Error 1021 B

Safety precautions:

- If you smell gas, leave the building immediately and call the gas emergency number — do not operate the boiler or electrical switches.

- Isolate the boiler electrically before opening the casing (switch off at the fused spur or consumer unit). Only qualified persons should isolate gas or perform internal repairs.

- If you are not competent with electrical or gas appliance work, stop at the initial checks and call a Gas Safe registered engineer.

Initial checks a homeowner can do (no tools or minimal tools):

1. Check the display and note the fault: if the boiler is locked out press the reset button as described in the manual (press and hold reset for 3 seconds or use the control dial method depending on your model). Record whether the 1021 error returns immediately or after trying to get hot water.

2. Turn on a hot tap to call for DHW and watch the boiler response. Note any unusual noises, long lockouts, or immediate error reappearance.

3. Check system water pressure and that the boiler has power. Low system pressure can cause unrelated faults—top up only if you know how and it is safe.

4. Inspect any external wiring and visible connector leads to the boiler for obvious damage, rodent bites or water ingress. If wiring to an external cylinder thermostat or external sensors is present, check those connectors visually.

If you are technically competent and confident with basic electrical checks (only proceed if you know what you are doing):

1. Isolate the boiler electrically at the fused spur or breaker and wait for capacitors to discharge (a few minutes).

2. Remove the boiler outer cover following the manufacturer’s instructions. Be aware removing covers may affect warranties—check your terms first.

3. Locate the hot water temperature sensor plug/lead. On Greenstar 8000 models this is typically sited near the DHW/plate heat exchanger or labelled on the wiring loom. Consult the boiler wiring diagram or service manual if available.

4. Visually inspect the sensor plug and wiring for corrosion, water ingress, broken pins or loose/crushed wires. Re-seat the connector firmly if it is loose and inspect the mating pins.

5. With the appliance still isolated, you can check the sensor wiring continuity with a multimeter. Also measure the sensor resistance (ohms) between the sensor pins and compare to the specification in the service manual. If you do not have the manual, a sensor that reads as an open circuit (infinite resistance) or a short (near zero ohms) is likely faulty. Do not rely on approximate resistance values unless you know the exact sensor type.

6. If the connector was corroded or wet, dry and clean it, re-seat it and reassemble the casing. Restore electrical supply and test: reset the boiler and call for hot water. If the error does not return, monitor for reoccurrence.

If the sensor shows no continuity or a resistance outside the expected range, or if reseating the connector does not clear the fault, the sensor should be replaced. Because the sensor replacement and definitive diagnosis involve working inside the boiler and verifying gas/combustion and safety interlocks, this should be done by a Gas Safe registered engineer.

When to call a professional and what to expect:

- If the fault persists after reset and simple connector checks, call a Gas Safe registered engineer. Replacement of the DHW temperature sensor, wiring repair, or further electrical/control unit diagnosis will usually require an engineer visit.

- The engineer will isolate gas and power, open the unit, test the sensor and wiring properly with calibrated instruments, fit a replacement sensor if required, clear the fault memory, run DHW demand tests and fully commission the boiler.

Do not attempt to work on gas valves, burners, or sealed hydraulic parts yourself. If in doubt, switch the boiler off and book a qualified engineer. Always follow the boiler manual and safety guidance from Worcester Bosch.