DIY Bosch Tankless Water Heater Troubleshooting Error Codes Explained.

There are many different types and sizes of Bosch tankless water heaters on the market. Most have an excellent error displayed to help you diagnose the problem.

As an electrician, I am often called to diagnose problems with instant hot water units. Also called tankless water heaters. These are very reliable and will tend to last 10-20 years or more.

I have listed some of the more common error code faults you will come across with some simple repair guides. So read on for our mega Bosch tankless water heater troubleshooting guide.

Bosch Tankless Water Heater Error Code 10

Abnormal combustion. Confirm blockage in either air intake or exhaust. Check the air fan for correct operation.

Bosch Water Heater Error Code 11

No detection of initial flame. The Bosch gas hot water fault code 11 signifies that there is a fault with the unit’s ignition. In this instance, your Bosch hot water heater will not operate and won’t produce hot water until Bosch fault code 11 has been investigated and repaired.

Bosch Water Heater Error Code 12

Flame failure safety device working. Check combustion condition, gas solenoid valve, flame failure safety device, earthing of PCB.

Bosch Water Heater Error Code 14

Overheating protection device activated. Check the wires to the over temp sensor. Check the water pressure and flow rate. Check the exhaust for restrictions. Activates at 184 deg Celsius.

Bosch Water Heater Error Code 20

The remaining flame safety device is activated. Check the cabling.

Bosch Tankless Water Heater Error Code 29

Problem: Something in the unit’s neutralizer is clogged.

Solution: Try resetting the controller by turning the main power off and on. A flush of the system may be required. You should plan on changing your filler for the condensate capsule at least once per year. Damage to equipment and drains could happen if you don’t replace the unit’s neutralizer regularly.

I use this tankless hot water flushing kit with some white vinegar for my client’s gas hot water units.

Bosch Tankless Water Heater Error Code 31

Abnormal inlet water thermistor. Measure the resistance of the water temperature thermistor. A short circuit is failed. May be hard to find this sensor as it may be behind other electronic parts and pipes.

Bosch Tankless Water Heater Error Code 32

Abnormal outlet water thermistor. Measure the resistance of the water outlet temperature thermistor. A short circuit is failed. Check terminals for corrosion.

Bosch Water Heater Error Code 61

Abnormal combustion fan. Check the fan pulse rate sensor. Also known as an under-speed proximity switch. Check all connections, possible restriction of the fan, or failure of the PCB. The target speed is 500 RPM.

Bosch Water Heater Error Code 65

Abnormal water volume adjusting valve. Confirm the limiter of water adjusting valve. Check connections. Make sure the water inlet volume and pressure are correct. The minimum water flow for correct operation is 2.5L minute.

Check for scale build-up and flush if necessary. In fact, if your hot water has never been flushed or it has been longer than 5 years between flushing I would get it done. I use this tankless hot water flushing kit from amazon. It could save you thousands in repair or replacement hot water plumbing bills.

Bosch Water Heater Error Code 71

The abnormal driving circuit of a gas solenoid valve. Control fault of the driving circuit of the gas solenoid valve. Check for ground faults or faulty PCB.

Bosch Water Heater Error Code 72

Abnormal flame failure safety device circuit (artificial flame.) Measure flame current at extinguish.

Bosch Water Heater Error Code 73

Abnormal PCB setup. Check all dip switches are set correctly to the operating manual for the country/area. Check gas changing connector and temp changing connector.

Bosch Water Heater Error Code 760, 76, 0

Abnormal remote control communication from the main controller to the gas hot water unit. Check the cable has not been damaged and is free of corrosion on all the terminals. These remote temperature controllers are prone to failure if moisture has found its way onto the internal PCB.

Disconnect the controller and clean the PCB of corrosion. Replace the Bosch temperature remote controller if required. You can still run the Bosch hot water systems but it will be at a set temperature. Often defaults to 55 deg Celcius.

You may have a jumper lead attached to the inside of the hot water until you can plug it into the PCB to set the temperature to 60 deg Celsius. I have done a post on how to repair the Bosch hot water wall controller if it fails.

Bosch Error Code 90

Problem: Abnormal combustion or low gas supply pressure.

Solution: Turn the control unit off, turn the hot water tap on, and turn the control unit on. Check the gas supply that it is up to pressure and that it is not isolated.

Bosch Error Code 92

Problem: The water heater’s neutralizer is in need of replacement.

Solution: Get in touch with a plumber to replace the water heater neutralizer. You should plan on changing your filler for the condensate capsule at least once per year.

Bosch Error Code 99

Problem: Your hot water system has shut down due to abnormal combustion.

Solution: Isolate the bosch water heater and clean the ignition terminals. Check that the flue is not clogged or restricted. If turning the main power off and on again does not work, you will need to get in contact with a qualified plumber with experience in hot water systems.

