Your Junkers has shut down and is displaying A4 (or A3, A2, or A6). This code isn't just any fault: it means that the combustion fumes aren't escaping through the chimney as they should, and the appliance has detected that they're flowing back into the kitchen. The sensor that monitors this, the well-known AGÜ sensor or flue gas sensor, has done exactly its job: cutting off the gas supply before carbon monoxide reaches the house.

Therefore, the number one rule with this code is simple: don't reset it repeatedly to see if it startstechnical manual says Junkers for each model, and when it's necessary to replace the sensor.

Quick summary

A4: The device has detected smoke escaping through the draft diverter (the gases return to the kitchen instead of rising through the chimney).
A3: The smoke probe is disconnected, cut, or short-circuited. This is a fault in the probe itself or its cable.
A2 and A6: only in Ceraclass Excellence / Cerastar natural draft boilers: fumes in the combustion chamber (usually dirty heat exchanger) and failure of the chamber sensor.
Most common cause of A4: the extractor hood turned on in a closed kitchen, or the chimney is obstructed or has a draft.
What to do now: turn off the device, open windows and wait about 10 minutes for the probe to cool down before reassembling.
If it happens again: don't try to fix it. Call a licensed gas fitter: there is a risk of carbon monoxide poisoning.

Content

What does the smoke probe monitor (and why are only some Junkersequipped with it)?

Natural draft boilers and water heaters (those without a fan that vent fumes through a conventional chimney) have a component at the top called a draft diverter . This opening balances the chimney's draft. If the chimney is working properly, the fumes rise. If there is insufficient draft, the fumes overflow through the draft diverter into the room.

The flue gas sensor is right there (at Junkers it's called the draft control device or AGÜ sensor, from its German acronym). It's a small thermostat: when the hot gas overflows and heats it, it shuts off the appliance. That's why the A4 indicator trips. And that's why you have to wait for it to cool down before it lets you reset.

Sealed boilers (with a fan and flue gas outlet through a pipe to the facade, versions AE or MFA) do not use this sensor: their equivalent is the pressure switch, which gives codes C1, C4, or C6. If your boiler is sealed, you can find what you're looking for in our guide to errors C6, C4, and C1 Junkers .

Error A4: The smoke is not coming out of the chimney

These are the causes, ordered from cheapest (and most frequent) to most expensive:

1. The kitchen extractor hood

It's the number one cause, and the one least suspected. A powerful extractor fan in a kitchen with the door and windows closed creates negative pressure: it draws air from wherever it can, and the easiest way is for it to escape through the water heater's chimney, in the opposite direction. The fumes don't rise, they overflow through the draft diverter, and the A4 fuse trips.

The clue: the problem occurs when you're cooking with the extractor fan on, or right after replacing it with a more powerful one. Try using hot water with the extractor fan off and a window open. If it doesn't happen then, you've found the cause.

2. Lack of air in the kitchen

A natural draft appliance needs air to burn the gas. Blocked kitchen ventilation grilles (whether with tape, a piece of furniture, or cardboard "to keep out the cold") or very airtight new windows have the same effect as a range hood: the appliance can't breathe and the chimney doesn't draw properly. If you recently replaced your windows and the A4 (presumably referring to a specific type of gas) started appearing this winter, suspect this.

3. Obstructed or plastered chimney

Bird nests (very common in autumn, after a summer without heating), accumulated soot, a loose section of the chimney, or a damaged cap can all cause this. Windbe a factor: on stormy days, if the chimney outlet is poorly positioned, the air pushes the smoke downwards. If the A4 code only appears on very windy days, it's almost certainly a problem with the chimney outlet. This should be checked by a chimney installer or a chimney sweep.

4. Heat exchanger dirty with soot

If the burner has been malfunctioning for some time, soot clogs the fins of the heat exchanger (the coil through which the exhaust gases pass). The gases cannot pass through, accumulate in the combustion chamber, and overflow. This is usually accompanied by a yellow flame and black stains on the top of the appliance. It requires thorough cleaning and combustion adjustment: a job for a qualified installer.

5. The smoke probe, finally

Only when all the above has been ruled out does it make sense to suspect the sensor: it might have a loose connection, a frayed wire from the heat, or the thermostat itself might be misadjusted and cutting off prematurely. It's an inexpensive part, but replacing it without first ruling out a leak is dangerous: if the sensor was tripping for a reason, the new one will trip just as well (in the best-case scenario).

Error A3: probe or cable failure

Unlike A4, A3 doesn't usually indicate a smoke problem, but rather an electrical one: the circuit board doesn't "see" the sensor because the circuit is open or short-circuited. The usual scenario:

A loose or corroded connector on the probe or circuit board. Unplugging, cleaning, and plugging it back in solves the problem more often than you might think.
Burnt or frayed cable: it passes too close to the draft diverter and the heat dries it out over the years.
Open probe: This is measured with a multimeter; it should show continuity at rest. If it does not, the probe is faulty.

In the Ceraclass Midi, the manual describes it as a faulty gas control device, and in the EurAcu-Hit, as a disconnected or shorted smoke sensor. The diagnosis is the same.

