Error E321

Safety precautions:

- If the boiler is showing E321, do not try to force it to run by tampering with gas controls. Work involving gas or live mains should only be done by a Gas Safe engineer.

- Before any visual inspection of wiring or connectors, switch the boiler off and isolate the electrical supply at the fuse box. If you must remove covers, ensure power is off and wait for hot components to cool.

- If you smell gas at any time, evacuate the property and call the emergency gas number immediately.

Initial homeowner checks you can safely do:

1. Note the exact fault code(s) and any other codes displayed. Write down model and serial number from the boiler label.

2. Try a simple reset: use the boiler reset button per the manual (hold for 3–5 seconds). If the code clears and normal hot water returns, monitor for recurrence. Multiple repeat lockouts indicate a persistent fault.

3. Check system water pressure on the boiler gauge. Low system pressure can cause other faults; top up to the recommended level if it is low (consult your manual). Do not proceed with internal checks if pressure is very low or you see leaks.

4. Check whether the problem is only hot water (DHW) or if central heating is affected too. E321 is specific to hot water temperature sensing.

5. Visually inspect any accessible sensor wiring and connectors for obvious damage, loose plugs or corrosion. Do this with power isolated.

Specific diagnostic and next-step checks (only if you are competent with basic electrical safety and have isolated power):

1. Locate the DHW/NTH sensor connector – it is typically on or near the hot water heat exchanger or a plug on the boiler wiring loom. Consult the boiler manual or label for the sensor location before opening the outer casing. If unsure, stop and call an engineer.

2. With power isolated, carefully disconnect and re-seat the thermistor connector. Look for green/white corrosion, bent pins, or signs of water ingress. Reassemble and power up to test if the fault clears.

3. If you have a multimeter and know how to use it safely, you can check continuity of the sensor wiring from connector to PCB and, if you can access the sensor body, measure the thermistor resistance. Do not attempt resistance checks if you do not have the correct reference values for that exact sensor; incorrect interpretation can lead to unnecessary part changes. If you measure open circuit (infinite resistance) or a short (near zero ohms), the sensor or wiring is faulty.

4. If wiring and connectors look fine but the error remains after a reset, the sensor itself is probably faulty or the control electronics are misreading it. Replacement of the NTC sensor or further electronic diagnosis is normally required.

When to call a professional and what they will do:

- Call a Gas Safe registered engineer if the E321 code persists after a reset or after you have visually checked and re-seated connectors. Also call an engineer if you are not confident performing the safe visual or electrical checks above.

- A trained engineer will safely isolate and open the boiler, confirm the fault with diagnostic tools, measure sensor resistance against the manufacturer specification, check wiring continuity, check for PCB faults and swap parts if needed (NTC sensor replacement, wiring repair or PCB repair/replacement). They will also test the boiler under load and clear stored fault codes.

Final notes:

- Do not attempt gas or live electrical repairs yourself. Replacing NTC sensors and working on the boiler internals should be performed by a qualified engineer.

- When you contact the engineer, tell them the boiler model and serial, the E321 code, what checks you have already completed (reset, pressure, visual connector inspection) and whether hot water is entirely absent or intermittently working. That information speeds diagnosis and repair.