Your Junkers is displaying d3, CC, or A8 , and the first thing to know is that, in most cases, the boiler isn't broken. These codes indicate something nearby: the thermostat or regulator, the outdoor temperature sensor, the cable connecting them, or a jumper wire in the terminal block. That's why they often appear right after changing the thermostat, carrying out renovations, or painting the facade.

In this guide we review all the external connection codes in the Junkers technical manual (CC, d3, d5, d1, d7, A8 and AC), what each one means according to the model and how to solve them, often without changing any parts.

Quick summary

d3: Terminals 8-9 (or the external controller contact TB1) are open. The factory jumper is missing or the underfloor heating safety thermostat has activated.
CC: The boiler does not detect the outdoor temperature probe (or the bus regulator).
A8: Communication between the boiler and the BUS-connected regulator or module has been cut off.
AC: interruption between a plug-in module and the board (Eur) or failure of the external temperature control (Eur).
d5: external drive probe, in installations with hydraulic compensator.
Most common cause: a cut or loose wire or a jumper removed when installing a new thermostat.

Content

Error d3: the bridge of terminals 8-9

Many Junkers have two terminals on their connection strip (8 and 9 on Eurand Cerapur; jumper 161 on connector ST8 on Ceraclass Excellence) that must be closed for the boiler to operate. They are factory-connected with a small jumper. If this contact opens, the boiler will shut down with d3.

So what's it for? To connect an external safety device, usually the TB1 limiting thermostat in underfloor heating systems. If the water going to the floor gets too hot, the TB1 opens its contact and the boiler shuts off to prevent damage to the flooring. Therefore, the d3 sensor has two very different readings:

If you don't have underfloor heating: it's almost certain that someone has removed or loosened the jumper wire, usually when installing or replacing a thermostat. Replacing it will fix the problem.
If you have underfloor heating: the d3 indicator is warning you that the underfloor water has overheated. Do not bypass TB1: check the underfloor water supply temperature, the mixing valve, and the regulator.

Touching the terminal block requires disconnecting the boiler from the power supply. If you're unsure which wire goes where, it's best to have a technician do it.

CC error: the external probe

Climate-controlled systems have an outdoor temperature sensor on the north-facing facade. The boiler uses this sensor to adjust the water temperature according to the outside temperature. The "CC" error occurs when the boiler (or the controller) expects this sensor and cannot find it.

Cable cut during construction work, while painting the facade, when changing windows or because of a blind.
Probe damaged by moisture or sun, or oxidized connector.
Probe removed without changing the regulator settings: If you remove the external probe, you must deactivate the operating mode that uses it on the regulator.

This is checked by measuring the probe's resistance: it should vary with temperature. If it shows an open circuit, the probe or the cable is broken.

Error A8: The regulator is not communicating with the boiler

Junkers thermostats and modulating controllers communicate with the boiler via a two-wire cable (BUS). The A8 error indicates that this communication has been interrupted. In order of probability:

Power outage or reset: sometimes simply removing the power to the boiler for a minute is enough for them to recognize each other again.
Loose or cut BUS cable, in the boiler or thermostat.
Interference: an obscure angle. If the BUS cable runs through the same conduit as 230V cables (for example, after an electrical upgrade), communication may fail intermittently.
Wireless thermostat: the receiver has lost connection with the transmitter or has lost power.
Finally, the regulator itself or the boiler's circuit board.

AC Error: Plug-in module or external control

Eur: Electrical interruption between a socket module and the Heatronic board. Check the cable and connector.
EurAcu-Hit: Outdoor temperature control failure TA211. Check the outdoor NTC probe and the remote control.

d5, d1 and d7: other peripheral codes

d5 (Ceraclass Excellence, Cerapur Excellence and Cerapur Acu): External drive probe failure , the one placed in the hydraulic compensator in installations with multiple circuits (IPM module). Check the probe and its cable.
d1 and d3 on the EurAcu-Hit: LSM module lock. The manual indicates checking the operating mode of that module.
d7 (Ceraclass Excellence / Cerastar): Regulator valve failure. Check its connection; if the problem persists, contact a technician.

What does the Junkers technical manual say about the model?

Eur

A8: CAN bus communication interrupted between the module and the control unit; check the cable. AC: Electrical interruption between the plug module and the Heatronic. CC: External sensor TA 211 E or bus controller interrupted; check sensor and cable. d3: Terminals 8-9 open; jumper or connector missing.

EurAcu-Hit

A8: Bus communication; check the regulation and connections. AC and DC: Faults in the outdoor temperature controller TA211 and its NTC probe. d1 / d3: LSM module lockup.

Ceraclass Excellence / Cerastar

A8: BUS interrupted. CC: external probe. d3: jumper 161 is missing on connector ST8. d5: external drive probe (IPM, hydraulic compensator). d7: regulating valve.

Cerapur, Cerapur Comfort, Excellence and Acu

A8: BUS communication. CC: external probe. d3: external controller TB1: missing jumper 8-9 or PR-PO. d5 (Excellence and Acu only): external drive probe.

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

Yes you can:

Disconnect the power to the boiler for one minute and then reconnect it.
Try to remember what was done before the code appeared: new thermostat, renovations, painting, window replacement. That's the best clue.
Visually inspect the outdoor probe cable on the facade.
Check that the wireless thermostat has batteries and is paired.

You shouldn't:

Bypassing TB1 of a radiant floor.
Open the terminal block with the boiler plugged in.

Spare parts

These codes almost never indicate a problem with the boiler itself. If it turns out to be the outdoor sensor, the regulator, or the circuit board, you can look up the part number among spare parts Junkers or boiler parts. If you can't find it, write to us with the part number and we'll find it for you. If the circuit board is suspected, first check our guide to circuit board codes F0, E0, and b1.

Frequently Asked Questions

I changed the thermostat and now it's showing d3. What did I do wrong?

Most likely, the jumper between terminals 8 and 9 (which are not the thermostat terminals) has been removed, or the new thermostat wire has been connected there. Check your boiler's wiring diagram or call a technician.

Can I use the boiler without an outdoor sensor?

Yes, if the regulator is configured not to use it. But it will work less efficiently: the outdoor sensor saves gas by adjusting the water temperature to the weather.

The A8 appears and disappears. Why?

Intermittent communication is usually due to a loose splice or interference from 230V cables. Check the BUS cable routing.

Where can I see the rest of the codes?

In the error code index Junkers.

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.