🛠️ Complete Guide: Bosch and Junkers Boiler Error Codes

When your heating system stops working and displays a flashing code on the screen, the self-diagnostic system is trying to communicate a specific fault. In this guide, prepared by the Termix.ro experts, we have centralized the most frequent error codes for Bosch (Gaz, Condens) and Junkers (Euroline, Ceraclass) models, along with recommended solutions.

⚠️ Warning: If an error persists after resetting, or if you smell gas, immediately turn off the gas supply and contact a certified HVAC technician!

🔍 Most Common Bosch & Junkers Error Codes

Error Code	Meaning	What to Check?
A7	Domestic Hot Water (DHW) sensor fault.	Check NTC sensor connections or replace the sensor.
C6	Fan or air pressure switch failure.	Inspect the flue/exhaust pipe and ensure the fan is spinning.
E9	Safety temperature limiter (STB) tripped.	Check for low system pressure or a blocked circulation pump.
EA	Flame not detected (Ignition failure).	Ensure the gas valve is open and the ignition electrode is clean.
F7 / FA	Flame detected after the burner should have stopped.	Check the ionization electrode and look for moisture on the PCB.
CE / [ ]	System pressure is too low.	Check the pressure gauge and refill the system to 1.5 bar.

💡 Quick Troubleshooting Steps (DIY)

Before calling a service engineer, try these simple steps to resolve minor faults:

Check the Pressure: Ensure the needle on the pressure gauge is in the green zone ( $1.2 - 1.5$ bar). If it is below $1.0$ , use the filling loop to add water.
Reset the Boiler: Press and hold the "Reset" button for 3–5 seconds. Often, a temporary software glitch or a power surge can be cleared with a simple restart.
Check Gas Supply: Verify if other gas appliances (like a stove) are working. Sometimes the external gas regulator may be tripped.
Power Supply: Ensure the boiler is properly plugged in. Note that some older Bosch/Junkers models are polarity-sensitive (Phase/Neutral).

✨ Models Covered in This Guide:

Bosch Condens Series: 2000, 2300, 2500, 7000, 8000, 9000i.
Bosch Gaz Series: 3000 W, 4000 W, 6000 W.
Junkers: Euroline, Eurostar, Ceraclass, Cerapur.

**Bosch & Junkers Boiler Error Codes**

A2 – The heat exchanger temperature limiter has tripped. ▶ Check the water side of the heat exchanger; replace if necessary. ▶ Check, clean, or repair the flue gas system.

A3 – The flue gas collector NTC sensor is interrupted. ▶ Check the NTC sensor and the connection cable.

A4 – Flue gas leaks at the flue gas collector. ▶ Check and clean the chimney/flue pipe.

A6 – The heat exchanger temperature limiter is defective. ▶ Check the heat exchanger temperature limiter and the connection cable for breaks or short circuits; replace if necessary.

A7 – Domestic Hot Water (DHW) temperature sensor is defective. ▶ Check the temperature sensor and the connection cable for breaks or short circuits; replace if necessary.

A8 – Communication interrupted. ▶ Check the BUS element connection link; replace if necessary. ▶ Check the control unit (automation) and replace if necessary.

AC – No electrical connection between the TA 211 E and the appliance electronics. ▶ Check the connection cable.

Ad – Tank temperature sensor not recognized. The tank temperature sensor was recognized as a BUS participant and then reconnected. ▶ Check the tank temperature sensor and connection cable; replace if necessary. ▶ Reset Heatronic 3 to default settings (> service function 8.E), reset IPM 1 or IPM 2 to default settings, and perform the automatic system configuration on the heating controller.

b1 – Coding plug not recognized. ▶ Insert the coding plug correctly; replace if necessary.

b2/b3/b4/b5/b6/b7 – Internal data fault. ▶ Reset the electronics to factory settings (Service Function 8.E).

C1 – The differential pressure switch disconnected during operation. ▶ Check the differential pressure switch and the exhaust ducting.

C4 – Differential pressure sensor does not open when the fan is off. ▶ Check the differential pressure sensor, wiring, and connection hoses. ▶ Check, clean, or repair the exhaust gas installation.

C6 – Fan is not running. ▶ Check the fan and fan cable; replace if necessary.

C7 – RPM sensor defective. ▶ Check the RPM sensor and connection cable for interruptions or short circuits; replace if necessary.

C8 – Fan speed outside tolerance range. ▶ Correct the fan stage setting; see "Flue gas exhaust instructions." ▶ Check the fan, plug, and cable; replace if necessary.

