Boiler
F.83
The F.83 error code on a Worcester Bosch boiler indicates a fault with the flue gas temperature sensor or a critically low flow temperature being detected — it’s a serious fault that will lock out your boiler and requires prompt attention.
What does this error mean?
F.83 signals that the boiler’s control board has detected an abnormally low or implausible temperature reading from the flue gas thermistor (temperature sensor). Worcester Bosch uses this code specifically when the sensor reading falls outside the expected operating range, suggesting either a failed sensor or a genuine problem with heat production in the heat exchanger. Because this could indicate a combustion or heat transfer problem, the boiler locks itself out as a safety measure.
Est. repair cost
£150–£300 for sensor replacement; £300–£500+ for PCB replacement — all work must be carried out by a Gas Safe registered engineer
🔍 Likely causes
🛠️ Step-by-step fix
Check your boiler’s pressure gauge — if it reads below 1 bar, repressurise the system to 1–1.5 bar using the filling loop (consult your boiler manual for instructions specific to your model).
Once pressure is correct, press and hold the reset button (usually marked with a flame and arrow symbol) for 3–5 seconds to attempt a restart — stand back and listen for normal ignition.
If the fault returns immediately or within a few minutes, do not keep resetting repeatedly — you risk masking a more serious underlying problem.
Visually check the external flue terminal (the white plastic vent on your outside wall or roof) for obvious blockages such as bird nests, debris, or ice in winter — do not attempt to dismantle it.
Do not attempt to access, test, or replace the flue gas sensor, heat exchanger, or PCB yourself — this is gas appliance work that must be carried out by a Gas Safe registered engineer.
📞 When to call a professional
If the F.83 fault returns after a single reset attempt, call a Gas Safe registered engineer immediately — do not continue resetting the boiler. Tell the engineer you have an F.83 lockout fault and describe whether the boiler fires briefly before locking out or fails to fire at all, as this helps them diagnose whether the sensor, heat exchanger, or PCB is the likely culprit.
⚖️ Repair or replace?
A flue gas sensor replacement typically costs £150–£300 including parts and labour; a PCB replacement costs £300–£500+. If your Worcester Bosch boiler is under 10 years old and otherwise reliable, repair is almost always the right call — Worcester Bosch boilers have a strong reputation for longevity. If the boiler is over 12–15 years old and this is not the first fault, it may be more cost-effective to invest in a new A-rated condensing boiler, with installed costs typically starting at £2,000–£3,000.