Worcester Bosch 24CDi, 28CDi, 35CDi II, 26CDi Xtra, 25Si/28Si, 24i/28i Junior, Si II, CBi and Highflow 400 fault codes
Worcester Heat Systems-era boilers with indicator lights and no display. A slow flash is a lockout, a fast flash a fault, and which light carries it says what has happened. Decoded on the red light page.
Codes on the 24CDi, 28CDi, 35CDi II, 26CDi Xtra, 25Si/28Si, 24i/28i Junior, Si II, CBi and Highflow 400 follow the same Worcester Bosch scheme as the rest of the range. Where a code behaves differently on this generation, the code page says so.
Worcester Bosch fault codes
Every published Worcester Bosch code. Open a code to see which models and years it applies to.
- EA 227The boiler tried to light and never detected a flame, so it has shut the gas off and locked out. This is the most common Worcester fault code.Stop — call an engineer
- CE 207System water pressure has dropped too low for the boiler to run safely. Topping the system back up to 1.2 bar usually clears it in five minutes.Get it checked
- E9 219The boiler has overheated and the safety thermostat has tripped. Water is not carrying heat away from the exchanger, and it should not be run until that is fixed.Stop — call an engineer
- C6 224The fan is not reaching the speed the boiler asked for, so combustion cannot be confirmed as safe and the burner has been prevented from firing.Get it checked
- EA 229The burner lit and then the flame was lost during operation. That is a different fault from failing to light in the first place.Get it checked
- H07A maintenance warning, not a lockout. The boiler is still running but water pressure has fallen far enough to limit how well it heats.Get it checked
- E9 224One of the two safety thermostats has tripped: the flue overheat thermostat or the high-limit stat on the heat exchanger. Worcester's own check list includes the burner gasket for flue gas leakage.Stop — call an engineer
- A1 229The boiler has detected a dry-fire condition — it is trying to heat water that is not moving. Turn it off and leave it off until an engineer has seen it.Stop — call an engineer
- Blue light flashingOn a Greenstar with no display, the blue light is the fault code. Steady means healthy; a flashing pattern, read with the reset button, is a diagnosis Worcester publish in the installation instructions.Get it checked
- F0The boiler has found a fault inside its own electronics and stopped. F0 covers the control board and the burner control unit, and it needs an engineer.Get it checked
- E5 218The flow temperature sensor has measured water leaving the heat exchanger far hotter than it should be. The boiler shut down to protect itself, and heat is not being carried away fast enough.Get it checked
- A7The hot water temperature sensor has failed. Heating usually carries on as normal while the hot water runs cold, scalding, or swings between the two.Get it checked
- C7The fan is not running at all, as distinct from running at the wrong speed. The burner is locked out until it is fixed.Get it checked
- F7 228A flame is being detected before the burner has started. Switch the appliance off and get a Gas Safe engineer rather than resetting.Stop — call an engineer
- FA 306The boiler detected a flame after it had closed the gas valve. That points to a valve letting gas past when it should be shut, and it must not be reset.Stop — call an engineer
- Red light flashingOn a Worcester built before the Greenstar era, the flashing lights are the fault code. Slow means a lockout you can reset once; fast means a fault the boiler is waiting out or an engineer's.Get it checked
- D5The condensate pump has failed or cannot discharge. Acidic water will back up inside the appliance, so it needs attention promptly.Get it checked
- 2951The flame has been lost too many times in a row and the boiler has locked out. Each loss was a 229; Worcester's instruction is to read the history and find what put it out.Stop — call an engineer
- E2 222The flow temperature sensor is giving a reading the appliance cannot trust. Usually the sensor rather than the temperature.Get it checked
- C1 264Airflow stopped part way through a firing cycle. The fan has quit or the flue is obstructed, and combustion gases may not be leaving safely.Get it checked
- A8The boiler has lost communication with a Worcester control on the two-wire bus — a Sense, FX or FW controller, or a Wave thermostat. Heating usually carries on at the boiler's own settings.Get it checked
- D1 240The return temperature sensor has short circuited. The boiler has lost its reading of how warm the water is when it comes back from the radiators.Get it checked
- 9U 233The heat control module is loose, missing or faulty. This small plug-in part carries the identity of your boiler, and nothing runs without it.Get it checked
- 2910Fault in flue system. The boiler has detected a problem with its flue and locked out. Worcester's checks are obstructions, the flue integrity test and water in the sump. Not a code to reset repeatedly.Stop — call an engineer
- 9F 855The oil burner could not establish or keep a flame and its control box has locked out. Worcester allow a reset and an oil-level check; four lockouts in one demand become 9F 856.Get it checked
- Fd 231Power to the boiler was cut while it sat in a lockout, so a fault happened before the interruption. On older Greenstars, Fd simply means the reset button was pressed by mistake.Get it checked
- 1065The system pressure sensor is faulty or not connected. The boiler cannot read its own water pressure, so it stops rather than guess. Worcester check the plug, the harness, the voltage, then the sensor.Get it checked
