Worcester Bosch Boiler F0 Error — Sensor Wire Fault

What Does This Error Mean?

F0 on a Worcester Bosch boiler is a generic sensor fault — usually the flow sensor or return sensor is reading out of range.
The boiler can't make a good control decision so it stops.
This is a parts-and-wiring fix that needs a Gas Safe engineer.
Power-cycle the boiler once to confirm it's not a one-off, then call for service if F0 returns.

Affected Models

Worcester Bosch Greenstar 25i
Worcester Bosch Greenstar 30i
Worcester Bosch Greenstar 4000
Worcester Bosch CDi compact

Common Causes

Flow or return sensor wire disconnected at the PCB
Sensor probe failed (open or shorted)
Moisture in the sensor connector
PCB sensor input damaged
Wire chewed by rodents (unconditioned cupboards)

How to Fix It

Power-cycle the boiler.

Switch the spur switch off for 30 seconds.
Switch back on.
Some F0 events are transient and clear on a fresh boot.
If F0 returns within minutes, treat as a real fault.
Don't open the boiler casing.

The sensors are inside the sealed casing alongside gas-side components.
Removing the front isn't legally a Gas Safe matter (no gas work is being done) but you also can't safely access anything useful from outside the casing.
Save it for the engineer.
Look for water damage around the boiler.

Walk around the boiler and look for any sign of leaks — drips, damp wall, white limescale stains.
Water in the wrong place is the most common F0 trigger because moisture corrodes sensor connectors.
Note this for the engineer.
Check the install age.

If the boiler is more than 10 years old and you're seeing first-time sensor faults, that's normal aging.
Sensors and connectors degrade over time.
The engineer will likely replace one or two sensors as preventative.
If under 5 years old, a one-off fault is more likely than aging.
Call a Gas Safe engineer.

Bring your model number (on the front or inside the manual cover) and how long F0 has been showing.
The engineer will test the sensors with a multimeter and replace whichever is faulty.
Sensors are 30–80 GBP parts plus an hour of labor.

Frequently Asked Questions

Will the boiler still work with F0 showing?

No — F0 stops both heating and hot water on most Worcester Bosch boilers.
The boiler relies on accurate temperature feedback to control its modulation.
Without it, running would risk overheating or producing the wrong water temperature.
You'll need the fault fixed to restore service.

Why does F0 sometimes fix itself overnight?

Marginal connectors expand and contract with temperature.
When the boiler cools overnight, a marginal contact can re-make briefly and clear F0.
It'll come back when the boiler warms up again.
This is a reliable signal that a connector is the cause — point the engineer at it.