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Bosch · Boiler

FA Flame detected after gas shut off

Critical severity Bosch Greenstar

What the control is telling you

A flame signal remains after the gas valve closes.

Applies to: Bosch Greenstar ZBR 21/28/35/42-3A boilers and ZWB 28/35/42-3A combi boilers

Likely causes

  • Gas valve leaking through
  • Dirty condensate trap
  • Electrode or lead fault
  • Flue problem

Test and repair steps

  1. Warning: Disconnect mains power and close the gas cock before opening the appliance or working on the gas train.
  2. Check the gas valve and replace if it does not close.
  3. Clean the condensate trap.
  4. Check the electrodes and leads; check the flue system.

Models that show this code

4 models in Bosch Greenstar

ZBR21-3AZBR28-3AZBR35-3AZBR42-3A

Other codes on the same units

  • E9 Safety high limit or flue gas limiter tripped
  • F0 Internal fault
  • F1 Internal data error
  • F7 Flame detected with burner off
  • Ad DHW tank temperature not detected
  • CC Outdoor temperature sensor not recognised
  • EA No flame detected
  • Fd Reset button pressed by mistake

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Source: Bosch Greenstar ZBR/ZWB 21-42-3A service manual 6721853955 (2025-11), Table 38 Faults shown on the display, p.74-75; Greenstar combi 100 p/151 p service manual 6721853953 (2025-11), Table 33. Summarized in our own words; check the original manual for your model.

Homeowner looking at this code? See what it means for you and when to call a technician.

For licensed technicians. Gas, electrical and refrigerant work must be done by a qualified, licensed person. Shut off power and gas before servicing, never bypass a safety control, and follow the manufacturer’s manual for the exact model; it and local codes always take priority. HVACopedia is not affiliated with any manufacturer; brand and model names only identify the equipment.