Bosch Tankless Error code A2 – Problem with the flue gas limiter

The temperature of the flue gases is too high and is above 230 F. Check for gas leakages or constant flame on the burner. This temperature sensor is located at the top-right corner of the unit.

Bosch Water Heater Error code A3 – Wire harness fault 

Measure the resistance on the wire harness; it must be between 115 and 120 ohms. Also, check the remote control unit if it is attached.

Bosch Water Heater Error code A4 – Backflow temperature sensor

Check the backflow temperature sensor connection for rust or damage. It could also be short-circuited. It is located below the heat exchanger and ignition electrodes. Look for the diagnostic menu 1P and nC code.

The Flue Safety Device (FSD) has activated indicating the heater is incorrectly flued causing spillage of flue gases into the room, or, the FSD is defective. Disconnect the wiring from the FSD, and measure the resistance. If the resistance is greater than 0.1 ohm, replace the FSD. If the resistance is less than 0.1 ohm, check the exhaust flue hardware on the appliance to ensure it is all on the vertical plane. Pay special attention to the flue cowl which should be an approved model.

Bosch Tankless Water Heater Error code A7 – Outlet temperature sensor fault

The outlet temperature sensor reacts when the temperature is either below 36 F when the unit is protected from freezing temperatures. Also above 210 F in a case of overheating due to the lime-scale deposits. Check the sensor and wiring for scale, rust, or damage. You might have to flush the unit to remove the mineral scale build-up.

I use this Kelaro tankless hot water flushing kit to descale my client’s hot water systems. It only takes around 1/2 to 1 hour to descale a tankless hot water system depending on scale build-up. White vinegar works extremely well and is cheap.

This fault results from the temperature thermistor on the right-hand side of the heat exchanger failing to give a temperature indication. This fault will not stop the heater from operating, but will first display ’00’ on the ICU display before changing to ‘A7’ on the ICU display. Ensure wiring connections to the thermistor. If this does not rectify the fault, test thermistor readings in ohms in conjunction with the thermistor chart to assess correct readings.

Bosch Water Heater Error code A9 – Outlet temperature sensor fault

This A9 error code is associated with the outlet temperature sensor, but this time, the sensor is not sensing the expected output temperature. This is not an error code; it is a flashing status message that tells you to check; the wiring connection, gas pressure as it might be too low, voltage as it doesn’t meet the required 120/240 VAC, the unit that might not be properly grounded or the issue with the control board.

Bosch Water Heater Error code A0

This fault results from the temperature thermistor on the right-hand side of the heat exchanger failing to give a temperature indication. This fault will not stop the heater from operating, but will first display ’00’ on the ICU display before changing to ‘A7’ on the ICU display. Ensure wiring connections to the thermistor. If this does not rectify the fault, test thermistor readings in ohms in conjunction with the thermistor chart to assess correct readings

Temperature °C	10	20	30	40	50	60	70	80	90	100
Resistance K ohms	22.75	14.74	9.78	6.64	4.60	3.25	2.33	1.70	1.26	0.95

Bosch Error Code C6 – Slow-spinning fan

The primary extraction fan spins slowly when the water heater is working. Check the wiring connection on the fan and control board. The voltage must be 120/240 VAC and the element correctly grounded. Does the venting system meet the code and manufacturer requirements? Ensure that the gas pressure is as required because the low pressure leads to fan rotation problems.

Bosch Error Code C7 – Fan rotational speed sensor

No signal from the fan’s rotational speed sensor. The sensor might be broken. Check the wire connection on the control board and fan. The voltage needs to be 120/240 VAC and adequately grounded. Ensure that the fan and control unit is not damaged.

Bosch Error Code CA – Excessive water flow

Make sure that the water pressure is less than 150 PSI and a flow rate below 10 GPM. Check the wire connections on the water valve and control panel.

Bosch Error Code E1 – Extremely high temperature

The temperature detected by the outlet temperature sensor is over 185 F. Ensure that the sensor is working correctly, and the wire connections are not loose. Another reason for the high temperature is the sediment build-up, so descaling and flushing will be required. I use this hot water descaling kit.

An indication that a water temperature in excess of 85°C has been detected. This will not cause the appliance to cease operation, however, service attention is required. Verify gas pressure to confirm pressure is not excessive. Also, confirm that the water flow rate did not become insufficient such that the 85°C settings were exceeded.

Bosch Error Code E2 – Faulty inlet water temperature sensor

Check the sensor and wiring connections for corrosion as this sensor is in charge of protecting the unit from freezing. 

Bosch Error Code E3 – Problem with the exhaust temperature sensor

The unit will shut down if the temperature of the exhaust gases is above 194 F. Check and clean the heat exchangers if needed. Reduce the maximum power and water temperature.

