Bosch tankless and combi units display alphanumeric error codes on the controller. The first letter indicates the system area (A = ignition/flame, C = combustion, E = exhaust, F = control board). Numbers narrow to the specific fault. Codes are consistent across Greentherm, Greenstar combi, and most Tronic Heat units (Tronic POU mini-tanks use a different scheme — typically just an indicator light + manual diagnosis).
The top codes you'll actually see
| Code | Meaning | Most common cause |
|---|---|---|
| Code A1 | Ignition failure | Gas supply / igniter / flame rod |
| Code A4 | Flame failure during operation | Dirty flame rod / vent |
| Code A7 | Outlet thermistor fault | Sensor or wiring |
| Code A9 | Heat exchanger overheat | Scaled heat exchanger |
| Code C7 | Combustion fan | Fan motor / RPM sensor |
| Code EA | Combustion abnormality | Vent / gas pressure |
| Code F2 | Condensate pressure switch | Drain blocked |
| Code F7 | Control board (PCB) fault | PCB or sensor wiring |
Full alphanumeric reference
| Code | Description |
|---|---|
| A1 | Ignition failure / no flame |
| A2 | False flame signal |
| A3 | Combustion air pressure |
| A4 | Flame failure during operation |
| A5 | Heat exchanger flue temperature |
| A6 | Inlet thermistor |
| A7 | Outlet thermistor |
| A8 | Heat exchanger thermistor |
| A9 | Heat exchanger overheat |
| B1 | DHW inlet thermistor (combi) |
| C1 | Modulating gas valve solenoid |
| C2 | Solenoid valve circuit |
| C4 | Burner sensor |
| C5 | Heating outlet (combi) |
| C7 | Combustion fan motor / RPM |
| E1 | Communication fault (remote controller) |
| E2 | Communication fault (Wi-Fi / Home Comfort module) |
| EA | Combustion abnormality |
| F1 | Internal memory error |
| F2 | Condensate pressure switch |
| F4 | Vent restriction / exhaust pressure |
| F5 | Flow sensor |
| F7 | Control board (PCB) fault |
| F9 | Boiler return temp too high (combi) |
| LC | Low water flow (insufficient GPM) |
How to reset
Three reset levels:
- Level 1 — controller reset: press the Reset button on the controller
- Level 2 — power cycle: turn off at the breaker, wait 30 seconds, restore
- Level 3 — service menu: requires Bosch installer password
If the code returns within 60 seconds of reset, the underlying fault is active. Work through the specific code's diagnostic walkthrough.
Bottom line
Each code has a specific cause and diagnostic sequence. Click the linked codes above for the detailed walkthrough. For cases without a displayed code, see general troubleshooting.