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E9

Worcester Bosch Boiler

Severity: Critical

Source: OnlyErrorCodes.com — Every error code. One place.

Worcester Bosch Boiler E9 — Overheat Lockout

What it means

E9 means the boiler reached an unsafe temperature and locked out for safety.
Common causes are blocked or sludgy heat exchanger, failed pump, or limescale buildup.
A Gas Safe engineer is required.

Affected Models

  • Worcester Greenstar 4000
  • Worcester Greenstar 8000
  • Worcester Greenstar i
  • Worcester Greenstar CDi

Common Causes

  • Sludgy or blocked primary heat exchanger restricting flow
  • Failed central heating pump not circulating water
  • Limescale buildup in the heat exchanger (hard water areas)
  • Failed temperature sensor reading falsely high
  • Air lock in the heat exchanger

How to Fix It

  1. Reset the boiler once.

    Press the reset button.
    A single reset is appropriate to clear a one-off overheat.
    If E9 returns within minutes or hours, do not reset repeatedly — call a Gas Safe engineer.

  2. Check pump operation (visible/audible).

    Listen for the central heating pump running when the boiler fires.
    A seized or failed pump is a leading cause of E9 — the heat exchanger overheats because flow has stopped.
    An engineer can replace the pump.

  3. Have the heat exchanger flushed.

    Sludge buildup in older systems restricts flow and causes overheat.
    A powerflush by a heating engineer cleans the system and prevents recurrence.
    Adding an inhibitor and a magnetic system filter helps long-term.

  4. Address limescale (hard water areas).

    Hard water deposits inside the heat exchanger reduce heat transfer and cause overheat.
    Descaling chemicals and scale-reducing devices help.
    Severe scale may require heat exchanger replacement.

When to Call a Professional

E9 indicates a safety lockout that requires a Gas Safe registered engineer to diagnose.
Do not bypass or repeatedly reset — the boiler is protecting itself for a real reason.

Frequently Asked Questions

Can I keep using my Worcester Bosch boiler if it shows E9?

No. E9 is a safety shutdown for overheat. Repeatedly resetting risks damaging the boiler and is unsafe. Switch the boiler off and call a Gas Safe engineer.

How much does it cost to fix Worcester Bosch E9?

Costs vary by cause. A pump replacement is around £200–£400 in the UK. A powerflush is £300–£700. Heat exchanger replacement can exceed £1,000. An engineer can quote after diagnosis.