Bosch Error Code E4 – Problem with the backflow temperature sensor

The backflow temperature sensor reacts when the temperature is over 309 F. Check the vent pipes if they meet the code and requirements, look for damages and obstructions.

Bosch Error Code E9 – Overheat sensor ECO has an open circuit 

Check the wire connections on the sensor and control board. Ensure the venting system is per codes and instructions. Descale the unit if necessary.

Disconnect the wires from the temperature thermistor (over-temperature sensor) on the right-hand side of the heat exchanger, and test the resistance of the thermistor. If the resistance is greater than 0.1 ohm, replace the thermistor.

Bosch Error code EA – No sparks for the electrodes

The ionization electrodes are not making sparks when water is flowing. There are many reasons for this problem and here are some possible issues; low gas pressure, the fan speed is low, wire connections are loose, the air in the gas line, no gas corroded connections, dirty electrodes.

Failure to ignite will cause Safety Time Out. Three possible causes are

1. Ignition failure due to insufficient gas supply. Check the gas supply to the unit, Correct if necessary.

2. Flame rod not detecting pilot flame. Check the wiring connection to the ICU. Measure the flame rod current (should be 10 – 12 µA DC). Check the condition of the pilot flame, and if dirty, clean or replace the injector. If the pilot is clean, and the voltage from the flame rod is correct, replace the ICU.

3. If the pilot is failing to light, measure the voltage to the orange pilot solenoid (should be 0.9 – 1.2 Volt DC). If the voltage is correct, replace the solenoid. If the voltage is incorrect, replace the ICU. (note:- always check the voltage with the wiring connected)

Bosch Error Code EC – Electrode failure

Ionization electrode failure during water heating. The reasons might be; obstructed venting system, low gas pressure, and CO2 readings, the fan speed could be too low.

Bosch Error Code E0 hardware/software issues

The issue with the hardware or software. First, try to reset the error code and turn the hot water unit off and back on. There might be an issue with the computer. Check if the wire connections are properly linked to the computer board or the board is not damaged.

Bosch Error Code F7 – Ionization rod problem

The problem is with the ionization rod at standby. Check connections to the electrodes for corrosion and tightness. It should be securely connected. Other reasons might include damaged elements or a faulty control board.

This fault can result from an earthing problem with the flame sensor rod or an ignition pack failure. Clean the flame rod and ensure the wires are adequately connected. If the fault remains, replace both the flame sensor rod and the ICU.

Bosch Error Code F0

This fault indicates the power switch has been activated while water was flowing. Rectify by closing all hot water outlets so that no water is flowing. Press the switch (the ON position), then open the hot water outlet.

Bosch Error Code FA – Gas valve circuit

The problem is with the gas valve circuit. Check the wire connections on the gas valve and control board. Measure the voltage on the wires when a water heater is ON… it should be 24 VDC. If the measurement is not as specified, the gas valve is malfunctioning. Check for correct gas pressure and that the gas valve is operating by putting 24v onto the valve.

Red LED Flashing

This indicates low water flow through the appliance. Check the water inlet filter is not blocked, and that the water supply to the water heater is adequate. Low pressure will also create a low-flow fault code so check your supply pressure. The recommended domestic water supply pressure should be around 500kpa.

Bosch Tankless Water Heater Troubleshooting Tips

• Is the electricity turned on for the hot water system?
• Is the gas turned on?
• Is the outlet flue clear of debris?
• Is the inside remote controller turned on?
• Does the internal fan spin when the hot water is running?
• Are all electrical connections free of corrosion?
• Are the water pressure and flow rate correct?
• Is the gas pressure correct?

How often should gas hot water be serviced?

You should service your gas hot water units every 2 years. They are very reliable but should be checked for water leakage, loose connections, and corrosion. With a well-serviced tankless hot water supply, you should get 10-20 years out of your Bosch hot water system.

These systems will go from cold to hot in a few seconds. This puts stress on pipes and connections which could fail over time. Servicing will identify any issues so they can be repaired before they fail completely.

Bosch High Flow 26e YS2670RA Repair

I have found the bosch instantaneous hot water a very reliable unit. Often it is something easy to fix like a corroded terminal or a faulty fan. Otherwise, you should be able to get 10-20 years out of these tankless hot water units.

If you follow the error codes it will give you a very good idea of what is wrong with these units. Often they just need a reboot as they have overheated and locked themselves out. Just turn the hot water unit off for 30 seconds and turn it back on.

The bosch high flow hot water unit will perform a self-check. You will be able to turn on the hot water tap after a few minutes and check if the unit is working again.

We have also done a post on Rinnai hot water error codes which have similar faults and issues to the Bosch unit. They are both built very similarly.

Let me know in the comments below if you need any help with your Bosch tankless water heater troubleshooting.