Error E9 in Junkersboiler or heater: why the limiter trips (and why it's not the culprit)

Your Junkers has stopped working and is displaying E9 (or, on models without a display, the red LED with a second green one, or the indicator light flashing three times). Almost every website will tell you the same thing: "The temperature limiter has tripped, replace it." And that's precisely the last thing you should do. The limiter is almost never the culprit: it's the indicator. It trips because something has caused the water to overheat, and if you replace the part without fixing the underlying issue, the E9 error will return within days.

In this guide we explain what the limiter actually monitors, why it trips, how to locate the real cause starting with what costs nothing, what the Junkers technical manual says for each family and when it is necessary to change the part.

What does the limiter monitor and why does it trip?

Inside your Junkers are two temperature sensors. One is the NTC probe, which continuously measures and tells the hob how much to heat. The other is the safety limiter, a thermal switch that doesn't measure anything: it simply opens when the metal it's mounted on exceeds a set temperature (in many Junkers, around 105–110 °C). When it opens, the hob cuts off the gas and displays E9.

Consider what this means: for the limiter to trip, normal regulation has already failed. The circuit board hasn't been able to prevent overheating. And this happens for two reasons: either the heat isn't dissipating the water (it's not circulating, or circulating very little), or the NTC sensor is misleading the circuit board, causing it to continue heating uncontrollably.

Boilers: why does water overheat?

Check them in this order, from free to expensive:

1. Circuit pressure. Check the pressure gauge with the boiler cold. If it reads less than 1 bar, there's not enough water in the circuit, it's circulating poorly, and overheating in the heat exchanger. Top it up to 1–1.5 bar using the filling valve. If the pressure drops again within a few days, you have a leak or an empty expansion vessel, and that's what really needs fixing.
2. Closed radiators. This is the cause no one looks at. If you have thermostatic radiator valves and all the rooms are heated, the valves close and the water from the boiler has nowhere to go. The same thing happens if you've closed radiators to save money. Open at least a couple of radiators completely and try again.
3. Air in the system. After refilling, after a repair, or if you hear gurgling, bleed the radiators, starting with those on the ground floor. A single air bubble in the pump or heat exchanger is enough to stop the water from circulating at that point.
4. Blocked pump. Very common during the first start-up in autumn: the pump sits idle for months, limescale builds up, and it won't turn. Many Junkers have an anti-blocking function, but it's not always enough. If the pump casing is hot and you don't feel any vibration, suspect the pump.
5. Flow and return valves. Check that the valves under the boiler are open, especially if someone has carried out a service or replaced radiators.
6. Blocked heat exchanger or filter. Sludge and limescale reduce water flow. If error code E9 appears mainly when heating domestic hot water and not with central heating, it points to the plate heat exchanger.
7. NTC probe faulty. If the NTC probe measures less than the actual value, the circuit board overheats. It doesn't register an error because it's not broken; it's simply giving a false reading. This can be verified by measuring its resistance and comparing it to a thermometer.
8. Check the circuit board limiter or fuse. Only after all the above. A faulty limiter will display error code E9 even when the boiler is cold; and in some models, the manual recommends checking the circuit board fuse.

There is a different E9 in condensing boilers

In the Cerapur, Cerapur Comfort, Cerapur Excellence, and Cerapur Acu, the E9 error code is not only triggered by the thermal block limiter but can also be triggered by the exhaust gas temperature limiter. If the flue gases are too hot, the boiler will protect itself with the same code. This usually indicates that the heat exchanger is dirty on the flue gas side or that the water is not cooling it sufficiently. The manual for the Cerapur and Cerapur Comfort also mentions a differential pressure switch. Installer diagnosis: requires opening the combustion chamber.

Water heaters: E9 is usually related to the water

A water heater doesn't have a closed circuit: cold water enters and hot water comes out. If the water comes out too hot, the limiter cuts off. Typical causes are different:

• Very low flow rate. Tap only half open, dirty inlet filter, tap aerator clogged with limescale, or low mains pressure. The burner heats only a trickle of water and overheats it.
• Limescale builds up in the heating element. In areas with hard water, the coil becomes calcified, water flows less efficiently, and very hot spots form.
• Very high temperature setting combined with mixing a lot of cold water from the tap. Lower the appliance's temperature instead of mixing.
• Incorrect or improperly adjusted gas supply. The Celsius Plus manual requires verifying that the appliance is compatible with the installed gas. A natural gas heater operating on butane, or vice versa, will have an incorrect flame.

