Boiler ionisation electrode: fault codes per brand, causes and which electrode fits

Short answer: the ionisation probe (ionisation electrode) tells the boiler that a flame is burning; the ignition electrode provides the spark. If the ionisation probe wears or gets dirty, the controls see no flame, close the gas valve and you get a code such as Intergas 4, Remeha 02, Nefit 227 or Vaillant F.28. The part itself costs between € 10 and € 50, depending on the brand. The probe sits on the burner, behind the appliance casing, so replacing it counts as work on a gas combustion appliance that must be done by a certified gas engineer. What you can do yourself: recognise the code, rule out the cheap causes and have the right part ready.

What an ionisation probe does

A gas flame conducts electricity. The boiler controls apply a small AC voltage to the ionisation probe; as soon as the flame touches the probe, a direct current of a few microamps flows to the earthed burner. That current is the flame signal. If it drops out, the gas valve closes within a second. This is the appliance's most important safety device: without a flame signal, no gas may flow.

On many boilers ignition and ionisation are combined in one electrode; Remeha calls it an "ignition/ionisation electrode", while Intergas has a separate ignition electrode and ionisation probe. Both wear: the tip burns away, a layer of oxide builds up, and the gap to the burner changes. An ionisation probe is therefore a service part that is checked every three to five years and replaced if necessary.

The symptoms and the codes by brand

There are three patterns. The boiler starts, sparks and drops out after a few seconds: no flame signal. The boiler fires and then drops out while running, often at part load: poor or fluctuating flame signal. Or the boiler reports a flame when it is not firing: false flame signal, usually leakage current due to moisture or a damaged cable.

Brand	Code	Message
Intergas	4	No flame signal after four ignition attempts
Intergas	5	Poor flame signal
Intergas	F007	No or insufficient ionisation current
Remeha	02	No flame formation or no ionisation after five start attempts
Remeha	77	Loss of ionisation during operation
Remeha	E14	Vijf mislukte branderstarts, wel vonk maar geen vlam
Nefit Bosch	227	Insufficient ionisation current after ignition
Nefit Bosch	228	Ionisation current measured before the burner started (false signal)
Nefit Bosch	229	Insufficient ionisation current while firing
Vaillant	F.28	Flame signal not detected during ignition
Vaillant	F.29	Vlam valt weg tijdens bedrijf, herstart mislukt
Vaillant	F.27	False flame signal while the gas valve is closed
Vaillant	F.68	Fluctuating ionisation signal
Bosch	227 V	No flame signal after ignition
ATAG	133	No flame after five ignition attempts
AWB	F 16	Vlam kort zichtbaar, valt weg; onvoldoende aarding
Ferroli	A01	No ionisation signal during ignition

If a code is not listed here, look it up in the fault codes by brand. The full story about a boiler that sparks but does not light, including gas pressure and burner, is in Boiler does not start and has no flame.

Why a new probe does not always help

An ionisation fault does not necessarily mean a broken probe. These are the other causes, in order of how often we see them:

• Earthing. The flame signal returns via the appliance's earth. A loose earth wire, an oxidised contact or a boiler on an unearthed socket gives exactly the same codes as a worn probe. AWB even says so literally in code F 16.
• Cable and plug cap. The ionisation cable runs past hot parts; the insulation becomes brittle, and the cap is loose or burnt. With a false flame signal (Vaillant F.27, Nefit 228), a leaking cable is the first suspect.
• Gas/air ratio. A blocked air inlet, a dirty burner or an incorrectly set CO2 value produces an unstable flame that does not touch the probe properly. A new probe then only helps for a few weeks.
• Gas pressure. With a low inlet pressure the flame burns too small. Intergas reports this separately; other brands only see the signal dropping out.
• Distance to the burner. A probe that was bent during a previous service is too far from the flame. You notice this mainly at part load: the boiler fires fine at high output, but not at low output.

An engineer therefore first measures the ionisation current (shown in the service menu on many brands) and the earthing before replacing the probe.