CC – Outdoor temperature sensor not recognized. ▶ Check the outdoor temperature sensor and connection cable for interruptions; replace if necessary. ▶ Correctly connect the outdoor sensor to terminals A and F.

CE – Heating system filling pressure too low. ▶ Refill with water.

CF – Pressure sensor tripped. ▶ Fill with hot water. ▶ Check the pressure sensor and connection cable for interruptions or short circuits; replace if necessary. ▶ Insert the coding plug correctly; replace if necessary.

d1 – Return temperature sensor is defective (low loss header/hydraulic separator). ▶ Check the temperature sensor and connection cable for interruptions or short circuits; replace if necessary.

d3 – External switching contact has tripped. ▶ Check the external switching contact and connection cable for breaks or short circuits; replace if necessary.

d4 – Gradient limitation: temperature rising too fast in heating mode. ▶ Open maintenance valves fully. ▶ Start or replace the heating circuit pump. ▶ Correctly set the pump speed at the pump terminal box.

d5 – Condensate retention sensor tripped. ▶ Check and replace the condensate retention sensor if necessary. ▶ Check and clean the secondary heat exchanger; replace if necessary.

d7 – Gas valve is defective. ▶ Check the connection cable. ▶ Check the gas valve; replace if necessary.

E0 – Internal fault on the electronic board (PCB). ▶ Replace the electronic board.

E2 – Flow temperature sensor is defective (interruption). ▶ Check the flow temperature sensor and connection cable for interruptions or short circuits; replace if necessary.

E9 – Flue gas temperature limiter, heat exchanger temperature limiter, or flue gas safety limiter has tripped. ▶ Check the temperature limiter; replace if necessary. ▶ Check the heat exchanger temperature limiter and connection cable for breaks or short circuits; replace if necessary. ▶ Check the heat exchanger on the water side; replace if necessary. ▶ Check, clean, or repair the flue gas system. ▶ Check the operating pressure. ▶ Check the pump startup; replace the pump if necessary. ▶ Bleed (air) the appliance. ▶ Check the fuse on the electronic board; replace if necessary.

EA – Flame not recognized. ▶ Check the protective conductor (ground) for correct connection. ▶ Check if the gas valve is open. ▶ Check gas supply pressure and correct if necessary. ▶ Check the mains (power) connection. ▶ Check the electrode and cable; replace if necessary. ▶ Check, clean, or repair the exhaust gas installation. ▶ Check the gas-to-air ratio; correct if necessary. ▶ For natural gas: check the external gas flow control device; replace if necessary. ▶ Check air circulation for room-dependent operation. ▶ Clean the condensate trap (siphon) outlet. ▶ Remove the diaphragm from the fan mixing system and check for cracks or impurities. ▶ Clean the heat exchanger block. ▶ Check the gas valve; replace if necessary. ▶ Two-phase network (IT): 2 MOhm – install a resistor between PE and N at the PCB power connection.

F0 – Internal fault. ▶ Press the button for at least 5 seconds (reset). The appliance restarts after release. ▶ Check electrical plug contacts and ignition cables; replace the PCB if necessary. ▶ Check the gas-to-air ratio; correct if necessary.

F1 – Internal data fault. ▶ Reset the electronics to factory settings (Service Function 8.E).

F7 – Flame detected even though the burner is off. ▶ Check electrodes for impurities; replace if necessary. ▶ Check, clean, or repair the exhaust gas installation. ▶ Check the electronic board for moisture; dry it if necessary.

FA – Flame recognized after gas shutdown. ▶ Check the gas valve; replace if necessary. ▶ Clean the condensate trap. ▶ Check the electrode and connection cable; replace if necessary. ▶ Check, clean, or repair the flue gas system.

Fd – Reset button held down for too long (over 30 seconds) by mistake. ▶ Press the button for 3 seconds. ▶ Check the wiring harness of the safety thermostat and gas valve for grounding issues.

H6 – Locking (5 minutes): Airflow problems. ▶ Check wiring, electrodes, and their positions. ▶ Check the exhaust gas installation for blockages or leaks; clean or repair if necessary. ▶ Check the selected fan speed based on flue length (Service function 2.b.d.) and correct if necessary. ▶ Check and increase minimum burner pressure if necessary. ▶ Check the main heat exchanger; clean or replace if necessary.

P – Appliance not defined. ▶ Set the appliance type (Service Function E.1).