Junkers recommends waiting approximately 10 minutes for their water heaters to cool down before restarting them. If the problem persists, a technician should be called.

If your Junkers doesn't have a screen

• Eurline (ZW…-1): Red LED and second green LED. The manual mentions overheating in the coil, a broken limiter, a seized pump, or air (bleed the system and the boiler's air vent).
• Ceraline Acu (ZWSE 28-5): the indicator flashes three times: the limiter has acted on the outbound journey.
• Celsius heaters: the appliance does not turn on and the fault LED flashes (on the Celsius Plus it shows E9).
• Ceraclass Midi: "E9" appears with the rearming symbol: the cause must be resolved and the system rearmed.

What does the Junkers technical manual say about the model?

• Eur: the limiter is activated. Check the NTC input, the pump, the printed circuit board fuse, and purge the appliance.
• Ceraclass Excellence / Cerastar: limiter activated on the flow line. System pressure, probe, pump, circuit board fuse and purge.
• Ceraclass Midi: Overheating with limiter activated. Check that the template valves are open, the system is purged, and the limiter is connected to its wiring.
• EurAcu-Hit: Primary overtemperature. 110°C limiter, primary NTC, heat sink, pump, and a specific board fuse.
• Cerapur, Comfort, Excellence, and Acu: Thermal block or exhaust gas limiter. Service pressure, limiter, pump, circuit board fuses, purge and check the water side of the thermal block.
• miniMAXX, miniMAXX Excellence, HydroPower-Plus and Celsius water heaters: temperature limiter. Wait approximately 10 minutes and turn on again; if the problem recurs, contact technical service.

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

Safe: Check and refill the pressure, open radiators and valves, bleed radiators, clean the tap aerator, lower the heater temperature and wait for it to cool down before reassembling.

For the installer: unblock or replace the pump, measure the NTC, descale the heat exchanger, check the flue gas side in condensation, check the gas and replace the limiter.

Spare parts related to the E9

Circulation pumps:

• Pump for boilers ZWC / ZWN 24/28 (8737604215)
• Pump for Ceraclass Midi, Euroline ZW24 and Neckar NW24 (8738725176)
• ZWSE 23/28 boiler pump (8716747369)
• ZWE 24 boiler pump (8717204264)
• ZW 24 boiler pump (87072040420)

Safety limiters and probes:

• Gas temperature limiter for condensation ZWB / ZWSB (87105062670)
• Safety probe for ZW boilers (87145000290)
• 110°C limiter for WR/WRD heaters (87072064350)
• 104°C limiter for WTD 11 heaters (87072064510)

If you suspect the NTC sensor, you'll find all the relevant information in our A7 and E2 error Junkers. And the complete list of spare parts Junkers.

Frequently Asked Questions

Can I bypass the limiter to make the boiler work?

No. The limiter is a safety feature: if you disable it, the boiler can overheat uncontrollably, risking serious damage and burns. No reputable technician would do that.

I reset it and it works for a while, then the E9 error returns. What's going on?

This is a classic sign that the water isn't circulating properly: the pressure limiter cools down, you reset it, the boiler heats up, and since the heat isn't being dissipated, it trips again. Check the pressure, radiators, air, and pump before checking the limiter.

Why does the E9 error code appear when I turn on the heating in autumn?

Because the pump has been out of service for months and may have seized up, or because some pressure was lost or air got in during the summer. Check the pressure gauge and bleed the system first.

What if it only happens when I use hot water, not with the heating on?

Point to a calcified domestic hot water heat exchanger or, in water heaters, to a water flow rate that is too low.

Is it related to the EA error?

No. EA means the boiler isn't detecting a flame, and E9 means the temperature is too high. If you get EA, you can find the EA error Junkers.