Bosch Tankless Water Heater Error Codes: EA, C2, A7

By Ray Bennett · Updated 18 October 2026 · 8 min read · Checked against the maker’s manual

Bosch Tankless Water Heater Error Codes: EA, C2, A7

Short answer

Bosch tankless code meanings depend on the model. EA concerns ignition on Therm 830 ES, 940 ES and Greentherm T9800 SE, but their sensor and safety codes differ. On T9800 SE, C2 reports insufficient combustion air at startup and E9 means a broken thermal fuse. Record the model and code before resetting.

Bosch tankless water heater error codes need the model name beside them. Bosch’s Therm 830 ES and 940 ES manuals describe E9 as an open overheat sensor circuit. Its Greentherm T9800 SE manual gives E9 a different meaning: a broken thermal fuse that requires replacement by a licensed professional.

This page covers those three US tankless models, with the T9800 SE table applying to the 160 and 199 versions. If your appliance is a Greenstar heating boiler, use the Bosch boiler code guide instead.

Before you start

What to carry: a phone to photograph the model label and the display. Include whether the code flashes and whether it appears as the tap opens or after hot water has started.

Gas or electric? These are Bosch’s gas tankless manuals. They do not provide codes for Tronic electric water heaters. Check your model paperwork without removing the cabinet.

Renting? Send the photo to whoever arranges your hot water repairs. Say whether every hot tap is affected.

Do this before considering a reset. Gas smell or a suspected leak. Bosch’s T9800 SE instructions prohibit lighting the appliance, smoking, flames, electrical switches and using a phone inside. Warn other occupants without ringing doorbells. Open doors and windows and close the manual gas valve if safe to do so. If you hear gas leaking, leave immediately. From outside, call the gas utility, a licensed professional, police and fire department. Keep others out.

Flue gas smell. Bosch says to switch the appliance off, open doors and windows, and contact the licensed installer. It warns that obstructed ventilation can release toxic flue gas. Do not restart while that danger remains.

Overheating or gas that will not shut off. Bosch’s T9800 SE manual directs you to shut the gas supply at the manual valve on the gas line. If any part has been under water, Bosch says not to use the heater. Call a qualified technician immediately to inspect it and replace any control system part or gas control that was under water.

Hot water and service work. Bosch warns of scalding at the fixtures. It says never to leave children, disabled people or older adults unattended in the bath or shower, or let children operate hot taps themselves. Bosch reserves installation, maintenance and repair to a locally licensed professional. Leave gas pressure, combustion adjustments, electrical tests and internal sensors to that person.

Therm 830 ES and 940 ES codes

Bosch gives E9 different meanings on these Therm and Greentherm models. Match the label before using a reset or ordering a part.

Check 1A display record; repair depends on the diagnosed cause

Use the Therm table only for 830 ES or 940 ES

These meanings come from table 41 in Bosch’s August 2021 manuals for the two models. A description identifies what the control detected; it does not prove that the named sensor itself has failed.

Therm code	Bosch’s indication for 830 ES and 940 ES
A2	Flue gas limiter fault; temperature above 230 degrees F inside the cabinet
A3	Wire harness defect
A4	Backflow temperature sensor disconnected or shorted
A7	Outlet temperature sensor fault, below 36 or above 210 degrees F
A9 flashing	Outlet sensor is not sensing the expected output temperature; a status message
C6	Primary fan rotation too low during operation
C7	No rotational speed sensor signal from the primary fan
CA flashing	Water flow signal above the specified maximum; a status message
E1	Outlet sensor detects water above 185 degrees F
E2	Cold water temperature sensor fault
E4	Backflow temperature sensor above 309 degrees F; burner closes and heater locks
E9	Overheat sensor, also called ECO, has an open circuit
EA	No flame ionization detected with water flowing
EC	Ionization failure during operation
E0	Internal hardware or software failure
F7	Ionization error while on standby
FA	Gas leakage error; gas valve circuit not closing properly

A7 is not automatically a descaling instruction. Bosch’s Therm chart includes freezing conditions as well as mineral buildup among the considerations for that code. The technician needs the actual temperature and sensor checks before choosing the remedy.

Greentherm T9800 SE 160 and 199 codes

Check 2Professional testing for a persistent fault

Use the Greentherm chart for the complete model

The following is Bosch’s June 2023 T9800 SE table 24, grouped to make it easier to scan. It is not a chart for every Greentherm model.

