for flue obstruction or combustion issues that could cause over-temperature or tripping of flue-related sensors.
When to call a professional and final notes:
1. Call a Gas Safe registered heating engineer if the E28 code persists after a basic reset and visual checks, if you find damaged wiring, if the flue terminal was blocked, or if you are not confident performing the safe checks described above.
2. Do not attempt to replace internal sensors, access combustion parts, or manipulate gas components yourself. These tasks require certification and specialist tools.
3. When contacting an engineer, give model, serial number, the E28 code, any other codes, what checks you have already done, and send photos if possible — this speeds diagnosis.
Remember: E28 is a sensor/circuit fault that frequently requires a trained engineer to repair safely. Avoid running the boiler when a safety-related sensor is reporting a fault.
Helpful Resources
Boiler Manual
Download the official PDF manual for the Potterton Assure Combi and system.