Worcester Bosch fault codes, explained
Worcester Bosch boilers display a fault code when something stops them running safely. Most codes fall into a handful of categories: ignition failure, low pressure, overheating, a sensor reading out of range, or a component that has stopped working. Knowing which category your code belongs to tells you whether you can clear it yourself or whether you need a Gas Safe engineer on the doorstep.
This guide covers the fault codes we see most often on Greenstar, CDi, Si, and i range boilers. The table below gives a plain-English translation. The safety notes below that tell you what you can and cannot do yourself.
Quick-reference table
| Code | What it means | What to do |
|---|---|---|
| EA / 227 | Ignition lockout. The boiler tried to light but could not detect a flame after three attempts. Causes include a gas supply issue, a faulty ignition electrode, a dirty flame sensor, or a frozen condensate pipe in cold weather. | Check that other gas appliances (hob, fire) are working. If the condensate pipe is frozen, thaw it with warm water. Try one reset. If the code returns, call a Gas Safe engineer. |
| E9 | Overheat protection has tripped. The boiler detected water temperature climbing too high and shut down for safety. Common causes: low system pressure, air trapped in the system, a sluggish pump, or a blocked heat exchanger. | Check the pressure gauge. If it is below 1 bar, repressurise via the filling loop. Try one reset. If the code comes back, do not keep resetting. Call an engineer. |
| A1 | Pump fault or low water pressure. The pump may be stuck or running dry, or pressure has dropped below the minimum threshold. | Check the pressure gauge (should be 1 to 1.5 bar when cold). If low, repressurise. Bleed radiators if you suspect air in the system. If pressure is fine and the code persists, the pump likely needs inspection by an engineer. |
| C7 | Fan not running on start-up. The boiler checks the fan before it fires; if it does not see the fan reach the correct speed, it shuts down. Possible causes: fan failure, wiring fault, air pressure switch fault, or a blocked flue. | This is an engineer-only repair. Do not attempt to reset repeatedly. Book a Gas Safe engineer. |
| C6 | Fan speed too low or outside the expected range during operation. Similar causes to C7. | Engineer-only. |
| D1 | Flow temperature sensor fault. The sensor is reading outside its expected range, which may mean a faulty sensor, wiring damage, or a circulation problem causing erratic temperature swings. | One reset is worth trying. If the code reappears, an engineer is needed to test or replace the sensor. |
| D4 | Primary flow temperature rising faster than expected. Often points to restricted water circulation: air locks, a partially closed valve, or sludge build-up in the heat exchanger. | Do not keep resetting. An engineer should investigate the cause, which may include a powerflush if there is significant sludge. |
| F0 | Internal electronic fault. Usually the PCB (control board) or gas valve electronics. | Engineer-only. Do not reset repeatedly. |
| F7 | Flame detected when the burner should be off. The boiler has seen a false flame signal after shutdown, which may indicate a faulty flame sensor, sticking gas valve, or moisture in the electronics. | Do NOT reset this one. Turn the boiler off and call a Gas Safe engineer promptly. |
| FA | Flame still detected after the gas valve has closed. Similar safety concern to F7. | Do NOT reset. Call an engineer. |
| 9U / 9A | Heat control module or code plug error. The boiler cannot read its configuration correctly. Usually a code plug issue or PCB fault. | Engineer-only. |
| A7 | Hot water NTC thermistor (temperature sensor) fault. Affects hot water output. | One reset is reasonable. If it returns, an engineer will need to test or replace the sensor. |
| FD | Power interruption recorded. Often a one-off after a power cut. | Reset the boiler. If it runs normally and the code does not return, no further action is needed. |
Before you reset: a safety note
Resetting a boiler once after a fault is reasonable. The boiler will try to restart and, if the fault was a one-off glitch, it should clear. What you should not do is reset repeatedly in the hope that the problem goes away. Modern boilers lock out for a reason: they have detected something that could cause a bigger failure or a safety risk if the boiler keeps firing.
If a fault code returns within an hour of resetting, stop resetting and book an engineer.
If you can smell gas at any point, do not touch the boiler at all. Leave the house, leave the door open behind you, and call the National Gas Emergency number: 0800 111 999. This line is free and open 24 hours a day.
Carbon monoxide has no smell. If you have a CO alarm sounding alongside a boiler fault, treat it as an emergency. Open windows and get outside. Call 0800 111 999.
What homeowners can safely do
A few fault codes give you a straightforward action to try before calling anyone:
- Recheck pressure. If the gauge reads below 1 bar, use the filling loop to bring it back to 1 to 1.5 bar. Your boiler’s user manual shows where the filling loop is and how to use it. Top up slowly and stop at 1.5 bar.
- Bleed a radiator. Air in the system can cause circulation problems that trigger E9 and A1 faults. Bleeding the highest radiator in the house releases trapped air.
- Thaw a frozen condensate pipe. In cold weather, EA / 227 codes are often caused by a frozen condensate pipe rather than a gas or ignition fault. The pipe is usually a white plastic pipe running outside at low level. Pour warm (not boiling) water along it, then reset the boiler.
- Check gas supply. Try another gas appliance. If the hob will not light either, the fault is upstream of the boiler. Call your gas supplier.
- One reset. For most codes, one reset is safe. Hold the reset button for the specified time (typically 3 seconds on Greenstar models) and give the boiler two minutes to attempt a restart.
Everything beyond these steps involves the boiler internals or the gas supply. Those are jobs for a Gas Safe registered engineer.
Time to call an engineer
If your fault code is not clearing, or if it is F7, FA, F0 or any code that keeps returning, BASI can usually get to you the same day across West Yorkshire. We have been repairing boilers for over 30 years and we are Worcester Bosch and Vaillant accredited, so we carry the right parts for Greenstar, CDi, and Si range boilers.
Boiler repair starts from £80 for the first hour, with fixed pricing agreed before work starts. No call-out fee surprises, no upselling to a replacement before we have properly looked at the fault.
Book a same-day repair or get a call back:
The best way to avoid fault codes altogether is an annual service. A Gas Safe engineer cleans the burner and heat exchanger, checks the flue, tests the gas pressure, and spots worn parts before they cause a breakdown. We offer annual boiler servicing from a fixed price with a written report:
If your fault code is not listed above, it is worth checking your boiler’s user manual (Worcester Bosch publish them all on their website) or giving us a call. We are Gas Safe registered (number 623525) and happy to advise over the phone if the fix turns out to be straightforward.