Error E321

Safety precautions:

- If you are not comfortable working around electrical equipment or plumbing, stop and call a Gas Safe engineer. Do not attempt gas-side repairs.

- Before touching any wiring or internal covers, switch the boiler off at the isolator and, if you will be working on wiring/connectors, isolate the electrical supply at the fuse box.

- Never work on the boiler while it is live unless you are suitably trained and competent.

Initial checks a homeowner can do:

1) Note the error: write down E321 and any other codes or lights shown, plus the circumstances (e.g., when you turned on a tap). This helps the engineer.

2) Reset the boiler: most Potterton boilers have a reset button—press and hold for 3–5 seconds. Try the hot tap again. If the fault does not return, monitor for recurrence (intermittent faults sometimes clear but may reappear).

3) Check system pressure: look at the pressure gauge. If system pressure is very low, repressurise to the manufacturer’s recommended level (usually around 1–1.5 bar) and then retry. Some sensors/circulation errors present alongside low pressure problems.

4) Check hot water demand and plumbing: ensure the tap is fully open so the flow switch registers demand. Verify other supply issues (cold mains, closed valves) are not the cause.

5) Look for obvious wiring/connectors: with the boiler switched off, check any visible external connectors to the boiler, thermostat wiring and external probes for loose plugs or damaged cables. Do not remove sealed panels or internal covers.

Specific diagnostic and basic fixes you can attempt (only if competent):

1) Reset and monitor: as above, sometimes a transient reading or communication fault is cleared by a reset—record whether E321 returns and under what conditions.

2) Visual inspection of accessible wiring: with the boiler isolated electrically, inspect only accessible connectors for corrosion, damage or loose pins. Re-seat connectors if they are external and accessible. Do not open gas valves or internal sealed compartments.

3) Flow/demand check: ensure the boiler is sensing DHW demand. If the flow switch is stuck or not operating, the boiler may not enter DHW mode or may mis-read temperatures. If you suspect the flow switch, note whether the boiler behaves differently at different taps and mention this to your engineer.

4) Multimeter check on sensor (only if you are experienced and the boiler is isolated): the NTC thermistor will show a resistance that changes with temperature. If you can access the DHW sensor plug and are confident, you can check for an open circuit (very high/infinite resistance) or short (very low resistance). If the sensor is open or shorted it will need replacing. Do not attempt to replace the sensor unless you are qualified; faulty replacement or incorrect wiring can be dangerous.

When to call a professional and what to tell them:

- Call a Gas Safe registered engineer if E321 persists after a reset, if the code returns intermittently, or if you find broken wiring, an open/shorted sensor, or other electrical irregularities. Also call an engineer if you are uncomfortable performing any of the checks above.

- Give the engineer the boiler model (Potterton Assure combi/system), the error code E321, any other codes present, what you observed (e.g., no hot water, fluctuating temps), and what steps you’ve already taken (reset, pressure check, visual inspection).

- A professional will usually check the DHW NTC sensor, wiring to the PCB, the PCB input, the flow switch and the diverter operation. If the thermistor is faulty it will be replaced; if wiring or PCB faults are found the engineer will advise the correct fix.

Important final notes:

- Do not attempt gas valve, burner or PCB repairs yourself. Those are controlled-service items requiring a Gas Safe registered engineer.

- If the fault is intermittent, record when it happens (time of day, hot water demand pattern) to help diagnosis. Persistent or recurring E321 should be handled by a professional to ensure safe, correct repair.