Protherm F.16 Fault Code: Causes, Fixes & Repair Costs
What does the Protherm F.16 fault code mean?
The F.16 code appears on Protherm boilers when the control board detects a short circuit in the flue draught sensor circuit. This sensor keeps a close eye on how well combustion gases are being expelled through the flue, and when the electronics receive a signal that falls outside acceptable parameters — in this case, one indicating a short — the boiler shuts down as a precaution. It is worth knowing that F.16 is the short-circuit counterpart to F.06, which signals an open or broken circuit in the same sensor; both codes point to the draught sensor and its associated wiring, just from opposite electrical directions. If the fault clears within about ten seconds of a reset, the problem may have been a temporary glitch caused by moisture or a momentary connection issue. A fault that returns promptly or refuses to clear at all will need hands-on diagnosis by a Gas Safe registered engineer.
General guidance only — not a substitute for professional advice, and not certified by a Gas Safe engineer. Always have your specific appliance assessed by a qualified professional. Any gas work must be carried out by a Gas Safe registered engineer. If you smell gas or suspect carbon monoxide, leave the property and call the National Gas Emergency line on 0800 111 999.
Common causes
- Failed draught or flue sensor Common
The sensor itself has broken down internally and is feeding the control board a short-circuit signal. This is the single most common root cause once wiring has been ruled out, and it typically requires straightforward sensor replacement.
- Damaged, pinched, or corroded wiring Common
The cable harness running between the draught sensor and the PCB may have been chafed against a panel, corroded at a terminal, or had its insulation compromised — all of which can create a short circuit without the sensor itself being at fault.
- Moisture or condensation on the connector Sometimes
Condensate or ingressed moisture at the sensor plug or on the control board connector can bridge contacts and produce a false short-circuit reading. On Protherm boilers this is flagged as a possible transient cause — if the fault clears after ten seconds and does not return, moisture was likely the culprit.
- Faulty PCB (control board) Rare
If the sensor tests within its correct resistance range and the wiring checks out cleanly, the fault may lie in the control board's input circuitry. A damaged PCB can misinterpret a healthy sensor signal as a short circuit.
How to fix it
- Reset the boiler once or twice DIY safe
Press the reset button on the boiler fascia and wait at least ten seconds. If the boiler fires up and runs without the code reappearing, the fault may have been caused by a brief moisture-related glitch. Do not reset more than two or three times in quick succession — repeated resets without success won't fix the underlying problem and can mask useful diagnostic information for the engineer.
- Check the gas supply to the boiler DIY safe
Confirm that your gas supply is active by testing another gas appliance in your home, such as a hob. If the gas supply is fine and the fault code persists after resetting, move on or call an engineer.
- Inspect wiring and sensor connections (engineer) Gas Safe engineer
A Gas Safe engineer will trace the cable harness between the draught sensor and the PCB, looking for signs of chafing, corrosion, pinching, or loose terminals. Any damaged section of wiring will be repaired or replaced before testing continues.
- Test the draught sensor with a multimeter (engineer) Gas Safe engineer
The engineer will disconnect the sensor and measure its resistance, comparing the reading against Protherm's published specification for the model. A reading that falls significantly outside the expected range confirms sensor failure.
- Replace the draught or flue sensor if faulty (engineer) Gas Safe engineer
If the sensor is confirmed defective, the engineer will fit a replacement part suitable for the Gepard model. After fitting, the boiler is powered up and the fault code checked to confirm it has cleared.
- Inspect and replace the PCB if the fault persists (engineer) Gas Safe engineer
Should the new sensor and repaired wiring still result in the F.16 code, the engineer will assess the control board. PCB replacement is a more involved and costly repair, so a thorough diagnosis of all other components first is important to avoid unnecessary expenditure.
- Call a Gas Safe registered engineer DIY safe
If the boiler has not recovered after one or two resets, arrange for a Gas Safe registered engineer to attend. All sensor testing, wiring work, and component replacement on a gas boiler is a legal requirement for a qualified professional in the UK. You can find registered engineers at the Gas Safe Register website.
Parts you may need
- Protherm Gepard flue/draught sensor · from £35
- Wiring harness / connector repair kit · from £20
- Protherm Gepard PCB (control board) · from £270
The exact spare depends on your boiler's GC number (on the data badge). Check this against the part before buying.
Typical repair cost
Expect to pay roughly £110–£280, depending on the underlying cause.
Frequently asked questions
Can I reset the boiler myself when F.16 appears?
Yes — a single reset is a perfectly reasonable first step. Press the reset button and wait around ten seconds to see whether the boiler recovers. If the code clears and the boiler runs normally, moisture may have caused a temporary false reading. However, if F.16 returns after one or two resets, stop there and call a Gas Safe engineer. Repeated resets will not fix a genuine sensor or wiring fault.
What is the difference between Protherm F.06 and F.16?
Both codes relate to the same draught or flue sensor, but they describe opposite electrical faults. F.06 means the sensor circuit appears open — either a broken wire or disconnected plug. F.16 means the circuit appears shorted — wires touching each other or a sensor that has failed in a way that drags the signal down. The diagnostic approach is similar for both: check the wiring, test the sensor, and replace whichever component is at fault.
How much does it typically cost to fix a Protherm F.16 fault in the UK?
For most households the repair comes to somewhere between £110 and £280 all-in, covering the engineer's call-out, labour, and the cost of a replacement sensor or wiring repair. Gas Safe engineers generally charge £60–£100 for a call-out plus £60–£80 per hour, and the draught sensor itself usually costs £20–£60 depending on the exact model. If the fault turns out to be a faulty PCB rather than the sensor, costs rise considerably — PCBs for Protherm Gepard boilers typically cost £200–£350 for the part alone, pushing the total repair closer to £350–£500. That is why thorough diagnosis before ordering parts is important.
Is the Protherm F.16 fault dangerous?
The boiler locks out as a safety precaution because it cannot confirm that flue gases are being correctly evacuated. While the lockout itself is not dangerous — it simply means the boiler won't run — you should not attempt to bypass the fault or force the boiler to operate. Leave the boiler locked out and arrange for a Gas Safe engineer to diagnose it promptly, particularly if it is your only source of heating or hot water during colder months.