Bosch Tankless Water Heater Error Codes List: Every Code Explained

Every Bosch water heater error code we could verify against Bosch's own documentation — what it means, what usually causes it, and the first fix to try — in one table. Codes differ between model series, so treat your model's tech sheet as the final word.

How to read and retrieve error codes on Bosch water heaters

Bosch tankless heaters report on a two-character LCD display — a letter plus a digit or a second letter (A7, C1, EA) — and the letter is the family: A codes are temperature sensors and cabinet heat, C codes are combustion air, venting, the fan and the water valves, E codes are over-temperature cut-offs, flame supervision and control-unit faults, and F codes are control, configuration and gas-path faults. Read the code exactly as shown, flashing or steady, and read it on the right generation: the Greentherm 9000 table and the older Therm table reuse letters with different meanings (A2 is an aquastat sensor on a Greentherm, the flue gas limiter on a Therm). Both generations keep a fault history you can quote to a technician — the Greentherm's FH menu lists the last 10 error codes with date and time, and the Therm diagnostic menu stores the last 10 failures at positions 1F through 10F.

The complete Bosch water heater error code list

How to reset / clear error codes on Bosch water heaters

Bosch documents two resets, one per generation. On Greentherm 9000 models, remove an error code by pressing and holding the ON/OFF button for more than 3 seconds (a long press); the manual adds that some codes can also be cleared by shutting off the water flow and turning it back on — the C-family note limits that method to 3 times per hour. On the older Therm / C generation there is a dedicated reset button: 'To remove error code from the display, press the reset button.' Both resets only clear the display — Bosch's tables pair every reset with its cause check, because a standing fault re-reports, and several Greentherm codes (the over-temperature E family, FA gas leakage) must never be repeatedly reset and run. Reset once, after the external checks, then run one hot tap and watch which stage the code returns at.

How to diagnose any Bosch water heater code

Work the problem in this order — and stop wherever gas piping, the burner compartment, venting or the temperature-and-pressure relief valve begins, because those belong to a licensed technician: (1) record the report exactly — a counted LED flash pattern, a numeric display code, or a smart-control alarm are three different languages, and a photograph or video beats a memory of any of them; (2) establish the water story: is there no hot water at all, water that runs cold mid-draw, or water but with a code — tankless codes split into ignition, flame, airflow/venting, flow and temperature families, and tank codes into ignition, sensor and control families, and the family picks the checks; (3) run the owner-level external checks for that family first: power and the unit's switch, the gas cock fully open, intake and exhaust terminations clear of snow, nests and debris, the inlet water filter or screen rinsed (tankless), the condensate trap and drain not blocked or frozen (condensing models), and on tank models the status light read before anything is reset; (4) reset once, the way the brand documents, and run hot water at one tap while you watch — a code that returns at the same stage (fan start, ignition click, flame light, sustained burn) is an active fault naming its stage, not a glitch; (5) everything past that line — flame sensors and igniters inside the burner, gas valves, heat exchangers, control boards — is professional work with the gas supply and a meter involved. Two standing cautions: never plug or cap a dripping T&P relief valve (it is reporting pressure or temperature, and it is the tank's last safety device), and treat any gas smell as an emergency — leave and call the gas utility. Scalding is the quiet hazard in every reset: a control fault that lets outlet temperature drift high is a reason to stop using the water on skin until the code is resolved.

