Worcester Bosch F0 fault code: internal error explained

What does the Worcester Bosch F0 fault code mean?

The Worcester Bosch F0 fault code means an internal error: the boiler's control box, the circuit board that runs the appliance, has failed one of its own checks, so the appliance stops and puts the code on the display. It is not a pressure, ignition or flue fault. In Worcester's servicing instructions for the Greenstar 25i and 30i ErP, the display text against every F0 row of the fault-finding table is "Internal error", and F0 always appears with a three-digit cause code beside it that names the part at fault.

The code is written F0, with the digit zero. Plenty of people search for it as F00, or type it as FO with a letter O, because a small display makes a zero and a capital O look almost identical. They are the same fault, so F0, F00 and FO all land you here.

Worcester gives homeowners the same code in plainer form. The Greenstar CDi Classic Regular user instructions (document 6 720 812 177) list it on page 11 as "F0 Internal error." and tell the householder not to try to rectify it. So this is not a nuisance code to work around. The boiler no longer trusts its own electronics, and clearing it is engineer work.

Key facts

The quick version, before the detail.

• Meaning: an internal error inside the boiler's control box, the main circuit board. Worcester's display text against every F0 row is "Internal error".
• Also searched as F00, or FO with a letter O. Same code, different typing.
• Always read the three-digit cause code beside F0 (238, 239, 242 to 256, 258, 262, 272, 280 or 290): it names the part at fault.
• DIY-fixable? No. The safe scope is both numbers written down, your model noted, and one reset attempt.
• Not a pressure fault. Worcester's low water pressure code is H07, so topping up will not clear F0.
• Sourced from Worcester's Greenstar i ErP installation, commissioning and servicing instructions (document 6 720 806 944, covering the Greenstar 25i and 30i ErP) and the Greenstar CDi Classic Regular user instructions (document 6 720 812 177). Check the code against the manual for your exact model.
• Worcester's online troubleshooting tool is a question and answer questionnaire rather than a code list, so F0 is explained inside the downloadable PDF manuals.
• Smart Plan cover: parts and labour up to £500 per 12-month period if the boiler is under 7 years old, or up to £200 if it is over 7. Cover is for future breakdowns, so an F0 on the display today counts as a pre-existing fault and needs a one-off repair instead.

Why does F0 appear, and what does the number beside it mean?

F0 points at the boiler's electronics rather than the water, the flue or the gas supply. The remedies Worcester prints name a control box that has failed, a defective or badly seated HCM (Heating Control Module, a plug-in module that has to match the control box), and loose or corroded connections at the control box. On one cause code only, F0 238, the gas valve is in scope as well.

The three-digit cause code is the half that tells you something. Worcester's installation, commissioning and servicing instructions for the Greenstar i ErP combi (document 6 720 806 944) list them in the fault-finding table on pages 66 and 67, like this.

1. F0 238: gas valve or control box error. It is the only F0 cause code where Worcester's remedy sends the engineer to the gas valve as well as the board.

2. F0 239, F0 242 to 256, F0 258, F0 262 and F0 272: the control box or the HCM is defective.

3. F0 280: the control box is defective.

4. F0 290: the control box is defective, logged as an analogue to digital converter reference error.

Two caveats. Those meanings are the ones Worcester lists for the Greenstar 25i and 30i ErP, so read yours against the manual for your exact model. And the same three-digit cause code can sit against more than one letter code: in that same table, 258 is listed against F0 and against EH. Quote both the letter code and the number when you ring anyone.

One thing F0 is not: it is not the same as F0 on an Ideal boiler, where F0 means no water flow thermistor. Check the badge on the front of your appliance first.

F0 causes and what to do about each

What you can safely check yourself

There is no homeowner remedy for F0, and Worcester says so. The Greenstar CDi Classic Regular user instructions (document 6 720 812 177) print this above the error code table on page 11: "Do not try to rectify any of the faults listed below by yourself." Worcester then routes the reader to a competent, qualified Gas Safe registered engineer. The same table gives householders things to check for some other codes, such as EA, and nothing at all for F0. That silence is the answer.

What you can usefully do is gather information and try the single reset Worcester documents, in this order.

1. Write down the code as displayed, including the three-digit cause code beside it. Worcester's user instructions say that when a fault occurs the reset button and the main fault indicator flash and the display shows the error code.

2. Note the model name from the front panel, for example Greenstar 30i ErP, because cause code meanings are published per range.

