Worcester Bosch Error Codes: Complete Diagnosis Guide

Quick Answer: Worcester Bosch Greenstar boilers display alphanumeric fault codes (e.g. EA, A1, D1) on the display panel, often paired with a numeric sub-code (e.g. EA 227). These indicate either a locking fault (manual reset required after repair) or a blocking fault (boiler may self-recover). Always note both the letter code and the numeric sub-code before beginning diagnosis.

Summary

Worcester Bosch is the UK's most widely installed boiler brand, and their Greenstar range dominates the domestic market. The fault code system uses a two-tier approach: a primary alphanumeric code visible on the display (EA, A1, C6, etc.) and a secondary numeric code (2xx, 3xx series) accessible via the diagnostic menu. Locking faults display a flashing code with a key/triangle symbol and require a manual reset — the boiler will not fire again until the fault is addressed and the reset button is pressed. Blocking faults cause the boiler to pause operation temporarily but it may restart automatically once the condition clears (e.g. frozen condensate pipe thawing). The numeric sub-codes provide more granular detail: for example, EA is "ignition lockout" but EA 227 specifically means "no ionisation detected after ignition attempts," while EA 228 means "ionisation detected before burner start." Always record the full code and sub-code before troubleshooting.

Key Facts

• Worcester Bosch uses locking faults (manual reset required, indicated by flashing display + key symbol) and blocking faults (self-recovering, temporary shutdown)
• The secondary numeric code is accessible via the info/spanner button on the boiler's control panel — always retrieve this before diagnosing
• EA (ignition lockout) is the single most common fault code across the Greenstar range
• Codes beginning with A relate to water pressure/flow, C to fan/flue, D to sensors/NTC, E to overheating/safety limiters
• The boiler attempts ignition up to 4 times before locking out on EA 227 — it waits 30 seconds between each attempt
• On newer Greenstar 8000 Life/Style models, the display shows a plain-language description alongside the code
• All fault codes involving gas supply, combustion, or internal components require a Gas Safe registered engineer (not just the homeowner pressing reset)
• Worcester Bosch technical support line for installers: 0330 123 3366

Quick Reference Table

Detailed Guidance

What does EA mean on a Worcester boiler?

EA is an ignition lockout. The boiler has attempted to ignite the gas burner and failed. It is the most common fault code across the entire Greenstar range.

Sub-codes:

• EA 227 — No ionisation detected after up to 4 ignition attempts (30-second intervals between each). This is the classic "no ignition" fault.
• EA 228 — Ionisation current detected before the burner starts. The electrode is sensing a flame that should not be there, or the electrode/PCB is faulty.
• EA 229 — An established flame has been extinguished mid-operation. Often intermittent.

Likely causes (ranked by probability for EA 227):

1. No gas supply — Meter off, prepayment meter run out, other gas appliances also not working. Check the gas meter and isolate other causes first.
2. Frozen condensate pipe — In winter, the condensate pipe (typically 21.5mm white overflow pipe running externally) can freeze, causing the boiler to lock out. Thaw with warm (not boiling) water and lag the pipe.
3. Faulty ignition electrode/lead — The spark electrode wears over time. Check for visible cracking, carbon build-up, or incorrect gap (should be approximately 3-4mm from burner). Replace electrode and lead as a pair.
4. Gas valve fault — Valve not opening fully or stuck closed. Measure gas pressure at the test point on the gas valve — you should see 20 mbar (natural gas) static and the correct burner pressure per the data badge. If the valve is not responding to PCB signal, replace.
5. PCB fault — If the spark generator is not firing at all (no audible clicking during ignition attempt), the PCB ignition circuit may have failed. Verify 230V supply to the boiler and check PCB connections before condemning.

Diagnosis steps:

1. Confirm gas supply — check other gas appliances, check meter, check emergency control valve is open
2. Attempt a single reset — if the boiler fires and runs, monitor. If it locks out again within minutes, proceed to step 3
3. Remove the burner cover and observe the ignition sequence — is there a spark? Is there gas flow? (use your nose and gas detection equipment, not a naked flame)
4. Check ignition electrode condition — gap, cracking, carbon deposits. Check lead continuity
5. Measure gas pressures at the gas valve test point — standing pressure, working pressure
6. Check condensate trap and siphon — a blocked sump causes moisture ingress to the burner area, which can extinguish the flame (EA 229)
7. If all above check out, suspect PCB or gas valve — measure valve coil resistance per manufacturer spec

Parts commonly needed: Ignition electrode and lead set, gas valve, condensate trap/siphon.

