Bosch Tankless Error Codes

    Bosch tankless error codes listed below are for reference only, please consult your Bosch tankless manual for a complete list of all error codes specifications for your system. Included below are error codes for the Therm series, the Greentherm series, and the ProTL series of tankless water heaters.

    How To Reset Your Bosch Tankless

    You can clear out the error code for your Bosch tankless by simply holding the on/off button for more than 3 seconds if your system has a touch control interface on the front. For Therm series tankless systems, press the RESET button on the front panel. Alternatively, for all models, you can try unplugging the system, waiting 2-3 minutes and then plugging it back in again.

    Choose Your Bosch Tankless for Specific Model Error Codes

    Skip to Bosch Therm Error Codes

    Skip to Bosch Greentherm Error Codes

    Bosch ProTL Error Codes

    Therm Tankless Error Codes

    Error Code A2

    FAULT = Fault in the flue gas limiter

    1. Fault in the flue gas limiter temperature above 230°F (110°C) inside the cabinet.
    2. Check continuity of the flue gas limiter. Determine flue gas limiter fault and repair ore replace.
    3. Check for flue gas leakage around the top and bottom seals of heat exchanger, use mirror to check around the rear as well as the viewing window.
    4. Check that flue gas limiter connector and spade connections are secure.
    5. Unplug heater and check the wiring harness connections on the control.

    Replacement Parts or Equipment

    • Not Available Currently

    Error Code A3

    FAULT = Wire harness defect.

    1. Check wire harness and wire connections
    2. Measure resistance loose on wire harness nearby the overmolding, must be 115-120 ohm.
    3. Check diagnostic menu > 1P, “nC” must be showed on the display.

    Replacement Parts or Equipment

    • Not Available Currently

    Error Code A4

    FAULT = Backflow temperature sensor defect

    1. This fault occurs or happens if sensor is disconnected or short circuit.
    2. Check backflow temperature sensor connection.
    3. Replace exhaust temperature sensor
    4. Check sensor

    Replacement Parts or Equipment

    • Not Available Currently

    Error Code A7

    FAULT = Outlet temperature sensor fault

    1. Outlet temperature sensor fault (Temperature below 36°F or above 210°F)
    2. Check wire connection at hot water temperature sensor
    3. Sensor may trip if water temperature drops below 36°F to protect heater from freezing conditions. Any damage due to freezing conditions is not covered under warranty.
    4. In areas where water has a high mineral content, periodic descaling should be performed.
    5. Check sensor

    Replacement Parts or Equipment

    Error Code A9

    FAULT = Outlet temperature sensor fault

    1. Outlet temperature sensor not sensing expected output temperature.
    2. This is a status message and not an error
    3. Check wire connection at hot water temperature sensor
    4. Check gas pressure. Low gas pressure may prevent the heater from reaching desired output temperature.
    5. Check supply voltage. It must be 120VAC and properly grounded.
    6. Possible defective control unit call Bosch Water Heating for further instructions.

    Replacement Parts or Equipment

    • Not Available Currently

    Error Code C6

    FAULT = Primary fan rotation too low

    1. Disconnect power supply cord and check wire connection on back side of fan and the connectors on the control board.
    2. Check supply voltage. It must be 120VAC and properly grounded.
    3. Check venting specifications are met. Long vent lengths, venting with more than three elbows, blocked vent or combination venting may cause this failure.
    4. Ensure intake and exhaust terminations maintain the required clearances stated in the manual. Cross contamination between intake and exhaust may cause the fan to alter its rotational speed.
    5. Check gas pressure. Low gas pressure may cause the fan to change its speed to meet desired temperature.
    6. Possible defective control unit call Bosch Water Heating for further instructions.

    Replacement Parts or Equipment

    • Not Available Currently

    Error Code C7

    FAULT = No rotational speed sensor signal from primary fan

    1. Disconnect power supply cord and check wire connections on back side of fan and the two connectors on the control board
    2. Check supply voltage. It must be 120VAC and properly grounded.
    3. Possible defective component in fan or defective control unit call Bosch Water Heating for further instructions.

    Replacement Parts or Equipment

    • Not Available Currently

    Error Code CA

    FAULT = FLASHING – Water Flow Fault

    1. Water flow signal over specified maximum value Water flow > 10 gallon/min.
    2. Disconnect power supply cord and check wire connections on water valve and the two connectors on the control board.
    3. Excessive water pressure/flow. Ensure water pressure is less than 150psi and flow rate is below 10 gallons per minute.

