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Worcester Bosch Greenstar 4000 Combi fault 1021

Cylinder charging or hot water temperature sensor faulty

Gas boiler

What it means

Fault category B. Cylinder charging or hot water temperature sensor faulty. A 1021 error can only be cleared by powering the appliance off and back on again after the hot water sensor has been changed if open or short circuit, or refitted to the pipe if it has fallen off.

Likely causes

  1. DHW temperature sensor disconnected or fallen off pipe
  2. Open or short circuit sensor
  3. Harness continuity fault

Where to start

  1. Check the domestic hot water temperature sensor is connected and the harness is fitted securely to the sensor
  2. Check the resistance of the temperature sensor
  3. Check the continuity of harness

The full answer, free

  • 7 more checks in order
  • How to put it right, step by step
  • The manual page for this fault (page 75), right under the answer
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Safety: Isolate gas and power before you open the case. If you have been into the gas side, do a tightness test, and a combustion and flue check before you hand back. Gas Safe (UK) or RGI (Ireland) work only.

From the Worcester Bosch manual for the Greenstar 4000 Combi. Check the model on the data plate before you order parts.

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