What does A1 mean on a Worcester boiler?

A1 indicates a water pressure or circulation fault. The boiler has detected that system pressure is too low or that water is not circulating correctly through the heat exchanger.

Likely causes (ranked by probability):

1. Low system pressure — Pressure gauge reading below 1 bar (should be 1.0-1.5 bar cold). Most common cause by far.
2. System leak — Radiator valves, pipe joints, heat exchanger weeping. Even a slow drip will cause gradual pressure loss.
3. Pump failure — Pump not running, seized, or running but not circulating (impeller detached from shaft). Listen for pump noise and check for heat differential across pump body.
4. Blockage — Sludge, magnetite, or debris restricting flow through the heat exchanger or system pipework. Common in older systems without a magnetic filter.
5. Pressure sensor fault — The pressure transducer on the boiler may be faulty, reading low when actual pressure is adequate.

Diagnosis steps:

1. Check the pressure gauge — if below 1 bar, repressurise via the filling loop to 1.0-1.5 bar (green zone). On the Greenstar range, the filling key is typically under the boiler or the external braided loop between the mains cold and heating flow
2. Reset the boiler and monitor — if pressure holds and the boiler fires, bleed radiators if needed and advise the customer to monitor
3. If pressure drops again within hours or days, there is a leak — conduct a full pressure test. Isolate the boiler from the system and test separately to determine whether the leak is on the boiler or the system
4. If pressure is adequate but A1 persists, check pump operation — is it running? Is there a temperature differential across it? Check for airlocks, particularly after system drain-down
5. On systems with heavy sludge, the heat exchanger may be partially blocked — check flow rates and consider a powerflush

Parts commonly needed: Filling loop/key, pump, pressure sensor/transducer, expansion vessel (if repeatedly losing pressure with no visible leak — vessel may have lost its charge).

What does C6 mean on a Worcester boiler?

C6 indicates a fan speed fault. The PCB has detected that the fan is not running at the expected speed.

Sub-codes:

• C6 215 — Fan speed too low. The fan is running but not reaching the target RPM.
• C6 216 — Fan speed too high. The fan is overspeeding.

Likely causes:

1. Worn fan bearings — Causes the fan to slow down gradually over time. Listen for grinding or irregular noise from the fan.
2. Restricted or blocked flue — Debris, bird nests, wasp nests, or ice in the flue terminal increases back-pressure, making the fan work harder or stall.
3. Damaged fan blades — Cracked or warped blades from heat exposure reduce airflow efficiency.
4. Air pressure switch fault — The APS monitors flue differential pressure. A faulty or stuck APS can report incorrect readings even if the fan is operating correctly.
5. Wiring fault — Corroded connections or damaged wiring between fan and PCB. Check the plug connections at both ends.
6. PCB fault — Incorrect speed signal being sent to the fan motor.

Diagnosis steps:

1. Listen to the fan during startup — abnormal noise (grinding, rattling, whistling) indicates mechanical failure
2. Check the flue terminal externally — any visible obstruction, damage, or incorrect termination position
3. Measure fan speed using the boiler's diagnostic menu (accessible via the control panel) — compare to manufacturer spec for the model
4. Inspect the air pressure switch and its tubing — check for splits, kinks, or moisture in the silicone tubing. Blow through gently to confirm it is clear
5. Check wiring connections at the fan plug and at the PCB
6. If the fan is confirmed faulty, replace the complete fan assembly — do not attempt to repair individual bearings

Parts commonly needed: Fan assembly, air pressure switch, flue extension sections (if flue run is marginal and causing back-pressure).

What does C7 mean on a Worcester boiler?

C7 (sub-code 214) indicates complete fan failure. The PCB is detecting no rotation from the fan motor at all.

Likely causes:

1. Fan motor seized — Bearings have completely failed. The fan will not spin even by hand (with boiler isolated from mains).
2. Wiring disconnected — The fan plug has become dislodged during previous servicing or due to vibration.
3. PCB fault — The fan drive circuit on the PCB has failed and is not sending power to the fan motor.

