Worcester Bosch fault codes, explained properly
Greenstar boilers show a letter and number when something stops them running. Here is what each one actually means, what you can safely check yourself, and when it needs a Gas Safe engineer.
What our customers say
Reviews from homeowners across the North East.
Reading a Worcester Bosch fault code
Greenstar boilers display a short code, usually a letter and a number, when a safety device or a sensor has stopped normal operation. On most models the code sits on the front display, and on some it alternates with the pressure reading.
The code tells you where the boiler stopped. It does not tell you why. EA means the appliance did not see a flame, which could be a frozen pipe outside, a worn electrode, or no gas at all. That is the difference between a job you can sort in an hour and one that needs parts, and it is why we diagnose before quoting rather than the other way round.
The reset rule, and why it matters
Almost every fault code can be cleared with the reset button, and almost everyone presses it more than once. That is the single most expensive habit in domestic heating.
A lockout is the boiler protecting itself. Resetting it tells the appliance to try again. Once is reasonable, because genuine one off glitches do happen. Repeatedly resetting a boiler that is locking out on ignition means it is pumping gas into the combustion chamber and failing to light it, over and over. Repeatedly resetting an overheat lockout means asking it to fire again into a system that cannot carry the heat away. Both do real damage, and both turn a known price repair into a bigger one.
Reset once. If it locks out again, stop and get it looked at.
Worcester Bosch codes in detail
EA, ignition failure
The boiler asked for gas, tried to light it, and did not detect a flame. It is the code we are called to more than any other Worcester Bosch fault.
Why it happens: In winter, a frozen condensate pipe is the most common single cause by a distance. Beyond that: no gas reaching the appliance, a worn ignition electrode, a gas valve not opening correctly, or a fault on the control board.
What you can do: Check other gas appliances in the house work, which rules the meter in or out. If the condensate pipe runs outside, pour warm water over it, never boiling. Reset once.
What an engineer does: Tests standing and working gas pressure at the appliance, checks the electrode gap and condition, verifies the gas valve operation and reads the fault history off the board rather than guessing at the part.
E9, safety limiter
The boiler has got hotter than it should and the overheat limiter has shut it down. This is a protection device doing its job.
Why it happens: Heat is being made faster than the system can carry it away. That is nearly always poor circulation: a pump that has failed or stuck, sludge restricting flow, an air lock, or a thermistor reading wrongly.
What you can do: Very little safely. Check the pressure gauge and whether radiators are heating at all. Do not sit resetting it.
What an engineer does: Measures the flow and return temperature difference, which tells you immediately whether water is moving. Checks the pump, the system water condition and the thermistors. If the system is full of magnetite, a clean is the fix rather than a part.
D5, low water pressure
System pressure has fallen below the level the boiler will run at.
Why it happens: Either water has left the system or the vessel that absorbs expansion has failed. Recently bled radiators, a leak somewhere on the pipework, a pressure relief valve passing water out through the discharge pipe, or a flat expansion vessel.
What you can do: Repressurise to between 1.0 and 1.5 bar with the filling loop and watch it. Topping up once is normal. Topping up every few weeks is not, and it means something needs finding.
What an engineer does: Checks the discharge pipe outside for signs of water, tests the expansion vessel charge, and pressure tests the system to separate a leak from a vessel failure. Those are three different repairs at three different prices.
C6, fan fault
The fan is not turning at the speed the board expects, or not turning at all.
Why it happens: A worn fan motor, a wiring or connector fault, debris in the fan, or a board not reading the speed signal correctly. On exposed and coastal properties a partly obstructed or corroded flue terminal makes the fan work harder and shortens its life.
What you can do: Nothing safely. The fan sits inside a sealed combustion chamber.
What an engineer does: Checks the fan current draw and speed signal, inspects the flue terminal and route for obstruction, and confirms whether the fan or the board is at fault before replacing anything.
A1, flue or air pressure fault
The boiler has detected a problem with how combustion gases are leaving, or with the air coming in.
Why it happens: A blocked or restricted flue, failed flue seals, a sensor fault, or persistent wind disturbing the terminal. Along the coast at Roker, Seaburn, Fulwell and South Shields the salt air corrodes terminals noticeably faster than inland.
What you can do: Look at the outside terminal for anything obvious across it, a nest, plastic sheeting or heavy debris. Do not dismantle anything.
What an engineer does: Inspects the full flue run including any concealed sections, checks the seals, and carries out a flue gas analysis to confirm combustion is correct once the cause is cleared.
CE, low pressure
A pressure fault in the same family as D5, shown on some models in place of it.
Why it happens: Identical causes: a leak, recently bled radiators, a failed expansion vessel or a passing pressure relief valve.
What you can do: Repressurise to 1.0 to 1.5 bar and keep an eye on the gauge for a fortnight. Note how quickly it falls, because that is genuinely useful information for whoever attends.
What an engineer does: Same diagnostic path as D5. The rate of pressure loss narrows it down quickly.
E1, flow thermistor fault
The sensor measuring the temperature of water leaving the boiler is reading something impossible.
Why it happens: A failed NTC thermistor, a corroded or loose connector, or a board fault. Thermistors are cheap parts that fail often, which is good news.
What you can do: Nothing. It needs testing with a meter.
What an engineer does: Measures the thermistor resistance against the manufacturer curve at a known temperature, which either condemns the sensor or points at the wiring or board instead.
What these repairs cost
We publish our prices rather than quoting them on your doorstep. There is one fixed diagnosis fee that covers attending and finding the fault, then a single fixed price for the repair itself, given to you before anything is ordered or fitted.
For context, a thermistor is one of the cheapest parts on the list, a fan sits in the middle, and a control board is at the top end. A frozen condensate pipe usually costs nothing beyond the visit. The full list is on our boiler repair cost page.
If your Greenstar is old enough that the repair is approaching a third of what a replacement costs, we will say so and give you a fixed price for a new boiler with nothing attached to it. Worcester Bosch warranties run up to 12 years when fitted and registered by an accredited installer, and the annual service is what keeps that valid.
Worcester Bosch questions we get asked
The ones that come up on the phone before we attend.
My Worcester boiler shows EA every morning but works after a reset. What is that? +
How many times can I reset it? +
Is a D5 pressure warning serious? +
Do you work on all Greenstar models? +
Will a repair affect my Worcester warranty? +
Which areas do you cover for Worcester repairs? +
Other brands and what to do next
- Vaillant fault codes
- Baxi fault codes
- Ideal boiler fault codes
- Navien fault codes
- Look any code up in the error code checker
- What a boiler repair actually costs
- Boiler repair across the North East
- Out of hours emergency cover
- Annual boiler servicing
Local pages: Sunderland, Durham, Washington, Chester le Street and Houghton le Spring.
There is a longer write up of one specific code here: the Worcester Bosch EA error code explained.