What you can check yourself

Without opening the casing:

• Is the gas tap under the boiler open and do other gas appliances work? A gas supply fault looks like an ionisation fault.
• Listen during a start attempt: fan, click of the gas valve, ticking of the ignition. If you hear ticking but no flame appears, the problem is in flame formation or flame detection.
• Reset once. If the boiler starts and runs normally for an hour, have it checked at the next service. If the code comes straight back, book an engineer.
• Check in the service menu whether the display shows an ionisation current; below 1 to 2 microamps it becomes critical. Your boiler's manual states whether this is possible.

The probe itself sits on the burner, in the sealed part of the appliance. Removing the burner, electrode or gasket counts as work on a gas combustion appliance and must be done by a certified gas engineer. After replacement, the combustion must be checked; that cannot be done without measuring equipment.

Which ionisation probe fits which appliance?

Order according to the exact appliance type on the rating plate; within a brand, the probes differ by series and year of manufacture. The parts below come from our catalogue; prices include VAT.

Brand	Part	Article number	Price
Intergas	Ionisation probe (Kombi Kompakt HR/HRE)	8062360	€ 9.95
Intergas	Ignition electrode replacement set HRE/HReco	8066010	€ 12.95
Remeha	Ionisation probe (Avanta, Calenta)	8740310	€ 32.95
Remeha	Ignition/ionisation electrode Tzerra and Tzerra Plus	8013343	€ 33.95
Remeha	Ignition and ionisation electrode Tzerra Ace	8016659	€ 32.45
Remeha	Ignition/ionisation electrode Selecta, Quinta, W21/28	8015040	€38.95
Nefit Bosch	Ignition electrode (TopLine, ProLine, TrendLine)	8035001	€ 19.95
Nefit Bosch	Ignition electrode ProLine NxT	8029507	€ 24.45
Nefit Bosch	Sealing ring ionisation probe, 5 pieces	8029129	€21.95
Vaillant	Ionisation electrode	80501427	€ 37.95
Vaillant	Gasket ionisation electrode	80501329	€ 15.95
AWB	Ionisation electrode	8078227	€ 49.95
Ferroli	Ionisation probe 27C	8113630	€ 34.95
ATAG	Gasket ionisation/ignition electrode	8135260	€35.95

With a new probe, always order the matching gasket or sealing ring as well; it is almost always damaged when the probe is removed. All ignition and ionisation parts are grouped together under ignition, and by brand under, for example, Intergas ignition of Remeha ignition. Not sure about the type? Send us a photo of the rating plate and of the old probe; we will check it before you order.

Frequently asked questions

What is ionisation in a boiler?

Ionisation is the way the boiler detects a flame. The controls apply a small AC voltage to the ionisation probe; as soon as the flame touches the probe, a small current of a few microamps flows to the burner. If that flame signal drops out, the gas valve closes within a second. An ionisation current that is too low (below 1 to 2 microamps) points to a worn probe, poor earthing or an unstable flame.

What is the difference between an ignition electrode and an ionisation probe?

The ignition electrode provides the spark that ignites the gas mixture; the ionisation probe then measures whether a flame is burning. On many modern boilers both functions are combined in one electrode with two or three pins.

How long does an ionisation probe last?

Three to eight years, depending on the number of burner hours and the gas quality. During a service the probe is checked for burn-off and gap; it is usually replaced preventively as soon as the ionisation current starts to fall.

Can I clean the ionisation probe myself?

No. The probe sits on the burner in the sealed combustion section of the appliance. Opening, removing and refitting it is work on a gas combustion appliance and must therefore be done by a certified gas engineer.

What does it cost to have an engineer replace it?

The part costs € 10 to € 50; labour for a separate call-out depends on your installer and country. If the probe is replaced during a service, you usually only pay extra for the part.

My boiler only shows the code in cold weather – why?

When it is cold outside, the boiler runs longer at full load and the flame changes shape. A probe that is just on the limit then loses the signal. It is an early sign of wear; have the probe replaced at the next service.