    Replacement Parts or Equipment

    • Not Available Currently

    Error Code E1

    FAULT = Over-Temperature

    1. Over-temperature detected by outlet temperature sensor. Temperature > 185°F
    2. Check wire connection at hot water temperature sensor
    3. In areas where water has a high mineral content, descale the tankless.
    4. Check sensor.

    Replacement Parts or Equipment

    Error Code E2

    FAULT = Cold water temperature sensor fault

    1. Check connector on wires coming from top of water valve for a secure connection.
    2. Sensor may trip if water temperature drops below 36°F (2°C). Protect heater from freezing conditions.
    3. Check Sensor.

    Replacement Parts or Equipment

    • Not Available Currently

    Error Code E3

    FAULT = Exhaust temperature sensor

    1. Exhaust temperature sensor – The appliance will close the burner and lock if it gets over 194°F (only applied for condensing units).
    2. Clean and check heat exchanger
    3. Clean and check condensing heat exchanger
    4. Reduce maximum power
    5. Reduce water temperature

    Replacement Parts or Equipment

    • Not Available Currently

    Error Code E4

    FAULT = Backflow Temperature Sensor

    1. Backflow temperature sensor over 309°F. The appliance will close the burner and lock (non-volatile).
    2. Check inlet/outlet vent pipes.
    3. Check if secondary fan connections are disconnected.
    4. Check venting specifications are met. Long vent lengths, venting with more than three elbows, blocked vent or combination venting may cause this failure.
    5. Call Bosch Water Heating for further instructions.

    Replacement Parts or Equipment

    • Not Available Currently

    Error Code E9

    FAULT = Overheat sensor (ECO)

    1. Overheat sensor (ECO) open circuit (resets when cooler temperatures are detected 220°F/104°C).
    2. Disconnect power supply cord and check wire connections on the overheat sensor and the two connectors on the control board.
    3. Check white wire connections at the overheat sensor. Clean terminals with an eraser. If badly corroded, replace sensor and wire harness.
    4. Check venting specifications are met. Long vent lengths, venting with more than three elbows, blocked vent or combination venting may cause this failure.
    5. In areas where water has a high mineral content, descale or flush the tankless.
    6. Unplug power supply cord to the water heater. Open a hot water tap for several minutes to allow cold water to pass through heat exchanger. Close hot water tap and disconnect lead wires to overheat sensor. Using a multimeter, check continuity through overheat sensor contacts. Replace sensor if open.

    Replacement Parts or Equipment

    Error Code EA

    FAULT = No flame ionization detected

    1. No flame ionization detected when water is flowing.
    2. Verify that all manual gas shut off valves are open.
    3. Check gas type.
    4. Reset error code and open a water tap to cycle the heater in an effort to purge air. Cycling hot water tap on and off multiple times may be necessary. If heater still faults with EA error code, have a licensed gas technician properly purge air out of the gas line leading to the water heater.
    5. Check three wire connections on the lower front of the heat exchanger are secure.
    6. Check gas pressure.
    7. Check venting specifications are met. Improper venting may cause premature failure of the flame sensor rod.
    8. Check that the minimum power fan speed has been adjusted to the proper value.
    9. Observe inside the viewing window of the heat exchanger when a hot water tap is opened. Sparking should be followed by a steady blue flame. If flame is unstable/yellow with proper gas pressure, confirm CO2 readings.

    Replacement Parts or Equipment

    • Not Available Currently

    Error Code EC

    FAULT = Ionization failure during operation

    1. Check gas type
    2. Check three wire connections on the lower front of the heat exchanger are secure.
    3. Verify that venting specifications are met. Improper venting may cause premature failure of the flame sensor rod.
    4. Check gas pressure.
    5. Check venting specifications are met. Improper venting may cause premature failure of the flame sensor rod.
    6. Check that the minimum power fan speed has been adjusted to the proper value.

    Replacement Parts or Equipment

    • Not Available Currently

    Error Code E0

    FAULT = Internal hardware/software failure

    1. Disconnect power supply cord and check the two wire and ground connections on control board as well as ground connection on heater chassis.
    2. Pressing the wrong combination of buttons on the control unit can create confusion among the microprocessors inside. In this case, the error code should not happen more than once or twice. Turn off the water heater. Turn water heater back on and try resetting error code. Use the reset button to reset any error codes.
    3. Possible defective control unit call Bosch Water Heating for further instructions.

