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Heat pumps · Error codes · Worcester Bosch
MB1Worcester Bosch · Greenstore ground source heat pump (original, System & Combi)

MB1 on a Worcester Bosch Greenstore ground source heat pump: Motor cut-out compressor

Short answer

On a Worcester Bosch heat pump, MB1 means: Motor cut-out compressor. The heat pump stops until the fault clears or is reset. An engineer is needed to fix the cause.

Reviewed 23 September 2026 · 1 source

What it means

The manual says: Motor cut-out compressor.

Part involved: compressor motor cut-out (Fuse 1).

When it shows: Motor cut-out compressor.

Likely causes:

  • Intermittent fault/overload on supply; motor cut-out current set too low; faulty contactor/cut-out or loose connections; compressor error.

Is it safe?

Switch the heat pump off at its isolator and call an engineer.

Does the heat pump stop?Yes
SeverityFault — needs a manual reset once the cause is fixed
Engineer needed?Yes

The heat pump stops until the fault clears or is reset.

Resetting itself: no.

Manual reset: Press Ackn. on the control panel; heat pump restarts within 15 minutes if there is demand. Some causes need a reset button/MCB on the heat pump (engineer).

What you can check

  • Read and note the full alarm text and code in brackets shown on the controller and pass it to your installer.
  • Check the heat pump isolator and consumer-unit breaker(s) are switched on; if a breaker has tripped more than once, leave it off and call an engineer.
  • Press the Ackn. button once as the manual describes (manual: no risk in resetting an alarm); contact technical support if the alarm repeats.

What the engineer will check

  • Reset motor cut-out (Fuse 1) on heat pump; check current setting, contactor, wiring, compressor.

Who to ask for: heat pump installer / electrician.

Certification: Electrical work to BS 7671 by competent person.

What you might notice

What owners and engineers report

3 owner and engineer reports mention this.

  • No heating on the Greenstore ground source heat pump (original, System & Combi): 2 reports
  • Keeps stopping / cutting out on the Greenstore ground source heat pump (original, System & Combi): 1 report

Sources

A code explains what the controller detected, not a confirmed root cause.

Reviewed: 23 September 2026