Platform & Control Notes
US Greenstar (ZBR/ZWB wall-hung and FS floor-standing). Codes verified on US contractor manuals. 'EA' (flame failure on ignition) appears mainly on UK/Worcester-Greenstar literature — it may show on some Bosch units; verify against your model.
Model Families & Platforms
- Greenstar ZBR / ZWB (Wall-hung condensing)
- Greenstar FS (Floor-standing condensing)
Diagnostic Characteristics & Common Service Topics
Bosch boilers utilize specific display codes and safety lockouts to protect the heat exchanger, combustion system, and hydronic loop. Everyday issues like pressure loss, flame sensing errors, or circulator faults trigger specific responses depending on the controller generation.
| Code | Meaning | Action Required |
|---|---|---|
| EA | No flame detected during ignition (flame establishment failure) — common flame-failure lockout. | Confirm the gas supply is on and the system has pressure. Gas/ignition fault — don't open the boiler. If a single reset doesn't clear it, call a licensed pro. |
| E9 | Heat-exchanger safety high-limit, or flue gas temperature limiter, tripped (overheat). | Check system pressure. Overheat safety trip — a pro should check the limiter, pump, and heat exchanger. Don't repeatedly reset. |
| C6 | Fan not running (combustion fan fault). | Combustion-fan fault — call a pro to check the fan and its leads. Not a homeowner repair. |
| d3 | Temperature high-limit fault / external guard tripped / temperature limiter locked out. | May involve an external LWCO/limiter — but the cause must be found. Wiring/jumper and limit checks are pro tasks. |
| E2 | Supply (flow) temperature sensor defective. | Sensor diagnosis/replacement is a tech task. Call a pro. |
| A7 | DHW (domestic hot water) temperature sensor defective. | Affects hot water; sensor replacement is a pro task. Heating may still run. |
| Ad | DHW tank temperature not detected (sensor recognized, then connection lost). | Check/replace the tank sensor and lead — technician task. |
| CC | Outdoor temperature sensor not recognized. | Generally a comfort/efficiency (weather-compensation) issue, not a safety lockout. Have an installer verify the outdoor sensor wiring. |
Complete Error Code Library
For the complete searchable error-code lookup table for Bosch equipment (including all 11 codes, exact lockout causes, and technician actions), visit the Phillips Hydronics Error Code Library.
Related Symptom & Diagnostic Guides
Sources & Documentation
- error-codes.json — https://www.manualslib.com/manual/3627447/Bosch-Greenstar-Zbr-21-3a.html?page=74