Protherm F.19 Fault Code: Causes, Fixes & Repair Costs
What does the Protherm F.19 fault code mean?
The F.19 fault code on a Protherm boiler indicates that the control board has detected a short circuit in the exhaust hood NTC (flue gas temperature) sensor circuit. The NTC sensor is a thermistor that tracks the temperature of gases leaving the boiler through the flue; as temperature rises, its electrical resistance falls in a predictable way. When the PCB reads a resistance value that is far too low — characteristic of a short circuit rather than a normal temperature reading — it locks the boiler out and flashes F.19 as a safety measure. On Protherm displays, the letter F flashes alongside the error number. You can also review the ten most recent fault records by pressing and holding both CH temperature buttons simultaneously for seven seconds.
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 or degraded NTC exhaust sensor Common
The thermistor inside the flue sensor can fail internally, causing its resistance to collapse and mimicking a short circuit to the PCB. This is the most common root cause of F.19 and usually means the sensor needs replacing.
- Moisture or condensation on the sensor wiring or connectors Common
Condensation can build up inside the boiler casing and find its way onto sensor connectors, temporarily lowering resistance and triggering a short-circuit reading. In some cases, drying out the affected area resolves the fault without any parts being replaced.
- Damaged, corroded, or loose wiring between the sensor and PCB Sometimes
The wiring harness running from the exhaust NTC sensor to the control board can suffer from oxidised terminals, loose plug connections, or physical chafing damage — all of which can produce a false short-circuit signal. An engineer will inspect connector tightness and check for obvious corrosion or pinched cables.
- Faulty PCB (control board) Rare
If the sensor tests within normal resistance tolerances and the wiring is sound, the fault can originate in the PCB itself misreading the sensor signal. A damaged or failing control board is a less common but possible cause, especially on older boilers.
How to fix it
- Reset the boiler DIY safe
Locate the reset button on your Protherm boiler (refer to your user guide for its exact position) and hold it until the boiler attempts a restart. Allow a few minutes for it to run through its ignition sequence. If the boiler fires up and runs normally, monitor it closely over the next few hours. Repeat no more than two or three times — persistent lock-outs mean the underlying fault needs professional attention.
- Check your gas supply DIY safe
Confirm that other gas appliances in your home (cooker, gas fire) are working normally. If they are not, contact your gas supplier rather than a boiler engineer, as the issue may be with the incoming supply rather than the boiler itself.
- Inspect for obvious condensation inside the boiler casing DIY safe
With the boiler off and cooled, remove the front casing panel and look for any signs of moisture on visible wiring or connectors near the flue area. Do not touch any components — simply note whether condensation is present and report this to your engineer. Do not attempt to direct warm air into the boiler yourself unless an engineer advises it is safe to do so in your specific situation.
- Arrange a Gas Safe engineer to inspect wiring and sensor connections Gas Safe engineer
A registered engineer will check all connectors between the exhaust NTC sensor and the PCB for oxidation, loose plugs, and physical damage to the cable run. Tightening or cleaning corroded connections can resolve the fault without replacing any parts.
- Have the NTC exhaust sensor resistance tested Gas Safe engineer
Using a multimeter, the engineer will measure the sensor's resistance at a known temperature and compare it against the manufacturer's specification. A reading that is far too low confirms a short circuit within the sensor itself, and replacement is required.
- Replace the NTC exhaust (flue) sensor if confirmed faulty Gas Safe engineer
The exhaust hood NTC sensor is a relatively low-cost part. Your engineer will source the correct replacement for your Protherm model, fit it, and clear the fault. The boiler should be tested across a full heating and hot water cycle after the repair.
- Replace or repair the PCB if the fault persists after sensor replacement Gas Safe engineer
If a new sensor is fitted and F.19 returns, the control board is the next suspect. PCB replacement or specialist repair is a more involved job and the most expensive outcome for this fault code.
- Call a Gas Safe registered engineer if the boiler does not reset or the fault returns Gas Safe engineer
If resetting the boiler does not clear F.19, or if it keeps returning after a short period of normal operation, do not continue to reset without investigation. Contact a Gas Safe registered engineer to diagnose and repair the fault safely and legally.
Parts you may need
- Protherm exhaust hood NTC temperature sensor · from £25
- Sensor wiring harness / connector set · from £18
- Protherm PCB (control board) · from £220
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 £120–£320, depending on the underlying cause.
Frequently asked questions
Can I fix a Protherm F.19 fault myself?
The safe homeowner actions are limited to resetting the boiler (up to two or three times) and checking for obvious condensation. Any further work — testing sensor resistance, replacing the NTC sensor, or inspecting internal wiring — must be carried out by a Gas Safe registered engineer. Working on boiler internals without Gas Safe registration is illegal in the UK and can void your warranty or home insurance.
How much does it cost to fix a Protherm F.19 fault in the UK?
For most people, the repair bill will fall between £120 and £320, covering the call-out/diagnostic fee plus the cost of replacing the exhaust NTC sensor and associated labour. If the PCB turns out to be the cause, costs can rise to £350–£500 or more depending on your engineer's labour rate and the price of the specific board for your Protherm model. Emergency or out-of-hours call-outs will also push the total higher.
What is the difference between the Protherm F.19 and a similar open-circuit sensor fault?
F.19 specifically indicates a short circuit — meaning the sensor's resistance has collapsed to an abnormally low level. An open-circuit fault (such as Protherm F.18 on some models) means the circuit is broken and no resistance reading can be taken at all. Both point to the exhaust NTC sensor circuit, but the diagnostic path and the most likely cause differ slightly. Your engineer will use a multimeter to confirm which condition is present.
Will F.19 go away on its own?
Occasionally, if condensation inside the boiler was the trigger, the fault may clear after a reset once the moisture has dried out. However, if the sensor itself has failed or wiring is damaged, the code will return — sometimes immediately. It is safest to have an engineer investigate any fault that comes back more than once rather than relying on repeated resets.