Worcester Bosch Heatslave Fault codes & diagnostics

5 fault codes with plain-English explanations, severity ratings, DIY guidance, and repair cost estimates.

CombiOil Oil 12-25 kW 1989-2005 Discontinued

Replaced by: Heatslave II

Download the Worcester Bosch Heatslave manual The official installation & service manual (PDF) — the exact document these fault codes were verified against. PDF

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Just want the most common Worcester Bosch Heatslave faults?
The top 6 problems in plain English — with causes, DIY checks and repair costs.
Fault codes

All 5 documented codes

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A1

Your boiler has detected that there isn't enough water moving through the system, which is usually caused by a leak or a pump that has become stuck or worn out.

High DIY-safe £180-350

What to check first

  1. Check the pressure gauge on the fascia and top up to 1.5 bar if it is low
  2. Look for any visible water leaks under the boiler or around your radiators
  3. Check that all radiator valves and internal isolation valves are fully open

Parts commonly replaced

  • Central heating pump
  • Auto air vent
  • Internal pipe seals

Related codes

Full explanation for A1
A7

Your boiler's temperature sensor for hot water is broken or has a loose connection, which means the system cannot correctly measure how hot your water is.

High Engineer only £90-160

Parts commonly replaced

  • Domestic Hot Water (DHW) NTC Sensor
  • Connecting Wiring Harness

Related codes

Full explanation for A7
A8

Your boiler has lost its connection to the external heating controls, meaning it cannot receive instructions on when to turn the heating or hot water on.

High Engineer only £90-180

Parts commonly replaced

  • RF Receiver
  • Room Thermostat
  • Wiring Harness

Related codes

Full explanation for A8
EA

Your boiler has stopped working because it cannot light the fuel required to provide heating and hot water.

High DIY-safe £100-£350

What to check first

  1. Check if your gas or oil supply is turned on and that you have credit on your meter
  2. Press and hold the reset button for five seconds to see if the boiler restarts
  3. Thaw your external condensate pipe with warm water if it is frozen during cold weather

Parts commonly replaced

  • Ignition Electrode
  • Flame Sensor
  • Gas Valve
  • Ignition Lead

Related codes

Full explanation for EA
FA

Your boiler is seeing a flame when it should be turned off, which means the fuel supply might not be closing properly.

Emergency Engineer only £180-450

Parts commonly replaced

  • Oil pump
  • Solenoid valve
  • Optical sensor
  • Control box

Related codes

Full explanation for FA
Status & diagnostic codes

5 operating states & engineer diagnostics

Not faults — these are normal operating states. Tap to expand.

200 Status Your boiler is operating normally and is currently providing heat to your radiators. 203 Status Your boiler is currently in standby mode and waiting for a signal from your thermostat or timer to begin heating. 203 0 Status Your boiler is working correctly and is simply waiting for you to turn up your thermostat or programmer to request heat. rE Status Your boiler is currently attempting to restart itself multiple times due to an internal system error. T4 Diagnostic Your boiler's internal switching valve is stuck or having trouble moving between the heating and hot water settings.