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Worcester Bosch Boiler A8 Error Code

Lost contact with the controls on Worcester Bosch boilers.

Worcester BoschAll Worcester Bosch boiler codes

A8
Moderate DIY

The boiler has stopped hearing from its programmer or room control over the data (BUS) link. On HighFlow models Worcester names the clip-in weather-compensating control and the Greenstar Comfort II RF. Broken wiring, a faulty control or BUS module, or the PCB are the causes in the engineer booklet.

Compatible models

Shown in the user instructions for Greenstar CDi Classic ErP combis and HighFlow 440CDi and 550CDi ErP boilers, and the CDi Classic engineer's booklet.

Also shown as

No other codes are listed for this fault.

Difficulty

Start with the safe checks below; a worn part may need replacing.

Not sure you are reading the code right? See how to read Worcester Bosch boiler codes.

Safety first: never take the casing off a gas boiler. If you smell gas, open windows, don't use light switches, turn the gas off at the meter if you can and call your gas emergency number from outside. A carbon monoxide alarm going off means the same: switch the boiler off, get fresh air and call.

How to fix Worcester Bosch A8

  1. Check the controls themselves

    Look at the programmer or wireless thermostat. Replace the batteries in a wireless room unit if its display is blank or shows a low battery symbol, and check it's still paired as its own instructions describe.

  2. Restart the boiler

    Switch off at the fused spur for a minute, then back on, and give the control a few minutes to reconnect.

When to call a technician

If the controls are powered and A8 stays, the wiring or the module needs an engineer. Mention which programmer or thermostat you have when you book.

Parts linked to A8

These are the parts an engineer usually checks for this code. They sit inside the sealed casing, so leave them to a Gas Safe registered engineer.

Frequently asked questions

Which Worcester Bosch models show the A8 code?

Shown in the user instructions for Greenstar CDi Classic ErP combis and HighFlow 440CDi and 550CDi ErP boilers, and the CDi Classic engineer's booklet.

Can I fix Worcester Bosch A8 myself?

Only the outside checks above. If they don't clear A8, the fix is inside the boiler and needs a Gas Safe registered engineer.

Other Worcester Bosch boiler codes

Last reviewed 2026-10-21. Codes and steps can vary by model; check your owner's manual.

Sources we checked