A2 and A6: only in Ceraclass Excellence and Cerastar natural draft

The Ceraclass Excellence / Cerastar boilers in the natural draft version (ZSC/ZWC… 3 MFK) have a second probe in the combustion chamber. This results in two additional codes:

A2: Smoke in the combustion chamber. The typical cause indicated by the manual is a dirty heat exchanger: the gases do not pass through and remain inside.
A6: combustion chamber sensor failure (cut or shorted), same as A3 but on the other probe.

If your Junkers doesn't have a screen

Eurline (ZW…-1): The red LED illuminated with the second and fifth green LEDs indicates poor drainage. Check the pressure switch (AE version), pressure switch connections, fan, and AGÜ sensor. This is explained in the fault light Eurline guide .
Ceraline Acu (ZWSE 28-5): In the natural draft version (MFK), four pilot light flashes indicate smoke is escaping through the draft diverter or the combustion chamber. More details on Ceraline Acu flashes.

What does the Junkers technical manual say about the model?

Euromaxx (only ZC 28 and ZWC 24/28-1 MFK, natural draft)

A3: Flue gas probe of the draft diverter interrupted or short-circuited; check the probe and its connections. A4: Flue gas outlet through the draft diverter; check the flue section.

Eurostar Acu-Hit (ZWSE 23-4 MFK, KE versions)

A3: Smoke sensor disconnected or short-circuited. A4: Smoke sensor cut off. In both cases, check the evacuation system and the AGÜ probe. Note: In this model, F0 may also appear during the AGÜ probe connection test.

Ceraclass Midi (ZW 24 KE)

A4: The gas control has been activated. Check the chimney, its layout, and any obstructions, and wait for the probe to cool down before resetting. A3: Gas control device faulty; check the device and its wiring.

Ceraclass Excellence / Cerastar (ZSC/ZWC… 3 MFK)

A2 and A3: smoke in the combustion chamber, usually due to a dirty heat exchanger; A3 could also be the smoke sensor itself. A4: smoke through the draft diverter. A6: combustion chamber sensor.

Heaters

miniMAXX (WR-2): A4, anti-backflow gas probe (AGÜ) open. Locks out the heater. The manufacturer recommends ventilating the room and waiting approximately 10 minutes.
HydroPower-Plus (WTD…KG): A4, the gas outlet control has been activated.

What you can do (and what you can't)

Yes you can:

Turn off the appliance, ventilate the kitchen well and wait 10 minutes before resetting.
Check if the fault occurs when the extractor hood is on or when the kitchen is closed.
Uncover the ventilation grilles and do not cover them again.
Check if it only comes out on windy days, and tell the technician: it saves him time.
Install a carbon monoxide detector in your kitchen. It costs little and is the best investment you can make if you have a natural draft appliance.

You shouldn't:

Reassemble repeatedly without ventilation.
Bypassing the smoke probe "to see if it starts" is literally disabling the system that prevents carbon monoxide poisoning.
Tampering with the chimney, draft diverter, or burner if you are not a qualified installer.

If the probe turns out to be the culprit, replacing it is simple for a technician, but it must be done after checking the draft with the appliance running.

Spare parts related to the A4 and A3

Smoke probes and draft control devices:

Can't find the part for your model? Check all spare parts Junkers or spare parts for water heaters. Always check the reference number of your old part before ordering.

Frequently Asked Questions

Is the A4 error dangerous?

It's possible. The appliance shut off precisely to protect you, but this indicates that, at some point, fumes escaped into the house. If this happens again, don't use it until a technician has checked it and ventilate the room well. If you experience headaches, dizziness, or nausea while it's running, turn it off, go outside, and call emergency services (112).

I only get A4 paper when I'm cooking. Why?

Almost certainly, it's the extractor fan: it creates negative pressure in the kitchen and pulls the fumes downwards. Use the fan with a window slightly open or consult an installer about an air intake solution.

Why does it appear more in autumn?

Because that's when people start using the heating again after months of disuse: bird nests, leaves or soot in the chimney, and they also tend to keep their windows closed more. A check-up before the start of the season prevents most of these problems.

I've replaced the sensor and it's still showing A4. Is the part faulty?

The old probe was most likely fine, and the problem is with the venting. Check the range hood, ventilation, and chimney before considering other parts.

Where can I see the rest of my Junkerscodes?

In our error code index Junkers, organized by code and by model.

Important notice

This article is a guide based on the manufacturer's technical documentation and our experience as a spare parts store. It is not a substitute for professional diagnosis. Every appliance and every installation is different, and the same error code can have different causes.

Any work on the gas circuit, combustion, or flue system must be carried out by a qualified installer, in accordance with the Technical Regulation for the Distribution and Use of Gaseous Fuels (RD 919/2006) and the Regulation on Thermal Installations in Buildings (RITE). Manipulating these elements without the necessary qualifications can be dangerous and will void the appliance's warranty.

The decision to purchase a replacement part based on this information is the sole responsibility of the customer. ElectroCholo is not liable for incorrect diagnoses or damage resulting from improper installation. Electrical and electronic parts that have been assembled or installed are non-returnable if the fault turns out to be something else, in accordance with our general terms and conditions.

Boilers and water heaters, Junkers

Error A4, A3, A2 and A6 in Junkers: flue gas outlet and AGÜ probe

What does the smoke probe monitor (and why are only some Junkersequipped with it)?