T9800 SE code	Bosch’s indication
A0	Water temperature sensors disconnected or damaged
A1	High cabinet air temperature; power reduced for protection
A2	External water sensor or aquastat disconnected, damaged or incorrectly installed
A3, A4, A5	Flue gas, cabinet air or condensing unit flue sensor disconnected or damaged, respectively
A6	Cold water inlet sensor disconnected or damaged
A7	Heat exchanger hot water sensor disconnected or damaged
A8	Hot water outlet sensor disconnected or damaged
A9	Output power too low
AA	High flue temperatures; power regulated against overheating
AD	Reduced appliance power and possible loss of hot water comfort
BB, BC	Fan calibration after missed ignitions, or repeated calibration with similar learned values
C1, C2	Excessive flue blockage detected at startup because of insufficient combustion air; safety shutdown
C3	Flow sensor does not detect water recirculation
C5, C8	Bypass water valve or main water valve disconnected or damaged, respectively
C7	Fan disconnected or damaged
C9	Barometric pressure sensor damaged
CA	Water flow above 9.2 gallons per minute
CE	Excessive flue blockage detected during operation, with excess pressure on the siphon; safety shutdown
CF	Excessive flue blockage detected during operation because of insufficient combustion air; safety shutdown
E1	Hot water above 194 degrees F; burner stops against scalding and can reactivate after cooling
E3, E4, E5	Flue gas, cabinet air or condensing unit flue overtemperature, respectively; burner stops
E9	Broken thermal fuse
EA, EC	Ignition failure or flame lost during operation, respectively
E0, E7, E8, EB, F2, F3, F7, F8	Internal electronic control errors; E8 occurs during the ionization test
EE	Gas valve modulation solenoid disconnected
EF	Propane connected when the appliance is configured for natural gas
F9	Internal gas valve driver fault
FA	Gas leakage in the gas valve or gas manifold shutoff path
FC	A touchscreen key has been held for more than 20 seconds
Red X	Display is not communicating with the electronic control unit

AD can describe the installation. Bosch says altitude above 2,000 feet and flue length can cause reduced output while the heater continues operating, with a warning that cannot be removed. A reset is not a promise of full rated output.

EA and C2 lead to different service checks

Check 3Gas supply and combustion testing if the fault remains

Describe when the flame problem occurs

EA versus EC. Bosch’s tables distinguish failure to establish flame from loss of its signal during a run.

For the Therm models, Bosch’s EA list includes the gas supply, gas type, pressure, ignition connections, venting and combustion. Its Greentherm EA and EC section also points to gas supply conditions, air supply and combustion settings. These are reasons to test the system, not reasons to buy an igniter from the code alone.

Check 4Air and vent checks, with repair determined on site

Keep a C2 investigation with the T9800 SE instructions

Bosch’s T9800 SE troubleshooting section groups C1, C2, CF and CE with excessive vent length or an obstruction. Its code table also includes the air filter and gas and air adjustment among the checks.

From a safe place on the ground, note any visible obstruction at the outdoor termination for the installer. Leave pipe removal, internal filters and adjustments to the professional. A C2 does not by itself establish that the fan needs replacing.

T9800 SE: THREE DIFFERENT REASONS FOR A STOP DISPLAY CODE AEA BC2 CE9 AIgnition was not established Gas supply and combustion checks. BAir supply at startup Detected flue blockage;insufficient combustion air. CThermal fuse broken Professional replacement required. T9800 SE 160 / 199 only.

T9800 SE: ignition, air supply and a broken thermal fuse require different investigations. None of these labels is a universal Bosch reset instruction.

Resetting the display after the fault is addressed

Check 5A documented button action; service if the problem persists

Use the button for your model

Therm 830 ES and 940 ES. Bosch’s operating instructions say to record the error and consult troubleshooting before pressing the separate Reset button firmly. For an ordinary displayed error, they say not to unplug the heater as the first response. A persistent problem goes to the installer.

Greentherm T9800 SE. Bosch specifies touching and holding On/Off for more than three seconds to clear a displayed error. Its table notes that a reset only takes effect while the error is visible. Contact a licensed professional if the problem persists.

Some faults need a repair. Bosch’s T9800 SE E9 remedy is professional thermal fuse replacement. FA requires professional gas path repair. A button press is not the remedy for either condition, and the safety warnings above take priority over reset instructions.

FC on T9800 SE is different. Bosch tells you to release the held touchscreen key and clean the display. Its operating instructions specify a damp cloth and hand operation, without tools.

What each one takes

What you have	What it takes
Model or code uncertain	A label photo and the matching manual
EA or EC that persists	A gas and combustion diagnosis
T9800 SE C2, CE or CF	Investigation of airflow and the vent system
A sensor code	Temperature, connection and sensor checks before replacing parts
T9800 SE E9 or FA	Professional repair of the identified safety or gas path fault

Questions people ask

Why is my Bosch tankless heater showing EA?

On the covered Therm models, Bosch describes no flame ionization with water flowing. The T9800 SE chart calls it ignition failure. The service checks must establish why ignition was not proved.

Does C2 mean low electrical voltage?

Not in the T9800 SE manual used here. Bosch identifies insufficient combustion air and detected flue blockage at startup. Use another model’s own chart if its label differs.

Is A7 always the outlet water sensor?

The Therm 830 ES and 940 ES tables use that description. T9800 SE separates the heat exchanger hot water sensor under A7 from the outlet sensor under A8.

Can I clear E9 by letting the heater cool?

Do not transfer the Therm overheat sensor description to Greentherm. Bosch’s T9800 SE chart calls E9 a broken thermal fuse and specifies replacement by a licensed professional.

What if the hot water stops without a code?

Use the separate tankless troubleshooting guide for help when the heater stops without displaying an error.

The other water heating guides: tankless codes by brand, Bosch Greenstar boiler codes, and no hot water.

Sources: Bosch US Therm 830 ES Installation and Operating Instructions 6720644936 (2021/08), table 41; Therm 940 ES Installation and Operating Instructions 6720644930 (2021/08), table 41; Greentherm T9800 SE 160/199 Installation and Operating Instructions 6720816815 (2023/06), table 24 and operating instructions.