Ad Space — Top Banner

F4

Worcester Bosch Water Heater

Severity: Minor

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

Worcester Bosch Boiler F4 Error — Return Sensor Fault

What Does This Error Mean?

F4 specifically points to the return temperature sensor — the probe that measures water coming back from the radiators.
The sensor is reading out of range (open, shorted, or implausible value).
The boiler stops because it can't manage temperature differential without that reading.
Fix is a sensor replacement by a Gas Safe engineer — usually under an hour on site.

Affected Models

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

Common Causes

  • Return sensor failed (open or shorted thermistor)
  • Sensor wire disconnected from the PCB
  • Sensor connector corroded by moisture
  • Sensor knocked loose during a previous service
  • PCB sensor input damaged

How to Fix It

  1. Power-cycle the boiler once.

    Spur switch off for 30 seconds, then on.
    Wait for the boiler to fully boot.
    If F4 returns within five minutes, it's a real fault.
    If it doesn't return, the sensor was reading momentarily strange and recovered.

  2. Don't open the boiler.

    The return sensor is inside the casing, mounted on the return pipe.
    You can't safely or productively access it from outside the casing.
    Leave it for the engineer.

  3. Note any recent service history.

    If anyone has serviced the boiler recently (annual service, repair), F4 may be a knocked-loose connector from that visit.
    Tell the engineer about recent service — sometimes a single connector reseat fixes it without parts.

  4. Check for visible water leaks.

    Walk around the boiler.
    Damp under the unit or limescale streaks down the casing point to a slow leak.
    Water plus an electrical connector equals corrosion.
    If you see signs, mention them — the engineer will check the sensor connector first.

  5. Call a Gas Safe engineer.

    Provide the model number and how long F4 has been showing.
    Most engineers carry replacement sensors for popular Worcester models.
    The fix is typically a sensor swap (30–60 GBP part) plus an hour of labor.
    Should be a same-day repair.

Frequently Asked Questions

Can I run with F4 in summer when I don't need heating?

Most Worcester Bosch boilers stop both heating and hot water on F4, even though you'd think hot water doesn't need a heating return sensor.
The boiler design uses the same logic chain for both modes.
So no — fix it before you need a hot shower.

Is F4 expensive to fix?

It's one of the cheaper Worcester Bosch faults.
Sensor itself is usually 30–60 GBP, plus an hour of labor.
Total bill should be under 150 GBP including the visit fee.
If a quote is much higher, get a second opinion — F4 doesn't typically need PCB-level work.