    Replacement Parts or Equipment

    • Not Available Currently

    Error Code F7

    FAULT = Ionization error at standby

    1. Loose connection to the flame ionization rod. Verify that the thinner wire leading from the control unit is securely connected to the set of electrodeslocated on the lower front of the heat exchanger.
    2. Flame ionization rod or control unit may be damaged. Contact Bosch Water Heating for further instruction.

    Replacement Parts or Equipment

    • Not Available Currently

    Error Code FA

    FAULT = Gas leakage error

    1. Gas leakage error or gas valve circuit not closing properly.
    2. Disconnect power supply cord and check wire connections on gas valve and the two connectors on the control board.
    3. Flow water out of a hot water tap above the minimum activation point of 0.5 GPM. Measure voltage at the gas valve wire plug connection. The voltage should measure 24VDC between the left pair of wires and 24VDC between the right pair of wires when the unit is operating. If voltage is not proper, contact Bosch Water Heating for further instruction.
    4. Gas valve may be defective, contact Bosch Water Heating for furtherinstruction.

    Replacement Parts or Equipment

    • Not Available Currently

    GreenTherm Tankless Error Codes

    Below are all the error codes associated with the Greentherm tankless system from Bosch.

    Error Code A0

    FAULT = Water Temp Sensor Disconnected or Damaged

    1. Check sensor and sensor wire connections
    2. Replace the sensor and/or wire connections if the problem persists

    Replacement Parts or Equipment

    • Not Available Currently

    Error Code A1

    FAULT = Air temperature inside the cabinet is above 158°F (70°C).

    1. External air temperature is above 140 °F (60 °C)
    2. Check for flue leakages inside the appliance or in concentric vent.

    Replacement Parts or Equipment

    • Not Available Currently

    Error Code A2

    FAULT = External water temperature sensor/aquastat disconnected, damaged or not properly installed.

    1. Check temperature sensor and associated connections.
    2. Check sensor position in the tank well.
    3. Verify if correct recirculation settings are selected (refer to manual for settings).
    4. Replace the sensor and/or wire connections, if the problem persists.

    Replacement Parts or Equipment

    • Not Available Currently

    Error Code A3

    FAULT = Flue gases temperature sensor disconnected or damaged

    1. Flue gases temperature sensor disconnected or damaged.
    2. Replace the sensor and/or wire connections if the problem persists.

    Replacement Parts or Equipment

    • Not Available Currently

    Error Code A4

    FAULT = Air temperature sensor disconnected or damaged.

    1. Check sensor wire connections.
    2. Replace the sensor and/or wire connections if the problem persists.

    Replacement Parts or Equipment

    • Not Available Currently

    Error Code A5

    FAULT = Condensing unit flue temperature sensor disconnected or damaged

    1. Check sensor wire connections.
    2. Replace the sensor and/or wire connections if the problem persists.

    Replacement Parts or Equipment

    • Not Available Currently

    Error Code A6

    FAULT = Inlet cold water temperature sensor disconnected or damaged.

    1. Check sensor wire connections.
    2. Replace the sensor and/or wire connections if the problem persists.

    Replacement Parts or Equipment

    • Not Available Currently

    Error Code A7

    FAULT = Heat exchanger hot water temperature sensor disconnected or damaged.

    1. Check sensor wire connections.
    2. Replace the sensor and/or wire connections if the problem persists.

    Replacement Parts or Equipment

    • Not Available Currently

    Error Code A8

    FAULT = Outlet hot water temperature sensor disconnected or damaged.

    1. Check sensor wire connections.
    2. Replace the sensor and/or wire connections if the problem persists.

    Replacement Parts or Equipment

    • Not Available Currently

    Error Code A9

    FAULT = Incorrect output power (too low).

    1. Check gas inlet pressure.
    2. Check water temperature sensors and water flow sensor wire connections.
    3. Check connections or replace sensor, replace any if it is not according to specification.

    Replacement Parts or Equipment

    • Not Available Currently

    Error Code AA

    FAULT = Condensing unit flue gases temperature too hot.