Diagnosis steps:

1. Isolate the boiler from the mains supply
2. Access the fan and attempt to spin the impeller by hand — if it is seized or extremely stiff, the fan needs replacing
3. Check the wiring plug is firmly seated at both the fan and the PCB
4. Restore power and check if the fan receives voltage when a heat demand is present — if voltage is present but the fan does not spin, the fan motor is confirmed faulty
5. If no voltage is reaching the fan, the PCB fan drive circuit has likely failed

Parts commonly needed: Fan assembly, PCB (if fan drive circuit failure confirmed).

What does D1 mean on a Worcester boiler?

D1 indicates an NTC temperature sensor fault. The boiler's PCB has detected an open-circuit, short-circuit, or out-of-range reading from one of the NTC thermistors.

Sub-codes:

• D1 218 — Flow NTC thermistor fault
• D1 219 — Return NTC thermistor fault

Likely causes:

1. Faulty NTC thermistor — The sensor has drifted out of calibration, or the internal resistance element has failed (open-circuit or short-circuit).
2. Wiring damage — Corroded terminals, chafed wires, or loose plug connections between the NTC and the PCB.
3. PCB fault — The analogue input circuit on the PCB is not reading the NTC correctly (less common).

Diagnosis steps:

1. Access the NTC sensor(s) — on most Greenstar models, the flow NTC is clipped to the primary flow pipe near the heat exchanger, and the return NTC is on the return pipe
2. Disconnect the NTC from the wiring harness and measure resistance with a multimeter — at room temperature (approximately 20 degrees C), a healthy NTC should read approximately 12 kOhms (check exact spec for the model). An open-circuit reading (OL) or very low reading (near 0 ohms) confirms a faulty sensor
3. Check the wiring harness and plug connections for corrosion or damage
4. If NTC resistance is within spec but the fault persists, the PCB input circuit may be at fault — swap the flow and return NTC connectors at the PCB to confirm

Parts commonly needed: NTC thermistor (flow or return), wiring harness.

What does E5 mean on a Worcester boiler?

E5 indicates the primary flow temperature sensor has detected overheating. The flow temperature has exceeded the safe operating limit, but the safety temperature limiter (STL) has not yet tripped.

Likely causes:

1. Restricted circulation — Pump running but flow rate too low due to sludge, debris, or partially closed valves.
2. Heat exchanger blockage — Limescale or magnetite build-up inside the primary heat exchanger, creating hot spots.
3. Pump speed too low — Pump set to incorrect speed setting for the system, or pump losing performance due to wear.
4. System too small — Insufficient radiation for the boiler output (e.g. after a boiler upgrade without checking radiator sizing).

Diagnosis steps:

1. Check system pressure and repressurise if low
2. Ensure all radiator valves (including TRVs and lockshields) are open — a common cause is a system where most radiators have been turned off, reducing the heat rejection capacity
3. Check pump operation — is it running at the correct speed? Is there adequate flow? Measure the delta-T across the boiler (flow minus return) — it should be 11-20 degrees C depending on design conditions. A very high delta-T (over 25 degrees C) indicates poor circulation
4. Inspect the heat exchanger for blockage — on a dirty system, consider a powerflush with appropriate chemical treatment
5. Check the flow NTC reading against an independent thermometer — the NTC may be over-reading if faulty (though this would more typically present as D1)

Parts commonly needed: Pump, heat exchanger (if severely blocked), magnetic system filter (MagnaClean or similar, if not already fitted).

What does E9 mean on a Worcester boiler?

E9 is a critical safety lockout. The safety temperature limiter (STL) in the central heating flow has tripped, meaning the heat exchanger has reached a dangerously high temperature. This is a hard lockout — the boiler will not fire until the cause is resolved and the boiler is manually reset.

Likely causes (ranked by probability):

1. Pump failure — Pump not running or not circulating despite running (seized impeller, airlock in pump body). This is the number one cause of E9.
2. Blocked heat exchanger — Heavy limescale or sludge build-up prevents adequate heat transfer, causing localised overheating.
3. System blockage — Closed isolation valves, blocked strainer/filter, or severely sludged pipework reducing flow to near zero.
4. Airlock — Air trapped in the heat exchanger or pump, preventing water circulation. Common after system drain-down or component replacement.
5. Faulty STL — The safety temperature limiter itself may have failed, reporting an overheat condition when the boiler is actually within normal range. Measure the actual flow temperature independently to confirm.
6. Expansion vessel failure — A waterlogged expansion vessel causes rapid pressure spikes and can contribute to overheating conditions.

