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Worcester Bosch Boiler A1 Error Code

Pump has run dry on Worcester Bosch boilers.

Worcester BoschAll Worcester Bosch boiler codes

A1
May need a pro

The pump is running with too little water around it. Low system pressure is the first thing Worcester's engineer booklet lists, followed by a seized pump or a worn pump bearing. The boiler blocks itself until the power has been switched off and on again.

Compatible models

Listed in the Worcester engineer's service booklet for Greenstar CDi Classic combi, system and regular boilers. The manufacturer doesn't publish a wider model list.

Also shown as

No other codes are listed for this fault.

Difficulty

Try the basic checks, but the usual fix is a technician's job.

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 A1

  1. Look at the pressure gauge

    Below 1 bar, top the system up through the filling link to between 1 and 1.5 bar the way your user manual shows.

  2. Power the boiler off and on

    Switch the boiler off at its fused spur, wait a minute and switch it back on. A1 is a blocking fault on CDi models, so a power cycle is what clears it once the pressure is right.

When to call a technician

If the pressure is fine and A1 comes back, the pump needs checking or replacing, and the air may need purging from inside the boiler. Both are Gas Safe engineer jobs.

Parts linked to A1

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 A1 code?

Listed in the Worcester engineer's service booklet for Greenstar CDi Classic combi, system and regular boilers. The manufacturer doesn't publish a wider model list.

Can I fix Worcester Bosch A1 myself?

Only the outside checks above. If they don't clear A1, 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