A7, E2, E4, Ad and A9 errors in Junkers: how to know if it is the NTC probe or the cable

Your Junkers is displaying codes A7, E2, E4, Ad, or A9, and it all points to the same problem: "faulty temperature sensor." The sensor is inexpensive, so the temptation is to order a replacement. But there's a detail that almost no one mentions: often the sensor itself is fine, and the problem lies with the cable, the connector, or how it's mounted. And if you replace the sensor without checking these issues, the code will return.

In this guide we explain what an NTC probe is, what each code in each Junkersfamily measures, how to check in five minutes with a multimeter whether the problem is with the probe or the cable, and what the manufacturer's technical manual says.

What is an NTC probe and how does it "break"?

NTC stands for "negative temperature coefficient": it's a resistor whose resistance decreases when heatedcircuit board Junkers applies a small voltage to it, measures the resistance, and translates it into degrees. This is how it knows how much to heat.

What's interesting is how it detects a fault. The board doesn't know if the probe is "bad": it only sees impossible values. If the resistance is infinite, it interprets the circuit as open (broken probe, cut wire, or loose connector). If it's almost zero, it interprets it as short-circuited (pierced probe, exposed wire touching ground, moisture in the connector). That's why the Junkers repeatedly says "check for continuity or a short circuit in the probe and the connecting wire": the code doesn't distinguish between the probe and the wire.

Which probe corresponds to each code?

A7

• Cerapur, Comfort, Excellence, Acu and Ceraclass Excellence (ZWC) boilers: domestic hot water temperature probe.
• Ceraclass Midi: Defective domestic hot water NTC.
• Eur: NTC interrupted or shorted, check probe and cable.
• EurAcu-Hit: Primary output NTC of the accumulator.
• miniMAXX, HydroPower-Plus, and Celsius Plus heaters: NTC output sensor, shorted or open. In the miniMAXX, this will lock the unit. For the miniMAXX Excellence, the manual refers directly to the sensor connections.

E2

• Boilers: Heating flow temperature sensor (primary NTC). Check the sensor and cable.
• HydroPower-Plus and miniMAXX Excellence water heaters: defective NTC water inlet probe.

E4 and Ad

• Cerapur Acu: E4 is the cold water probe.
• Eur: E4 is the NTC sensor for the preheated water inlet of the solar kit. Note the nuance: the manual doesn't recommend replacing the sensor, but rather checking the 24V fuse. It's a minor fault that many people mistake for a sensor issue.
• Ad (boilers with storage tank): the storage tank probe is not detected. In the Cerapur Excellence, there is a curious case: the probe may have been detected as a BUS user and "switched." The solution is not a replacement part, but rather to reset the Heatronic 3 to its factory settings (service function 8.E) and reconfigure the regulation.

A9: The probe works, but it measures incorrectly

This is the most treacherous code. On the Ceraclass Excellence and Cerapur Acu, it means the hot water sensor is incorrectly installed. These sensors are attached to a pipe and transmit heat using thermal paste. If the paste has dried out, is missing, or the sensor has shifted, it will measure less than the actual temperature. The manual for the Ceraclass Excellence instructs you to reinstall it with thermal paste, and for the Cerapur Acu, to check the mounting location. In neither case is a new part required.

In water heaters, the A9 has other nuances: in the miniMAXX Excellence it can also be very low water inlet pressure, and in the Celsius Plus it appears flashing without blocking and suggests checking the NTC assembly and the gas pressure.

If your Junkers doesn't have a screen

• Eurline: Red LED with the third and fifth green: Primary NTC open or shorted. Red LED with the fourth and fifth green: Hot water NTC open or shorted.
• Ceraline Acu: the pilot light flashes twice: temperature sensors interrupted or short-circuited.

How to tell if it's the probe or the cable (with a multimeter)

If you have a multimeter and some basic electrical skills, this check will save you from buying blindly. With the boiler unplugged from the power supply:

1. Check the connector. Look for algae, rust, burnt plastic, or a bent pin. Disconnect and reconnect firmly. Sometimes that's all it takes.
2. Measure the probe with it disconnected. Set the multimeter to ohms on both terminals of the probe. If it reads infinity (or "1" on the display), it's open. If it reads practically zero, it's shorted. In either case, replace it.
3. Test the heat. If the reading is reasonable, hold the probe in your hand or heat it with a hairdryer while taking the measurement. The reading should decrease gradually. If it jumps to unusual values, the probe is faulty.
4. Measure the cable. If the probe is working correctly, measure from the other end of the cable (at the circuit board). If you don't see the same reading there, the problem is with the cable or a splice.
5. Compare with the manufacturer's table. Each NTC model has its own resistance curve according to temperature, which is found in the service manual. If the probe gives a value significantly different from the actual temperature, it's measuring incorrectly, even if it's not broken.

Note: A probe that "measures incorrectly" without being broken won't trigger any probe codes, but it can cause other problems. If it measures less than the actual temperature, the boiler will overheat and eventually display error code E9. If it measures more than the actual temperature, the water will come out lukewarm.

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

Without risk and with the boiler disconnected from the mains: check visible connectors, measure resistances and observe if there is moisture or dripping on the plate or cables.

For the installer: anything that involves opening the watertight chamber, draining the circuit to change immersion probes, or touching the electronic board.

Junkers NTC probes in stock

• NTC temperature sensor for ZWC, ZWN and ZWSE boilers (87160110730)
• NTC hot water probe for ZWC 24/28 boilers and WTD 12 heaters (87160108090)
• Probe for condensing boilers ZWBE 25 (87145000810)
• NTC probe for ZWR 18 and ZWC 24 boilers (87145000540)
• Temperature probe for boilers ZW 20 (87145000430)
• NTC probe for ZW boilers (87004000140)
• Temperature probe for WRD and WTD heaters (87004000260)
• Water temperature probe for Hydronext water heaters (8738722007)

Always check the reference number on your appliance's rating plate. Don't see yours? Browse spare parts Junkers or contact us with the exact model number.

Frequently Asked Questions

Can I continue using the boiler with the A7?

It depends on the model. In many boilers, error code A7 only affects domestic hot water, while the heating continues to function; in water heaters like the miniMAXX, it blocks the entire unit. In any case, it's best to have it repaired soon.

I changed the sensor and the same code is still appearing. Why?

Because the problem was with the cable, the connector, moisture, or the circuit board. That's why it's worth measuring before buying.

Will any NTC probe work, even if it's from another brand?

No. Each manufacturer uses a specific resistance curve and mounting type (contact, immersion, clip). A probe with a different curve would cause the boiler to give an inaccurate reading.

What is the probe's thermal paste and is it necessary to apply it?

It's a paste that improves the contact between the probe and the pipe. It's essential for contact probes: without it, the probe will measure less than it actually is, and error code A9 may appear.

What if I get EA or E9?

These are different faults: you have the guides for error EA (no flame detected) and error E9 (overheating).