Diagnosis steps:

1. Do not simply reset and walk away — E9 indicates a genuine safety condition in most cases. Diagnose the root cause.
2. Check system pressure — repressurise if below 1 bar
3. Check the pump — is it receiving power? Is it running? Is the impeller turning? Tap the pump body gently (a seized pump sometimes frees temporarily). Measure the temperature differential across the pump with pipe thermometers
4. Check for airlocks — bleed the pump via the bleed screw (have a towel ready). Bleed nearby radiators
5. Check the heat exchanger — measure flow rate if possible. On a badly sludged system, the delta-T across the boiler will be very high (over 30 degrees C) or the return will be barely warm
6. Inspect the STL — if all circulation checks are satisfactory and the flow temperature is genuinely within range, the STL may have failed. Measure its resistance and compare to spec
7. Check the expansion vessel pre-charge pressure (typically 0.75-1.0 bar cold, system drained) — a flat vessel causes pressure fluctuations that can contribute to overheating

Parts commonly needed: Pump, heat exchanger, safety temperature limiter, expansion vessel.

What does H07 mean on a Worcester boiler?

H07 indicates low water pressure on display-equipped models (Greenstar 8000 Life/Style and similar). This is functionally equivalent to the A1 code on older models.

The boiler has detected system pressure below approximately 0.5 bar and has blocked operation to prevent dry-firing the heat exchanger.

Fix: Repressurise the system to 1.0-1.5 bar via the filling loop. If pressure repeatedly drops, investigate for leaks or a failed expansion vessel. See the A1 section above for full diagnosis steps.

What does 224 mean on a Worcester boiler?

Code 224 can appear in two contexts:

1. As a sub-code of E9 (E9 224) — Safety temperature limiter tripped due to overheating. See E9 section above.
2. As a standalone blocking code — Flue gas thermostat overheat. The flue gas temperature has exceeded the safe limit, which can indicate a heat exchanger issue, incorrect gas rate, or flue restriction.

Diagnosis: Check gas rate against the data badge. Inspect the flue for restrictions. Check the heat exchanger for sooting (indicates incomplete combustion). Perform a combustion analysis — CO readings above 200 ppm in the flue gas warrant immediate investigation.

What does 228 mean on a Worcester boiler?

Code 228 (EA 228) indicates parasitic ionisation — the boiler is detecting ionisation current (flame signal) before the burner has started. This is a safety lockout because the boiler believes gas is burning when it should not be.

Likely causes:

1. Faulty ionisation electrode — Leaking current to earth due to cracked ceramic insulator or carbon tracking.
2. Moisture in the combustion chamber — From a blocked condensate trap or a leaking heat exchanger.
3. PCB fault — The flame detection circuit is reading a false signal.

Diagnosis: Disconnect the ionisation electrode lead and reset — if the fault changes to EA 227 (no flame detected), the electrode or its wiring is the issue. If 228 persists with the electrode disconnected, suspect the PCB.

What does 229 mean on a Worcester boiler?

Code 229 (EA 229) indicates flame loss during operation. The boiler successfully ignited but the flame was extinguished while running.

Likely causes:

1. Blocked condensate trap/siphon — Moisture backs up into the combustion chamber and extinguishes the flame. Very common.
2. Wind at the flue terminal — Strong or gusty wind causing downdraught. Check flue terminal position relative to prevailing wind direction, nearby walls, and roof features. Consider a plume management kit or repositioning the terminal.
3. Intermittent gas supply — Fluctuating gas pressure, particularly if a gas meter regulator is failing.
4. Dirty or marginal ionisation signal — The flame is present but the ionisation current is too low for the PCB to maintain the "flame proven" signal. Clean or replace the electrode.

Diagnosis: Check the condensate trap first — remove, clean, and refit. Observe the boiler running with the case off — watch for flame instability. Measure ionisation current via the diagnostic menu (typically should be above 3 microamps; below 1 microamp is unreliable).