    1. Condensing unit flue gases temperature above 392°F (200°C) and/or flue gases temperature above 145°F (63°C) at the exhaust sensor.
    2. Check flue gas temperature sensor.
    3. Check water inlet temperature sensor.
    4. Descale the tankless water heater

    Replacement Parts or Equipment

    Error Code AD

    FAULT = Flue blockage detected during startup

    1. Flue blockage detected during startup due to insufficient combustion air. Appliance power derate and possible reduction of comfort level.
    2. Reset error (press and hold on/off button for 3 seconds).
    3. If the problem persists:
    4. Check air openings and exhaust opening for obstructions and reset the error.
    5. Check if flue length is according to manual specification and reset the system to clear error.
    6. Check if the air filter is dirty or clogged, replace it if needed.
    7. Perform gas and air tuning according to manual.

    Replacement Parts or Equipment

    • Not Available Currently

    Error Code BB

    FAULT = Fan calibration triggered due to missed ignitions

    1. Check gas inlet pressure
    2. Perform gas and air tuning

    Replacement Parts or Equipment

    • Not Available Currently

    Error Code BC

    FAULT = Successive fan calibration with similar learned values.

    1. Check gas inlet pressure
    2. Perform gas and air tuning

    Replacement Parts or Equipment

    • Not Available Currently

    Error Code C1 & C2

    FAULT = Excessive flue blockage detected

    1. Excessive flue blockage detected during startup due to insufficient combustion air. Appliance shuts down for safety.
    2. Reset error (touch and hold on/off button for 3 seconds).
    3. If the problem persists:
    4. Check air intake and exhaust pipes and terminals for obstructions and reset the error.
    5. Check if flue length is according to specification and reset the error.
    6. Check if the air filter is dirty or clogged, replace it if needed.
    7. Perform gas and air tuning

    Replacement Parts or Equipment

    • Not Available Currently

    Error Code C3

    FAULT = Flow sensor does not detect water recirculation.

    1. Check pump and connections.
    2. Check recirculation circuit for any blockage.

    Replacement Parts or Equipment

    • Not Available Currently

    Error Code C5

    FAULT = Bypass water valve disconnected or damaged.

    1. Check bypass water valve wire connections.
    2. If the problem persists:
    3. Replace the bypass water valve and/or wire connections. After valve replacement, proceed with bypass water valve calibration.

    Replacement Parts or Equipment

    • Not Available Currently

    Error Code C7

    FAULT = Fan disconnected or damaged.

    1. Check fan wire connections.
    2. Reset error (touch and hold on/off button for 3 seconds).
    3. If the problem persists:
    4. Replace the fan and/or wire connections.

    Replacement Parts or Equipment

    • Not Available Currently

    Error Code C8

    FAULT = Main water valve disconnected or damaged.

    1. Check main water valve wire connections.
    2. Proceed with main water valve calibration.
    3. If the problem persists:
    4. Replace main water valve and/or wire connections. After valve replacement, proceed with main water valve calibration

    Replacement Parts or Equipment

    • Not Available Currently

    Error Code C9

    FAULT = Barometric pressure sensor damaged.

    1. Reset error (touch and hold on/off button for 3 seconds).
    2. If the problem persists:
    3. Replace electronic control unit (ECU).

    Replacement Parts or Equipment

    • Not Available Currently

    Error Code CA

    FAULT = Water flow above 9.2gpm

    1. Disconnect the appliance and purge hydraulic circuit to remove air.
    2. Install water flow restrictor.
    3. Check water pressure does not exceed 150psi

    Replacement Parts or Equipment

    • Not Available Currently

    Error Code CE

    FAULT = Excessive flue blockage detected

    1. Excessive flue blockage detected during operation due to excessive pressure on the siphon. Appliance shuts down for safety.
    2. Reset error (touch and hold on/off button for 3 seconds).
    3. If the problem persists:
    4. Check exhaust pipes and terminals for obstructions and reset the error.
    5. Check if flue length is according to specification in manual and reset the error.
    6. Check if the air filter is dirty or clogged, replace it if needed.
    7. Perform gas and air tuning.

    Replacement Parts or Equipment

    • Not Available Currently

    Error Code CF

    FAULT = Excessive flue blockage detected.

    1. Excessive flue blockage detected during operation due to insufficient combustion air. Appliance shuts down for safety.
    2. Reset error (touch and hold on/off button for 3 seconds).
    3. If the problem persists:
    4. Check air intake and exhaust pipes and terminals for obstructions and reset the error.
    5. Check if flue length is according to specification in manual and reset the error.
    6. Check if the air filter is dirty or clogged, replace it if needed.
    7. Perform gas and air tuning.