- EA 261An ignition fault that Worcester define two ways: on the Greenstar i it points at the heat control module, on the Compact range it means the flame was not established inside the four-second ignition window.Stop — call an engineer
- D4 341The water temperature in the boiler is climbing faster than it should, which means very little of it is moving. Worcester's checks are pressure, air, circulation and the pump, in that order.Get it checked
- 2980Super lock-out. Five locking faults were reset inside fifteen minutes while the boiler saw a potentially safety-critical condition, so it has locked itself against further resets. Only an engineer can clear it.Stop — call an engineer
- b1The control board cannot find its code plug, the small module that tells it which boiler it is fitted to. Without it the board will not run. Usually follows a board or plug being disturbed.Get it checked
- H03Planned maintenance required. The Greenstar i is saying a service interval has passed — H03 counts burner hours, H08 a set date, H13 a number of months. A reminder, not a fault.Usually simple to fix
- 9F 856Four 9F 855 burner lock-outs have been reset in one demand, so the controls have locked the reset out to protect the burner. A power-down clears the count; the cause is still there.Get it checked
- C4 273The fan has run continuously for 24 hours, so the boiler has paused firing for two minutes as a precaution. Something has been calling for heat non-stop, and that is what to find.Usually simple to fix
- SESE alternating with the temperature on a CDi Classic is a service reminder, not a fault. The burner has run the hours Worcester equate to a year, and the boiler wants its annual check.Usually simple to fix
- 2970The heating system's pressure has dropped too quickly too often. On an 8000 with the intelligent filling loop this usually follows venting after an automatic fill; otherwise it is a leak that needs finding.Get it checked
- E9 210One of three manual-reset safety devices has opened: the flue gas overheat thermostat, the air pressure switch or the tank overheat thermostat. Overheated flue gases or a restricted flue are the usual reasons.Stop — call an engineer
- EA 234The control board cannot see the gas valve coil, so it will not attempt to light. Worcester call it a gas valve error, coil disconnected. No gas is released, but it is a gas job.Get it checked
- E9 220The safety temperature sensor on the heat exchanger has failed electrically — 220 is a short circuit, 221 an open circuit. Nothing is overheating; the boiler has stopped because it can no longer prove that.Get it checked
- 2948The burner lit but the flame signal was not there at low output. The boiler purges and tries again itself; Worcester's checks are the CO2 setting, the flue and the terminal, and they mention wind.Get it checked
- D1 286The return water coming back into the boiler is too hot, and the boiler has locked out on overheat at the return sensor rather than the flow. Worcester describe it simply as boiler overheat.Get it checked
- Fb 365The gas valve failed its own leak test. Worcester publish Fb 365 for the first valve seat and FA 364 for the second; both mean gas may pass a valve that should be shut.Stop — call an engineer
- H09The control board has lost its data link to the pump. The boiler keeps running, but Worcester flag it because a pump the board cannot talk to is either wired wrongly or the wrong pump.Get it checked
- A0 809The sensor reading the Heatslave II's internal hot water store has failed or lost its connection, so the boiler cannot tell how hot the store is and locks out until it is checked or replaced.Get it checked
- A6The boiler thinks an external limit thermostat has opened — usually the underfloor heating stat — or the link that stands in for one is missing. Nothing runs until that circuit is closed again.Get it checked
- EF 349The boiler is on its lowest flame and the water is still leaving far hotter than it returns — more than 18 °C hotter. Worcester call it boil detect, and it means circulation has collapsed.Get it checked
- CCThe outdoor temperature sensor for weather compensation is not being read — open circuit, short circuit or disconnected. The boiler keeps heating but loses its weather-adjusted control.Usually simple to fix
- F1An internal data error in the control board — Worcester call it a ROM error on the Highflow. The board's own memory check has failed and the remedy is the board itself.Get it checked
- E2 233The heat control module, the code plug that tells the board which Heatslave II it is running, is missing, loose or faulty. 9A 360 is its sibling: a plug fitted for the wrong boiler.Get it checked
- b7 257An internal error in the burner control. Worcester's remedy is the control board, after checking the heat control module is seated. There is nothing to try beyond one reset.Get it checked
- 2945Too many short heat demands in a short time. The Greenstar 2000 has locked out because it kept being asked to fire and stop — usually a system with nowhere for the heat to go.Get it checked
- 2911Calibration failed. The Greenstar 2000 sets its own combustion by reading the flame, and the reading did not add up. Worcester check the electrode, its earth, the siphon, then the gas valve or board.Get it checked
- A5The cylinder or heat bank temperature sensor has failed — open or short circuit. Heating carries on; the boiler just no longer knows how hot the stored water is, so hot water control suffers.Usually simple to fix
- H27A warning triangle with H27 means the sensor on the incoming cold mains has failed. Worcester say operation is not affected, so heating and hot water carry on until the sensor is replaced.Usually simple to fix