Two Bosch generations, one alphabet: read the code on the right platform

Bosch has sold two tankless generations in North America under overlapping names, and they share a code alphabet without sharing all of its meanings. The current platform is the Greentherm 9000 series — T9900 SE and T9900i SE, T9800 SE and the outdoor T9800 SEO, in 160,000 and 199,000 BTU sizes — condensing heaters with a colour display, auto-tuning combustion, an optional fan air filter, and on the i models a built-in recirculation pump. Behind it sits the older Therm generation — Therm 830 ES, 920 ES and 940 ES, the condensing C 1050 and C 1210 ES/ESC, and the GWH models those designs descend from — with a simpler two-character LCD, a dedicated reset button and a Program key. Both generations write codes as a letter plus a digit or second letter, and both sort them into the same broad families. But the generations were programmed separately, and at least six letters changed meaning between them: A2, A3, A4, A7, E3 and E4 do not mean the same thing on a Greentherm as on a Therm. Bosch's own US FAQ demonstrates the trap — its A2 answer describes the older flue gas limiter, while the Greentherm manual's A2 is an external aquastat sensor that has nothing to do with flue gas. The hardware tells the generations apart at a glance if you know where to look: Greentherm models carry their rating as T9900 or T9800 on the cover and run their menus from a colour display, while Therm models carry the 830/920/940 ES names and the older control layout with its separate reset and Program buttons. The table on this page therefore scopes every colliding row to its generation, and the model name is part of every lookup: quote code and model together, in any search and any service call, because each half alone can send you to the wrong generation's table and a confident, wrong repair.

The A family: a map of the heater's temperature sensors

On a Greentherm 9000, the A codes are the temperature-sensor census. The heater watches its water and air with a chain of named sensors — Bosch's component list numbers them: outlet hot water sensor [17], heat exchanger hot water sensor [19], inlet cold water sensor [20], flue gases sensor [15], condensing unit flue sensor [16], cabinet air sensor [18], and the external aquastat [21] that reads a storage tank or recirculation loop — and the A family reports them in order: A0 is the group report (water temperature sensors disconnected or damaged), A2 the external aquastat, A3 the flue gases sensor, A4 the cabinet air sensor, A5 the condensing unit flue sensor, A6 the inlet sensor, A7 the heat exchanger sensor and A8 the outlet sensor. Almost every A solution in Bosch's table is the same sentence — check the sensor wire connections; replace the sensor and/or wire connections if the problem persists — because an A code is a continuity report, not a temperature report: the control can no longer trust a reading it needs, so it stops rather than fire blind. Two A codes break the pattern. A1 is a genuine temperature event — cabinet air above 158 F (70 C), with the appliance derating to protect itself, caused by very hot ambient air or flue gas leaking inside the cabinet — and A9 is an output report, power too low, pointing at gas inlet pressure or the sensor chain. On the older Therm generation the A family is shorter and partly different: A2 is the flue gas limiter (cabinet temperature above 230 F), A3 a wire harness defect Bosch tests at 115–120 ohms, A4 the backflow sensor, and the hot-sensor fault that Greentherm calls A7/A8 is reached there through the E2 bulletin swap test. Same letters, different map — the generation decides.

The C family: combustion air, venting, the fan and the water valves

If the A family is the heater listening to its sensors, the C family is the heater reporting on its breathing and its plumbing. The blockage codes are the heart of it, and Bosch splits them by when the blockage is proven and how bad it is: AD is a derate — flue blockage detected during startup, power reduced, the unit still runs (Bosch's US FAQ calls AD an informational code, and lists its causes as insufficient air supply, blocked intake or exhaust, a clogged fan air filter, long vent runs and altitude over 2,000 ft). C1 and C2 are the same startup detection at shutdown level — excessive blockage, appliance off for safety. CE and CF are the operating pair: blockage proven during the burn, CE specifically from excessive pressure on the condensate siphon, CF from insufficient combustion air. Bosch's troubleshooting section groups C1, C2, CF and CE together and gives them one owner recipe — verify there are no venting obstructions, clean or replace the air filter, reset — which is why the external terminations and the fan filter are the first checks for any C code. Around the blockage codes sit the component reports: C7 the fan disconnected or damaged, C3 the flow sensor seeing no recirculation, C5 the bypass water valve and C8 the main water valve disconnected or damaged, C9 a damaged barometric pressure sensor (whose fix is the control unit), and CA a plain flow report — water flow above 9.2 gpm, answered with purging air from the circuit and checking pressure against the 150 psi limit. Read a C code as a sequence: is it a derate or a shutdown, at startup or during operation, and is it about air in, gas out, or water through? Those three answers pick the checks.

