Error E321

Safety first: do not attempt internal repairs on a gas boiler unless you are a Gas Safe registered engineer. If you smell gas, leave the property immediately and call the gas emergency number. Before doing any checks, switch off the boiler at the programmer and at the isolator (or at the fused spur), and do not open the boiler casing. If you are not comfortable with any step below, stop and call a Gas Safe engineer.

Initial homeowner checks you can safely do:

1) Record the fault: write down the exact error code (E321), any other codes or lights, and the time it happened. Take a photo of the display if you can. This information helps the engineer.

2) Try a reset: follow your boiler’s reset procedure (press and hold the reset or selector as per your manual). If the fault clears and the boiler runs normally for a while, note how long it stays OK and if it returns. Repeated resets are not a permanent fix.

3) Check basic system conditions: confirm the boiler has power and the house hasn’t tripped a fuse or RCD. Check the system pressure on the boiler gauge—very low pressure can cause faults elsewhere (top up to the recommended pressure if you know how). Check for obvious signs of water leaks around the boiler.

4) Look for related issues: if you have other error codes shown, or if the hot water is simply cold or fluctuating, mention this to the engineer. If pipes or condensate runs are exposed to freezing temperatures, consider whether freezing might be affecting sensors or pipes (although E321 specifically points to the DHW NTC).

What a qualified engineer will do (and why you should call one):

1) Professional diagnosis: the engineer will isolate the appliance, remove the casing, and visually inspect the hot water NTC sensor, its connector and wiring for damage or corrosion. They will measure the sensor’s resistance with a multimeter at ambient and running temperatures to confirm open-circuit or short-circuit conditions or out-of-spec readings.

2) Wiring and PCB checks: they will check continuity of wiring between sensor and PCB and inspect the PCB connector for faults. If wiring and connector are fine but readings are wrong, the sensor will be replaced. If readings are correct at the sensor but the PCB shows a fault, the PCB may need further testing or replacement.

3) Sensor replacement and sealing: if the sensor is faulty the engineer will replace the DHW NTC thermistor and any O-ring/seal required, then reassemble and run the boiler through tests to confirm safe operation and correct hot water control.

4) Final checks and commissioning: the engineer will run hot water and check temperatures, confirm no secondary faults, and clear fault history. They will also update any service records and advise on warranty implications.

Important notes: do not attempt to open or work inside the boiler’s casing or attempt to replace sensors yourself unless you are Gas Safe registered — this can be dangerous and will likely void warranties. If the boiler is under warranty, contact the manufacturer or your installer and use a Gas Safe registered engineer. When you call an engineer, provide make, model, serial number, the E321 code, any other codes, what you tried (reset/no reset), and photos if possible to speed diagnosis.