Error E321

Safety precautions:

- If you smell gas, get everyone out, call the gas emergency number and do not operate the boiler.

- Do not attempt to work on the gas supply, burner, or internal sealed components unless you are Gas Safe qualified.

- Before any visual checks, switch the boiler off at the programmer and isolate electrical supply at the fused spur or consumer unit if you are near the appliance casing. If you are unsure, leave further work to an engineer.

- Stop using the boiler for hot water until the fault is resolved to avoid unpredictable behavior.

Initial homeowner checks you can safely do:

1. Note the full error code and any additional LED indications or messages on the display.

2. Try a single reset of the boiler following the manufacturer’s procedure (use the reset button or selector R, hold as instructed). If the fault clears and does not return, monitor for intermittent recurrence. Do not repeatedly reset more than once or twice in a short period.

3. Check the boiler pressure gauge — although E321 is a hot water sensor fault, very low pressure or other system faults can cause cascade errors. If pressure is very low, do not overfill; consult your manual for repressurising steps or call an engineer.

4. Check for obvious external issues: visible water leaks around the boiler, damp or corrosion on the casing, or frozen/blocked condensate pipe (condensate freezing causes other codes but is worth ruling out in cold weather).

5. Check hot water outlets: turn a hot tap on and observe whether hot water is produced, whether the boiler tries to fire, and whether the code appears as soon as hot demand is made.

Diagnostic steps a Gas Safe engineer will perform (for information):

1. Confirm the fault history in the boiler logs and reproduce the fault safely.

2. Isolate electrical supply and remove the boiler cover to visually inspect the hot water NTC sensor, wiring loom and connector for damage, loose pins, corrosion or water ingress.

3. Measure the NTC sensor resistance with a multimeter at the sensor and at the PCB connector to check for open/short circuit or intermittent wiring faults. Compare readings to the manufacturer’s resistance/temperature chart.

4. If wiring and connector are OK, test continuity through the wiring harness back to the PCB to rule out broken wires.

5. If the sensor reads out of tolerance, replace the hot water NTC sensor and seal (O-ring) with the correct OEM part. Ensure the sensor is fitted to the correct depth and torque to make good thermal contact and sealing.

6. If sensor and wiring are good but readings are still incorrect, check the boiler PCB and connector for damage or corrosion; replace PCB if found defective.

7. After repair, restore power, clear fault codes, run the boiler through hot water and heating cycles, and verify correct temperature readings and stable operation.

When to call a professional and other notes:

- Call a Gas Safe registered engineer if the fault persists after a reset or if you find any wiring damage, water ingress, or you do not feel confident doing the safe visual checks above.

- Do not attempt to replace sensors, open burner compartments, or modify gas or electrical connections yourself — these operations must be carried out by a qualified engineer.

- Mention warranty status when you call an engineer; unauthorised DIY repairs may void warranty cover.

- Record when the fault appeared, what you tried, and any other symptoms (intermittent hot water, boiler locking out) to help the engineer diagnose quickly.

In short: E321 usually means the hot water NTC sensor or its wiring has failed. Try a single safe reset and visual checks, but arrange a Gas Safe engineer to perform electrical checks and sensor replacement if the code remains.