Frequently Asked Questions

How do I reset my Worcester Bosch boiler after a fault code?

On most Greenstar models, press and hold the reset button (marked with a flame/reset symbol) for 3-5 seconds. On the Greenstar 8000 range, press the reset button on the front panel. Only reset once. If the same fault code returns after a reset, there is an underlying issue that needs professional diagnosis. Repeatedly resetting a boiler that is locking out on E9 (overheating) risks damage to the heat exchanger.

What is the difference between a locking fault and a blocking fault?

A locking fault shuts the boiler down completely — it will not attempt to restart until you manually press the reset button. These are indicated by a flashing code with a key or triangle symbol. Examples: EA 227, E9, C7. A blocking fault causes the boiler to pause temporarily but it will attempt to restart automatically after a set period (typically 10-15 minutes). These are generally less severe — for example, a momentary fan speed deviation or a brief pressure dip.

Can I fix Worcester Bosch fault codes myself?

Homeowners can safely: repressurise the system via the filling loop (A1/H07), thaw a frozen condensate pipe with warm water (EA 229 in winter), and perform a single reset. Any fault involving gas supply, ignition, combustion, overheating, or internal components must be diagnosed and repaired by a Gas Safe registered engineer. It is illegal for an unregistered person to carry out gas work under the Gas Safety (Installation and Use) Regulations 1998.

How do I access the full numeric sub-code?

On most Greenstar models, when a fault code is displayed, press the info button (spanner/wrench icon) or the return button to access the detailed diagnostic code. On the Greenstar 8000, navigate to the service menu via the control dial. The numeric sub-code (e.g. 227, 215, 224) provides significantly more diagnostic detail than the letter code alone.

Worcester Bosch boiler keeps losing pressure — is that a fault code issue?

Repeated pressure loss will eventually trigger A1 or H07, but the root cause is usually a system leak (radiator valves, pipe joints, heat exchanger), a failed expansion vessel (pre-charge lost, diaphragm ruptured), or a faulty pressure relief valve (PRV) dripping into the condensate pipe. Check the external discharge pipe (tundish and copper pipe, usually visible outside) — if water is dripping from it, the PRV is lifting, which often indicates a waterlogged expansion vessel.

Regulations & Standards

• Gas Safety (Installation and Use) Regulations 1998 — All work on gas appliances must be carried out by a Gas Safe registered engineer. It is a criminal offence for an unqualified person to carry out gas work.

• BS 7593:2019 — Code of practice for the preparation, commissioning, and maintenance of domestic central heating and cooling water systems. Relevant to system cleanliness (powerflushing, inhibitor treatment) which directly impacts fault codes E5, E9, and A1.

• BS 5440-1:2008 — Flueing and ventilation for gas appliances. Relevant to fault codes C6, C7, EA 229 where flue performance is implicated.

• Building Regulations Part J (England & Wales) / Part F (Scotland) — Combustion appliance installation standards including flue terminal positions and air supply requirements.

• Benchmark commissioning checklist — Worcester Bosch boilers must be commissioned per the Benchmark scheme to validate the manufacturer warranty. Incomplete commissioning can void the warranty, which is relevant when diagnosing faults on newer installations.

• Worcester Bosch official troubleshooting: https://www.worcester-bosch.co.uk/support/troubleshooting

• Worcester Bosch installer technical helpline: 0330 123 3366

• Boxt fault code reference: https://www.boxt.co.uk/boilers/guides/worcester-bosch-fault-and-error-codes

• Heatable complete code list: https://heatable.co.uk/boiler-advice/worcester-bosch-error-codes

• Boiler Central fault code guide: https://boilercentral.com/guides/worcester-boiler-error-codes/

• iHeat C6 guide: https://iheat.co.uk/boiler-help/worcester-bosch-c6-boiler-error-code-guide

• Edinburgh Boiler Company fault codes: https://theedinburghboilercompany.com/boiler-advice/worcester-bosch-boiler-error-fault-codes/

• 0800 Homefix EA 227 guide: https://www.0800homefix.com/heating/worcester-ea-227-fault-no-ignition-error-explained/

• 0800 Homefix E9 guide: https://www.0800homefix.com/heating/worcester-e9-overheat-lockout-causes-and-solutions-for-boiler-error-code/

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