The E and F families: over-temperature cut-offs, flame supervision and the control itself

The E family mixes three different kinds of seriousness, and telling them apart matters more than the digits. First are the over-temperature cut-offs, where the heater stops the burner to protect itself or you: E1 is water temperature — above 194 F (90 C) on a Greentherm, above 185 F at the outlet sensor on a Therm — with the burner cut off against scalding and reactivation only after cooling; E3, E4 and E5 are the flue and cabinet versions (flue gases above 167 F, cabinet air above 176 F, condensing flue gases above 428 F, each with a two-minute lower threshold), and Bosch's own descaling section names E3 — with AA and A5 — as a scale code, because a scaled heat exchanger runs its flue side hot. E9 is the thermal fuse, broken: a one-time safety device that is replaced, never bypassed. Second is flame supervision: EA, ignition failure — no flame ionization detected with water flow, which Bosch's FAQ quantifies as three failed ignition attempts — and EC, flame lost during operation, with Bosch grouping the pair as a combustion-setting problem (gas pressure, gas type, tuning). Third is the control reporting on itself: E0, E7 and EB are electronic control unit internal errors, E8 an ionization-test error whose Bosch solution is control replacement, and EE and EF pull the gas path back in — the gas valve modulation solenoid disconnected, and the wrong gas connected for the configuration. The F family is the control's second drawer: F2, F3, F7 and F8 are further internal errors (F3 allows that configuration may be required), F9 is the gas valve driver, FC is housekeeping — buttons pressed over 30 seconds — and FA is the family's emergency: gas leakage in the gas path, the valve or manifold not sealing, answered by shutting the heater down and replacing the valve or manifold. An FA is never a reset-and-watch code.

Diagnosis order on a Bosch: external air and water first, one reset, then the stage test

Work a Bosch code in the order the heater itself experiences a call for hot water, and stop at the line Bosch draws around licensed work. Start by recording the report exactly — code, flashing or steady, model generation, and what the taps were doing — and, on a Greentherm, open the FH failure-history menu: the last 10 codes with dates often show whether today is a first offence or the end of a pattern (the Therm stores the same history at 1F–10F in its diagnostic menu). Then run the external checks that match the family. For C codes and AD: walk the venting — intake and exhaust terminations clear of snow, nests and debris, no crushed or disconnected runs visible — and clean or replace the fan air filter. For ignition codes (EA, EC, BB, BC): confirm every manual gas shutoff is open and another gas appliance works, which separates a house supply problem from a heater problem without touching the gas path. For flow and temperature complaints: rinse the inlet water filter screen — Bosch lists it in annual maintenance and in its low-flow troubleshooting — and check the condensate trap and drain on condensing models, which is also the CE check. Only then reset, once, the documented way for your generation (Greentherm: hold ON/OFF for more than 3 seconds; Therm: the reset button), and run one hot tap while you watch and listen: fan start, ignition clicks, flame established, sustained burn. The stage at which the code returns is the diagnosis the code alone could not give you — a C code at fan start is air, an EA after clicking is gas or electrodes, an E code minutes into a long draw is heat and scale. Everything past that stage test — gas pressure and tuning, electrodes, sensors inside the cabinet, valves, the control unit — is the licensed technician's half of Bosch's own table, and quoting the code, the generation and the stage it failed at is the most useful thing an owner can hand over.

Less common and model-specific codes

Search data shows owners also meet these codes on Bosch water heaters: A-Family, C-Family, E/F-Family. They are not part of the core code list Bosch publishes for most models — several are series-specific, abbreviated display readings, or codes carried over from a sister platform — so we do not publish meanings for them rather than guess. Treat them by symptom: note what the machine was doing when the code appeared (filling, draining, heating, spinning), run one full power cycle (5 minutes off), and if the code returns, match the symptom to the closest documented family in the table above and check the tech sheet stored inside your machine for the model-specific definition.

Related guides

Facts on this page were checked against manufacturer documentation: www.bosch-homecomfort.com · www.bosch-homecomfort.com · www.bosch-homecomfort.com · www.bosch-homecomfort.com · www.prowaterheatersupply.com · www.prowaterheatersupply.com · www.globaltowne.com