Worcester Bosch Boiler Fault Codes (General Guide)

Meanings, Likely Causes & What To Do Next

When your Worcester Bosch boiler flashes a fault code, it’s your boiler’s way of telling you what part of the system needs attention. Below is a clear, homeowner-friendly guide to the general codes used across many Greenstar models(wording varies slightly by model). We’ve grouped them by theme, added plain-English meanings, quick checks you can safely try, and when to call an engineer.

⚠️ Safety first: If you smell gas, turn off the gas at the meter, ventilate the area, and call the emergency gas number. Do not open the boiler casing yourself. Any work on gas components must be carried out by a Gas Safe engineer.

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How to use this guide

1. Note the exact code on the display (letters + numbers).
2. Find it in the relevant group below.
3. Try the safe checks.
4. If the code returns or you’re unsure, book a qualified engineer.

Worcester Bosch Boiler Fault Codes

EA / Ignition & Flame Detection

• EA 227 — No flame detected after ignition attempts.
  Likely causes: No gas to the boiler, ignition failure, blocked or frozen condensate, faulty flame sensor.
  Try: Check other gas appliances; ensure gas supply is on. If it’s freezing outside, inspect the condensate pipe for ice and safely thaw (warm cloth/hot-water bottle—never boiling water). Then reset the boiler once.
• EA 229 (on some models) — Often linked to condensate/firing issues.
  Try: Same steps as above.
• F7 228 — False flame detected before ignition.
  Likely causes: Flame-sensing/PCB issue, gas valve letting by (engineer task).
  Try: Power cycle once only. If it returns, call an engineer.
• FA 306 — False flame after burner stops.
  Likely causes: Flame signal persisting; needs diagnosis.
  Action: Engineer visit recommended.

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E / Temperature, NTC Sensors & Overheat

• E2 222 — Flow temperature sensor (NTC) fault/short.
  Try: Power reset once. Persistent? Engineer to test/replace sensor.
• E5 218 — Flow temperature too high (overheat).
  Likely causes: Pump/circulation issue, air lock, closed valves, sludge.
  Try: Bleed nearby radiators, confirm system pressure (see below), ensure valves are open. Re-try once.
• E9 219 / E9 224 — High-limit stat/flue thermostat triggered; excessive temperature.
  Action: Do not keep resetting. Call an engineer to check circulation, sensor health, and heat exchanger.

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C / Fan & Air System

• C7 — Fan not running on start-up.
• C6 — Fan speed out of range (too high/low).
• C1 — Fan stopping during operation.
• C4 — Fan failed to stop (control/fan anomaly).
  Try: One controlled reset. If recurring, this is engineer territory (fan, wiring, controls).
• CC — Outdoor sensor defective (if fitted).
  Try: System will usually continue but weather compensation won’t work correctly. Engineer can replace sensor.

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D / Temperature Behaviour & Condensate

• D1 240 — Return sensor error (often wet/damaged connection).
  Action: Engineer to check sensor and wiring.
• D4 271 — Temperature rising too fast / large delta between sensors.
  Likely causes: Circulation issues (pump/air/sludge) or scaling.
  Try: Check pressure; bleed rads near the boiler. Engineer if it persists.
• D5 — Blocked/frozen condensate.
  Try: Thaw the external condensate pipe safely (see EA 227 tips), then reset.

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A / Sensors, Hydraulics & Comms

• A1 — Low water content / poor flow on some models (pump or flow issue).
  Try: Check system pressure (see “Quick homeowner checks” below). Top up if low, then reset once.
• A7 — Hot-water NTC sensor defective.
  Action: Engineer to test/replace.
• A8 — Control/HCM communication or module invalid.
  Action: Engineer to diagnose controls/PCB.

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B & 9 / Configuration & PCB

• B1 (b1) — Code plug not detected.
• B7 — Control board (PCB) fault indicated.
• 9A — Incorrect or invalid HCM fitted (control module mismatch).
• 9U — Code plug problem (faulty/not connected).
  Action: These are configuration/PCB matters—book an engineer.

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F / Internal Faults & Mains

• F0 237 — Internal error (PCB/internal fault).
• FD 231 — Mains power interruption detected.
  Try: For FD 231, check power to the boiler/spur, look for tripped breakers. If frequent or F0 237 appears, get an engineer to test supply and PCB.

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FA / FB / Gas Valve Self-Tests (selected models)

• FA 364 — Gas valve EV2 leak test failed.
• FB 365 — Gas valve EV1 leak test failed.
  Action: Do not repeatedly reset. Call a Gas Safe engineer to test the valve and combustion settings.

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Quick homeowner checks (safe & simple)

• System pressure: On most combi/system boilers, cold pressure should sit around 1.0–1.5 bar.
  If it’s low, top up via the filling loop to ~1.2–1.5 bar, then reset the boiler once.
• Radiator bleed: If some rads are cold at the top, carefully bleed to remove air (protect carpets/towels ready). Re-pressurise after.
• Condensate pipe in freezing weather: If the white plastic pipe outside is iced, gently warm it (warm cloths/hot-water bottle). Never use boiling water or open flame.
• Power cycle: Turn the boiler off at the fused spur for 30 seconds and back on. Don’t keep resetting a lockout repeatedly—if the fault returns, call a pro.

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When to call a Gas Safe engineer

• Codes repeating after a single reset.
• Any E9/E5 overheat, C-codes (fan/air), A-sensor faults that persist, or FA/FB gas-valve test failures.
• Suspected PCB/configuration issues (B/9/F0 codes).
• You’re unsure and want a safe diagnosis.

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Model-specific notes

Wording and extra codes vary across models (e.g., Greenstar i/Si/CDi Compact/4000, Heatslave II (oil)).

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If you’re in the UK and need help, our Gas Safe engineers can visit today, diagnose the fault, and carry out repairs at fair mid-market rates (clear call-out + hourly or fixed-price options). We fix no-heat and no-hot-water emergencies 24/7—rapid diagnosis, clear pricing, and a qualified engineer on the way fast. Our goal is simple: restore your heating and hot water the same day whenever possible, with friendly updates from start to finish and no hidden surprises.