Central heating thermistor sensor fault
The E28 error code on Baxi boilers indicates central heating thermistor sensor fault.
Baxi E28 means: central heating thermistor sensor fault. On this component the documented failure modes are the heating thermistor has failed — short circuit or open circuit or wiring, connector or harness of the heating thermistor damaged or loose. A Gas Safe registered engineer should diagnose the root cause.
What Does E28 Mean
The E28 code is shown by the boiler’s control board when it detects central heating thermistor sensor fault. The boiler then stops instead of continuing to run in that condition: the code is the appliance reporting a problem, not proof that the boiler is broken.
| What you see on the boiler |
|---|
| The display shows E28 and the boiler stops until the fault is cleared |
Common Causes
The manufacturer does not publish a cause list for this specific code. The causes below are the failure modes documented for the same component on other boilers, confirmed by the service literature of 8 different manufacturers.
| Cause |
|---|
| The heating thermistor has failed — short circuit or open circuit |
| Wiring, connector or harness of the heating thermistor damaged or loose |
Safety Warning
E28 is a gas-related fault. Never attempt to repair or adjust the gas valve, burner assembly, or flue system yourself. All gas work must be carried out by a Gas Safe registered engineer (UK) or licensed HVAC technician (US). Repeated resets after ignition failure can cause unburnt gas to accumulate inside the combustion chamber — this is a serious safety hazard. If you smell gas at any point, turn off the gas supply at the meter, open windows, leave the property, and call your gas emergency service immediately. Gas-related fault. Do not open the boiler casing or attempt repairs to the gas valve, burner, ignition or flue. Repeated resets are unsafe. Diagnosis and repair must be carried out by a Gas Safe registered engineer.
Parts & Prices
Prices shown are indicative. Actual prices and availability vary by region.
Step-by-Step Diagnosis
1. Visually inspect the flue terminal outside — check for blockages, birds' nests, or ice. 2. Check the flue run — verify it is within the manufacturer's maximum length and number of bends. 3. Perform a flue gas analysis — CO and CO2 levels should be within specification. 4. Check the condensate drain — ensure it is not blocked. 5. Verify the air intake is not obstructed (for room-sealed appliances). 6. Check for signs of flue gas spillage in the room (for open-flue appliances)
How to Fix It
Measure the resistance of the heating thermistor with a multimeter and compare it against the manufacturer’s resistance-versus-temperature table; replace it if it is open circuit, short circuit or out of range.
Unplug and re-seat the connector of the heating thermistor, then check the harness for chafing, corroded pins and breaks in continuity.
Refer to the Baxi service documentation for the detailed procedure. Only a qualified engineer should carry out these repairs.
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Affected Models
Baxi models that can display fault code E28:
| Model Family | Output Range | Frequency |
|---|---|---|
| All Baxi models | — | Possible |
Frequently Asked Questions
Measure the resistance of the heating thermistor with a multimeter and compare it against the manufacturer’s resistance-versus-temperature table; replace it if it is open circuit, short circuit or out of range. According to the technical documentation, this is the first check on the list for E28.
E28 is classified as a high-severity fault: it involves gas, combustion or flue components. Do not keep resetting the boiler and do not attempt the repair yourself — call a Gas Safe registered engineer.