    Replacement Parts or Equipment

    • Not Available Currently

    Error Code E0

    FAULT = Electronic control unit internal error.

    1. Reset error (touch and hold on/off button for 3 seconds).
    2. If the problem persists:
    3. Replace electronic control unit.

    Replacement Parts or Equipment

    • Not Available Currently

    Error Code E1

    FAULT = Hot water temperature above 194°F (90°C)

    1. Hot water temperature above 194°F (90°C) Appliance burner cut off to prevent scalding and will restart after cooling down.
    2. Disconnect the appliance and purge hydraulic circuit to remove air.
    3. If the problem persists:
    4. Reduce set point temperature to minimize temperature instabilities.

    Replacement Parts or Equipment

    • Not Available Currently

    Error Code E3

    FAULT = Flue gases temperature too high

    1. Flue gases temperature above 167°F (75°C) (residential) or 2 minutes above 145°F (63°C). Appliance burner cut off to prevent overheating.
    2. Descale the Bosch tankless.
    3. Reset error (touch and hold on/off button for 3 seconds).
    4. If the problem persists:
    5. Check sensor resistance and replace the sensor if it is not according to specification.

    Replacement Parts or Equipment

    Error Code E4

    FAULT = Air temperature inside cabinet too hot.

    1. Air temperature inside cabinet above 176°F (80°C) or 2 minutes above 158°F (70°C). Appliance burner cut off to prevent overheating.
    2. Check for flue gas leakage inside the cabinet or concentric vent.
    3. Reset error (touch and hold on/off button for 3 seconds).
    4. If the problem persists:
    5. Check sensor resistance and replace the sensor if it is not according to specification.

    Replacement Parts or Equipment

    • Not Available Currently

    Error Code E5

    FAULT = Condensing unit flue gases temperature too hot.

    1. Condensing unit flue gases temperature above 428°F (220°C) or 2 minutes above 392°F (200°C). Appliance burner cut off to prevent overheating.
    2. Descale the Bosch tankless.
    3. Reset error (touch and hold on/off button for 3 seconds).
    4. If the problem persists:
    5. Check sensor resistance and replace the sensor if it is not according to specification.

    Replacement Parts or Equipment

    Error Code E7

    FAULT = Electronic control unit internal error.

    1. Reset error (touch and hold on/off button for 3 seconds).
    2. If the problem persists:
    3. Replace electronic control unit.

    Replacement Parts or Equipment

    • Not Available Currently

    Error Code E8

    FAULT = Error during ionization test

    1. Error during ionization test. Electronic control unit internal error.
    2. Replace electronic control unit.

    Replacement Parts or Equipment

    • Not Available Currently

    Error Code E9

    FAULT = Thermal fuse broken

    1. Replace thermal fuse.

    Replacement Parts or Equipment

    • Not Available Currently

    Error Code EA

    FAULT = Ignition failure.

    1. Check gas inlet pressure
    2. Perform gas and air tuning

    Replacement Parts or Equipment

    • Not Available Currently

    Error Code EB

    FAULT = Electronic control unit internal error

    1. Reset error (touch and hold on/off button for 3 seconds).
    2. If the problem persists:
    3. Replace electronic control unit

    Replacement Parts or Equipment

    • Not Available Currently

    Error Code EC

    FAULT = Flame lost during operation.

    1. Check gas inlet pressure
    2. Perform gas and air tuning

    Replacement Parts or Equipment

    • Not Available Currently

    Error Code EE

    FAULT = Gas valve modulation solenoid disconnected

    1. Check gas valve wire connection.
    2. If the problem persists:
    3. Replace the gas valve and/or wire connections and/or electronic control unit. For gas valve replacement perform gas and air tuning.

    Replacement Parts or Equipment

    • Not Available Currently

    Error Code EF

    FAULT = Wrong gas connected.

    1. Wrong gas connected (LP instead of NG) for current appliance configuration.
    2. Check gas type

    Replacement Parts or Equipment

    • Not Available Currently

    Error Code F2

    FAULT = Electronic control unit internal error

    1. Reset error (touch and hold on/off button for 3 seconds).
    2. If the problem persists:
    3. Replace electronic control unit

    Replacement Parts or Equipment

    • Not Available Currently

    You have found Bosch tankless error codes but are you looking for other models or brands?

    We have a full list of all tankless error codes and the corresponding parts needed to fix or repair your tankless or combi boiler systems. Check them out here.