Your Junkers stops and displays "AA", often just when you're using hot water. The Junkers isn't usually alarming (it's not related to gas or fumes), but it typically indicates a growing problem: the water in the boiler is overheating because it's not circulating properly or transferring heat effectively. And in many cases, the culprit has a name: limescale.

This guide explains what the boiler is usually measuring when the AA code appears, why limescale is the usual suspect (but not the only one), what you can check yourself, and what this code typically indicates depending on the model. At the end, we include two related codes: d4 for the Ceraclass Excellence and dE for the EurAcu-Hit.

Quick summary

AA error mean Junkers? It usually indicates that the boiler detects an excessive temperature difference between its probes, or a primary temperature that is too high, because the water is not circulating enough.
Models: Eurand Ceraclass Midi. In Ceraclass Excellence the equivalent is usually the d4, and in EurAcu-Hit, the dE (accumulator).
Most common cause: plate heat exchanger (the one that heats the shower water) clogged with limescale.
Other causes: pump that moves little, air in the circuit, low pressure or an NTC probe that measures incorrectly.
Key clue: if it appears mainly when using hot water and the water comes out in temperature shocks, it is most likely due to limescale.

What is the boiler measuring?

A combination boiler has two water circuits: the primary circuit (the heating water, which always circulates inside the boiler and radiators) and the domestic hot water circuit (the water from your taps). These circuits intersect in the plate heat exchanger, a block of thin plates where the hot water from the primary circuit transfers its heat to the tap water without mixing.

The boiler monitors both with NTC probes. If the primary heats up too quickly and the domestic hot water temperature doesn't rise as it should, or if the primary temperature spikes, the electronics will most likely interpret this as inadequate heat transfer and activate the air conditioning system for protection before the limiter trips (which would result in an E9).

Causes of the JunkersAA error, from cheapest to most expensive

1. Low pressure or air in the circuit

Check the pressure gauge with the boiler cold: it should normally read between 1 and 1.5 bar. With low pressure or air in the system, the pump usually moves less water and the primary circuit overheats. Refilling and bleeding the radiators is free and should be the first thing you do.

2. Radiators or valves closed

Very common at the beginning of autumn: boiler valves closed after a repair, or all radiators with their thermostatic valves closed. If the water has nowhere to circulate, it overheats.

3. Limescale in the plate heat exchanger

This is the most common cause. Over time (and much sooner in hard water areas), limescale builds up between the heat exchanger plates on the domestic hot water side. Heat doesn't flow properly, the primary side overheats, and the JunkersAA error code may appear. Symptoms that usually indicate this problem include:

The hot water comes out in bursts of temperature: very hot, then lukewarm, then hot again.
It's difficult to get hot water with low flow.
The error almost always occurs when using hot water, and rarely only with heating.
Boiling noise or small knocks inside the boiler.

The usual solution is to disassemble the heat exchanger and descale it with a specific product, or replace it if it's heavily clogged or leaking. A time-saving tip: if the boiler is old and the limescale buildup is intensa, it's often more cost-effective to install a new heat exchanger than to clean the old one.

4. The bomb

A pump that spins weakly, seizes up after summer, or has a dirty impeller can move very little water. A common sign is that the heating takes a long time to warm the radiators furthest away. Sometimes it can be freed by unlocking the shaft; other times it needs to be replaced.

5. An NTC probe that is measuring incorrectly

A hidden angle: if one of the probes is miscalibrated or has poor contact with the pipe (lacks thermal paste or the clamp is loose), the boiler may calculate a temperature difference that doesn't exist. This can be checked by measuring its resistance when cold and hot. It's an inexpensive part, but it's best to confirm this before replacing it. We explain this in the NTC probe Junkers.

Junkers AA error according to the model: what it usually indicates

Eur

AA: This usually indicates an excessive temperature difference between the heating and domestic hot water supply. The usual procedure is to check the condition of the NTC sensors for both heating and hot water, and for limescale buildup in the tank or heat exchanger.

Ceraclass Midi (ZW 24 AE/KE)

AA: In many cases, this is due to excessive temperature in the primary circuit or insufficient flow rate. It's advisable to check the pump, the heating NTC, and whether the plate heat exchanger or coil is calcified.

Ceraclass Excellence / Cerastar: code d4

d4: This usually indicates excessive temperature. In our experience, the usual steps are to check the pump, the bypass, and the system pressure. The logic is the same as with the air conditioning: the water isn't circulating enough.

EurAcu-Hit: dE code

dE: This usually indicates an excessive temperature in the storage tank. In practice, with a boiler with a storage tank, it's advisable to check the storage tank probe first and then the valve that distributes the water between the heating system and the tank.

Euronline (without screen)

It doesn't display code AA, but the red LED with the fifth green one fault lights Eurusually indicates slight overheating, with the same suspects: plate heat exchanger and primary NTC. More on .

What you can do about the AA Junkers error (and what you can't)

Yes you can:

Check the pressure and refill to 1–1.5 bar with the boiler cold.
Bleed the radiators.
Check that the valves under the boiler and those on the radiators are open.
Clean the faucet filters and the cold water inlet filter, if accessible.

It's best to have it done by an authorized technician:

Disassemble and descale the plate heat exchanger: the circuit must be drained and gaskets replaced.
Measure the NTC probes and check the pump.

If you live in a hard water area, a water softener or limescale filter at the boiler inlet usually extends the life of the heat exchanger considerably.

Spare parts related to Junkers AA error

Plate heat exchangers:

NTC probes:

Circulation pumps:

More parts available in spares Junkers. Always check the reference number of your old part before ordering.

Frequently Asked Questions

Can I descale the heat exchanger without disassembling it?

There are kits available for circulating limescale remover through the sanitary side of the pipes without disassembly, but these require shutting off the water supply and connecting a pump. It's a job for a technician, and if the blockage is severe, it's not always enough.

The Junkers AA error only appears with hot water, not with heating. Is this normal?

This is the typical pattern of a plate heat exchanger with limescale: with heating, the heat goes to the radiators and there is usually no problem; when hot water is requested, the heat has to pass through the heat exchanger and it cannot.

Could AA end up in E9?

It's possible. If the cause isn't corrected, overheating can trigger the temperature limiter. We explain this in the E9 error Junkers.

Where can I see the rest of the codes?

In the error code index Junkers.

Important notice

This article is a guide based on 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

AA error in Junkers (and d4, dE): temperature difference, cal and low flow rate

What is the boiler measuring?