3. Reset once. On the Greenstar i ErP, Worcester's instructions (document 6 720 806 944, page 35) say to press the reset button, and the tick symbol is displayed briefly for a successful reset. Reset once only.

4. Do not use the blue indicator light to reset. The same page carries a notice headed RESET: "Do not press the blue operation/fault diagnostic indicator to reset the appliance." That blue item is the heat demand and fault indicator light, not the reset button, and pressing it is a common mistake.

5. Glance at the pressure gauge as good practice, but do not expect it to matter. Low water pressure shows as H07, and re-pressurising will not clear an internal error.

Then stop. If F0 comes back, every remedy Worcester lists is engineer work, and most F0 cause codes route the engineer to Worcester's technical helpline or to replacing the control box. All of that sits behind the casing at mains voltage, so do not open the boiler yourself.

When to call a Gas Safe registered engineer

Call a Gas Safe registered engineer as soon as F0 returns after a single reset, and call straight away if the number beside it is 238. Every remedy Worcester documents is engineer work: checking the gas valve coils and their wiring and replacing the valve if needed, checking the HCM is seated properly and replacing it, remaking the control box connections, replacing the control box, or ringing Worcester's technical helpline. None of it is reachable without opening the appliance.

Worcester does not make this easy to look up. Its online troubleshooting tool is a question and answer questionnaire rather than a list of lettered codes, and we could not find F0 explained anywhere on the public site outside the downloadable PDF manuals. That is why it is hard to find, and it is not your search technique.

If you would rather we sorted it, call 0333 772 6247 and book a one-off repair, and we will arrange a Gas Safe registered engineer to diagnose the fault. One-off repair visits run Monday to Friday, 09:00 to 17:00, a £95 call-out fee applies and is paid in advance, and the engineer tells you what is needed and what it costs before any work goes ahead. One-off repairs do not carry a workmanship guarantee.

A Smart Plan boiler and central heating module covers parts and labour up to £500 per 12-month period if your boiler is under 7 years old, or up to £200 if it is over 7, and you pay anything above that limit. Cover is for future breakdowns rather than a fault your boiler is already showing, so an F0 on the display today counts as a pre-existing fault and a new plan would not pay for it. Book the one-off repair for this one. A £95 call-out fee, paid in advance, applies in your first 30 days, for faults the plan does not cover, no-access visits and early annual-service requests, and your cover still applies in those first 30 days. Cover-plan call-outs run Monday to Friday, 08:00 to 18:00, and outside those hours you are only seen sooner for a genuine emergency breakdown, so a Friday-evening fault can wait until Monday. Within those hours we aim to get an engineer out to you within two working days, which is a target rather than a promise. Using a module starts a 12-month agreement period the first time you use a service. There is a 14-day cooling-off period, which ends the moment a service is carried out, and leaving early after that costs the remaining months or 75% of the outstanding balance. Smart Plan is a service plan, not insurance.

One safety note. F0 points at the boiler's electronics rather than the gas supply, but cause code 238 can involve the gas valve, so treat an F0 238 as gas-side work for a Gas Safe registered engineer. And if you ever smell gas or suspect carbon monoxide, do not wait for an engineer. Call the National Gas Emergency Service on 0800 111 999 straight away.

Worcester Bosch F0 fault code FAQs

Boiler showing F0? Get a Gas Safe registered engineer to it.

Book a one-off repair and we will arrange a Gas Safe registered engineer to diagnose the fault. One-off repair visits run Monday to Friday, 09:00 to 17:00, a £95 call-out fee applies and is paid in advance, and one-off repairs do not carry a workmanship guarantee. A Smart Plan boiler and central heating module covers future breakdowns instead, with parts and labour up to £500 per 12-month period if the boiler is under 7 years old, or up to £200 if it is over 7, so an F0 on the display today counts as a pre-existing fault that a new plan would not pay for. A £95 call-out fee, paid in advance, applies in your first 30 days, for faults the plan does not cover, no-access visits and early annual-service requests, and your cover still applies in those first 30 days. It is a service plan, not insurance.

Helpful links

• All Worcester Bosch fault codes
• Worcester Bosch boiler guides
• H07 fault: low water pressure explained
• EA / 227 fault: no flame detected
• F7 fault explained
• Book a boiler repair
• Boiler & central heating cover
• More boiler guides on the blog
• Worcester Bosch literature and manuals
• Check an engineer on the Gas Safe Register