Worcester Bosch boiler error codes explained: EA, E9, D5, C6 and what to do
- Checked September 2026
- 5 min read
- By PROS AI editorial team
Quick answer
The short version, before the detail.
A Worcester Bosch fault code tells you which safety check stopped the boiler, not what to replace. EA means no flame was detected, E9 that the overheat limiter tripped, D5 that the condensate pipe has frozen, and C6 that the fan is running too slowly. A homeowner may check the gas supply, thaw the condensate pipe, top up the pressure and press reset once. Everything else on a gas boiler is work for a Gas Safe registered engineer.
What is the likely cause?
One code per item, most common first, in Worcester Bosch's own wording. If your display shows a code that is not here, look it up in the user instructions rather than guessing.
EA (no flame): Worcester Bosch defines EA as "Flame not detected".
The usual causes are no gas or low gas pressure, a frozen or blocked condensate pipe, or a blocked flue. In a freeze the boiler often gurgles before EA appears. The one safe thing: check that another gas appliance lights, top up a prepayment gas meter, thaw the condensate pipe with warm water if it is iced, then reset once. Ignition electrodes, the gas valve and the flue are engineer work.
E9 (overheat limiter tripped): the code means the "safety temp.
limiter in CH flow has tripped", so the water leaving the boiler got too hot. That usually happens because water is not moving: low system pressure, air in the system, or a pump that is blocked or seized. Worcester's own fault-finding for E9 starts with the heating pressure, then top up, vent and reset. The one safe thing: read the gauge, top up to 1 to 1.5 bar, reset once. If E9 comes back, the pump or the limiter is the fault.
D5 (frozen condensate): Worcester Bosch's support page says a frozen condensate pipe can show as "EA fault code, D5 fault code or a blue slow flashing light".
The one safe thing: thaw the outside pipe with warm, not boiling, water, but only where it is at ground level, then reset once. Our frozen condensate pipe guide has the full method.
C6 (fan speed too low): the fan is not reaching the speed the boiler expects, so it refuses to light.
The usual cause is the fan itself or its wiring. The one safe thing: reset once; there is nothing else a homeowner can check.
A1 (pump has run dry): on Greenstar CDi boilers, A1 means the "controlled characteristic pump has run dry", and the first step in Worcester's fault-finding is whether the system pressure is below 1.2 bar.
The one safe thing: top up to 1 to 1.5 bar and reset once. If it drops again within days there is a leak, covered in boiler losing pressure.
Numbered codes on Greenstar 4000 and 8000 boilers: newer models show a number with a short text.
227 is "No flame signal after ignition", with the same causes as EA, including a frozen condensate pipe and a blocked flue. 224 is the overheat equivalent of E9. 1017 and 2971 are "System pressure too low", and a Greenstar 4000 shows a low-pressure message when the gauge sits between 0 and 0.79 bar.
What can I safely check myself?
Everything here is done from the front of the boiler, the pressure gauge and the outside wall.
- Write the code down, and whether the boiler is silent or trying to fire. The engineer will ask for the code, the model and the serial number.
- Reset once. On older Greenstar boilers press the reset button for about three seconds; on a Greenstar 4000 or 8000 press the arrow keys until Reset shows. Wait two to three minutes. If the same code returns, stop there.
- Gas. Check a hob or another gas appliance lights, top up a prepayment gas meter, and make sure nobody has turned off the gas cock at the meter.
- Pressure. With the heating cold the gauge should read 1 to 1.5 bar. Below 1 bar, open the filling loop until it does, then close both valves fully. Worcester Bosch says topping up once or twice a year is normal; more often is a leak.
- Condensate pipe. In a freeze, or if the boiler gurgles before it stops, find the white plastic pipe where it leaves the wall. Pour warm water along it from a jug, from the ground only, and clear leaves off the drain it feeds. Never use boiling water, and expect the spilt water to freeze on the path.
- Power and controls. Check the fused spur beside the boiler is on, the timer is set and the thermostat is above room temperature. After a summer off, controls stop more boilers than faults do, see boiler not firing up after summer.
Do not remove the casing, do not touch the gas valve, the fan or the pump, and do not keep pressing reset.
When do I stop and call an engineer?
- If you smell gas, leave the property, do not switch anything on or off, and call the National Gas emergency line on 0800 111 999 from outside.
- If water is dripping near the fused spur, sockets or the consumer unit, switch off at the main switch only if it is dry and safe to reach.
- Stop if the code returns after one reset, if E9 or 224 appears at all, if C6 appears, if the pressure falls again within days, or if the boiler is noisy, banging or leaking. Worcester Bosch's own instruction is plain: do not try to rectify the listed faults yourself; contact a Gas Safe registered engineer.
- Anything behind the casing is gas work. Check the seven-digit number on the engineer's ID card at gassaferegister.co.uk, or text Gas and the number to 85080.
What does it cost in London?
A weekday call-out with diagnosis in London is £60 to £125, often waived if the repair goes ahead, and labour is £80 to £140 an hour. The parts behind these codes run from £120 to £280 for a pressure fault, £180 to £420 for a pump, £200 to £420 for a fan and £290 to £650 for a circuit board; most repairs land between £150 and £400 with the part. Out of hours the first hour is £140 to £260, so a code that is not urgent is cheaper the next morning.