Junkerserror codes C6, C4 and C1: pressure switch, fan or chimney (how to know which one is faulty)

Your Junkers stops with error code C6 (or C4, C1, C2, or C7), the fan makes a noise, and it doesn't restart. Almost everything you find says "pressure switch faulty." But the pressure switch is only what triggers the warning: in most cases, the problem lies in the pipes, the chimney, or the fancondensing boilers Junkers, error code C6 doesn't even refer to the pressure switch.

In this guide we explain what the pressure switch does, what each code means exactly according to your model, what to check before buying anything, and which parts are usually responsible.

What does the pressure switch do?

In sealed or forced-draft boilers and water heaters, a fan pushes the flue gases outside. Before turning on the gas, the hob needs to know that the fan is actually working and that the flue isn't blocked. That's what the differential pressure switch: a small switch with a diaphragm, connected to the flue circuit by one or two silicone tubes. When the fan blows and creates the appropriate pressure difference, the diaphragm moves and the contact closes. The hob then understands, "Everything's fine, I can turn on the gas."

The logic of the board is simple and can be seen in the codes:

• With the fan stopped, the pressure switch must be open. If it is not, the board thinks it is stuck or bypassed: C4.
• With the fan running, it must close. If it does not close: C6.
• If it closes but opens while fully operational: C1 (in some heaters, C2).

Before blaming the pressure switch: what usually fails

1. The flue. Bird nests, leaves, a sooty grate, a crushed facade fitting, or a dislodged duct. If the smoke isn't escaping, the pressure difference isn't reaching the building. Check the fitting from outside the house.
2. Wind. If C1 or C6 errors appear mainly on windy days, the flue terminal is incorrectly positioned or needs a deflector. This is not a boiler malfunction.
3. The silicone tubing. It's the classic thing no one pays attention to. It cracks with the heat, fills with condensation, or gets dirty inside. The Euroline manual Eurmentions it: dirty pressure switch ports. A single drop of water in the tubing is enough to trigger a C6 error.
4. The tubes were switched. After a check, if someone connects them backwards, the pressure switch never registers the correct pressure.
5. Weak fan. If it spins but slower than before (worn bearing, dirt on the blades, damaged diaphragm), it won't create enough pressure. This is usually indicated by a different, deeper, or whirring noise.
6. Low voltage. Some Eurmodels have a specific code, UU, for voltage below 185V. With low voltage, the fan will rotate slowly. If the light is flashing in your home or your boiler is connected to a long circuit, be aware of this.
7. The pressure switch. Now we've got it. Perforated diaphragm or stuck contact. Typical of the C4.
8. Wiring or motherboard. Last, and only if everything else is okay.

What does the Junkers technical manual say about the model?

Sealed boilers: EurMFA, Ceraclass Excellence, Ceraclass Midi AE and Eur

• Eur(MFA versions): C1 pressure switch that activates during operation; C4 pressure switch that does not open when at rest; C6 pressure switch that does not close; and C7, no fan speed signal. Check pressure switch, extractor, pipes, and electrical connections.
• Ceraclass Excellence / Cerastar: C1, C4, and C6 refer to the differential pressure switch. The manual specifies checking the pressure switch, its wiring and hoses, the fan with its ducts, and the exhaust duct.
• Ceraclass Midi (AE): C1, C4, and C6 have the reset symbol. Check the electrical and pneumatic connections of the pressure switch and ensure there is no obstruction in the gas extraction.

Condensing boilers: Cerapur, Comfort, Excellence and Acu

Here , C6 means the fan isn't working. The manual instructs you to check the fan cable, the plug, and the fan itself, and replace them if necessary. These boilers regulate the gas and air mixture by controlling the fan speed, so they don't rely on a pressure switch like traditional sealed boilers. If you're looking for a pressure switch for a Cerapur with a C6 code, you're looking for the wrong part.

Heaters

• miniMAXX Excellence: C2, the pressure switch opens during operation (check the flue and remove dust or particles); C4, it does not open (pressure switch, cables and chimney connections); C6, it does not close the circuit (flue and pressure switch connections).
• Celsius / Celsius Plus: If the appliance does not turn on and the fault LED is illuminated (C4, C6 on the Celsius Plus), the manual indicates that with the water inlet filter, the flow sensor, and the pressure switch. In other words, a dirty water filter can cause the same symptom. If it shuts off and locks up while running (C2), the issue points to the flue, condensate drain, and the pressure switch and its associated pipes.

If your Junkers doesn't have a screen

• Eurline (ZW…-1): Red LED with the second and fifth greenLEDs: poor gas evacuation. The manual mentions the pressure switch cut-off, dirty pressure taps, a fan that does not rotate, and the AGÜ probe.
• Ceraline Acu MFA: the pilot light flashes four times: pressure switch that does not open, does not close, or is activated during operation.

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

Safe to check: look at the flue from the outside, see if the problem coincides with windy days, clean the water inlet filter in water heaters, and check if the fan sounds different.

For the installer: Open the boiler to check the pipes and pressure switch, remove the fan, and perform any work on the flue. Proper flue gas evacuation is a safety feature: a combustion gas leak in the home is dangerous.

Related spare parts

Pressure switches:

• Pressure switch for ZWC 24/28 boilers (87167712610)
• Pressure switch for boilers ZWC 24 (87167711750)
• Pressure switch for boilers ZWE 24/28 (87174060490)
• Differential pressure switch for boilers ZWR 24 (87174060410)
• Differential pressure switch for WRD 11 and WTD 11 heaters (8738709731)
• Pressure switch for Celsius Plus, miniMAXX and WTD heaters (8738709732)

Fans:

• Fan for boilers ZWC 24 (87167711010)
• Extractor for Ceraclass ZW 24 boilers (87161432010)
• Fan for ZWE and ZWR boilers (87172042240)
• Europur condensation fan, ZWBC 30 and ZWB 32/35 (87172045290)

Always check the reference number on your rating plate. More parts available at spares Junkers.

Frequently Asked Questions

Can I bypass the pressure switch to get around this?

No, never. The pressure switch is what prevents the boiler from turning on if the flue gases aren't escaping. Bypassing it can fill your house with combustion gases, including carbon monoxide.

I only get a C6 error code when it's windy. Is the boiler broken?

Probably not. The wind creates backpressure at the flue outlet. An installer can assess whether to move the terminal or install a deflector.

I've replaced the pressure switch and it's still showing C6. What else can I do?

Pipes (water inside, cracks, poorly connected), flue, weak fan, and finally, the cooktop. That's why it's a good idea to check all of that before buying.

The fan is making a strange noise, could that be related?

That's probably true. A fan with worn bearings or dirty blades rotates more slowly and doesn't generate the pressure the pressure switch needs.

Are there more Junkers codes explained?

Yes: you have the guides for error EA, error E9 and NTC probes (A7, E2), and the error codes Junkers.