Worcester Bosch D5 Fault Code: Causes, Fixes & Repair Costs
What does the Worcester Bosch D5 fault code mean?
The D5 code on a Worcester Bosch boiler does not have a single universal meaning — its interpretation depends on which model you own. On most Greenstar condensing models it signals a blocked or frozen condensate pipe preventing safe drainage. On some models it indicates low system water pressure or a pump circulation problem. On certain CDi variants it points to high flue gas temperature or an internal sensor or component fault. Finally, on the Greenstar CDi SE specifically, D5 is not a fault at all but a service-due reminder triggered by accumulated burner hours. Check your boiler's manual to confirm which interpretation applies to your appliance before taking any action.
General guidance only — not a substitute for professional advice, and not certified by a Gas Safe engineer. Always have your specific appliance assessed by a qualified professional. Any gas work must be carried out by a Gas Safe registered engineer. If you smell gas or suspect carbon monoxide, leave the property and call the National Gas Emergency line on 0800 111 999.
Common causes
- Frozen or blocked condensate pipe Common
Condensing boilers produce acidic condensate water that drains through a plastic pipe — typically white or grey, 21–32 mm in diameter — to an external drain. In sub-zero temperatures this water can freeze solid inside the pipe, blocking drainage and forcing the boiler to shut down. This is the most frequent cause of D5 appearing during cold spells and is also associated with the related fault code EA 229 on some Worcester Bosch displays.
- Low system water pressure Common
If the pressure gauge reads below 1.0 bar the boiler lacks sufficient water to circulate safely and will lock out. Pressure can drop gradually through minor leaks, a faulty pressure relief valve, or simply through normal expansion and contraction over time. Some Worcester Bosch models display D5 for this condition where others might show CE.
- Pump circulation failure or seized pump Sometimes
The circulating pump moves heated water around the central heating system. If the pump is seized, air-locked, or failing, water flow drops below the level the boiler considers safe and a shutdown follows. Sludge or debris accumulation in the system can restrict flow to the point where even a working pump cannot maintain adequate circulation.
- High flue gas temperature or flue blockage Sometimes
On certain CDi models, D5 is triggered when flue gas temperature exceeds a safe threshold. This typically happens when there is a partial blockage in the flue outlet or air intake restricting proper draught, causing hot gases to back up rather than exhaust freely. A Gas Safe engineer is needed to inspect the flue safely.
- Service reminder — CDi SE models only Sometimes
On the Greenstar CDi SE, D5 is not a fault code but an indication that the boiler has accumulated enough burner hours to require a service. The boiler may still operate normally. The reminder can be acknowledged via the control panel using a specific button sequence, but the underlying service should still be carried out to protect the manufacturer's guarantee.
- Internal sensor or PCB fault Rare
On some CDi variants D5 can be a catch-all code indicating an issue with an internal sensor, control board, or associated wiring that the boiler's diagnostics cannot attribute to a more specific cause. This requires professional diagnosis and is not DIY-safe.
How to fix it
- Identify which D5 interpretation applies to your boiler model DIY safe
Before doing anything else, check your boiler's installation and user manual or the manufacturer's website to confirm what D5 means on your specific model. The action you take differs significantly depending on whether you have a condensate blockage, a pressure issue, or a CDi SE service reminder.
- Check the system pressure gauge DIY safe
Look at the pressure gauge on the front of the boiler. The needle should sit between 1.0 and 1.5 bar when the system is cold. If it is below 1.0 bar, you can repressurise the system yourself using the filling loop — a small braided hose usually found beneath the boiler. Open both valves slowly, watch the gauge rise to around 1.2–1.3 bar, then close both valves fully. Do not overfill beyond 2.0 bar.
- Inspect the condensate pipe for freezing DIY safe
If the weather is at or below freezing and your boiler has a condensate pipe running outside, locate it — it is usually white or grey plastic pipe emerging from the bottom or side of the boiler casing and running to an external drain. Feel along its length for a frozen section. If frozen, pour warm (not boiling) water slowly and repeatedly along the pipe until you hear gurgling as the ice clears. Never use a naked flame or boiling water, as these can crack the pipe.
- Reset the boiler DIY safe
Once you have addressed a pressure shortfall or cleared a frozen condensate pipe, reset the boiler by turning the mode selector dial to the reset position (often marked with a flame and a cross or a specific reset symbol) and holding it for a few seconds, then release. The boiler should attempt to fire. Limit resets to two or three attempts — repeated resetting without a successful fire can stress components further.
- Acknowledge the service reminder on CDi SE models DIY safe
If your boiler is a Greenstar CDi SE and the D5 is a service-due reminder, you can clear the display alert by pressing and holding the Spanner button until it turns orange, then pressing and holding the Eco button until 5A appears on the display. Follow the remaining control panel sequence as described in your user manual. Note that clearing the reminder does not replace an actual annual service — have the boiler serviced by a Gas Safe engineer to comply with the conditions of your Worcester Bosch guarantee.
- Call a Gas Safe registered engineer if the fault persists or relates to internal components Gas Safe engineer
If D5 returns shortly after a reset, if the pressure drops repeatedly suggesting a leak, if the cause is unclear, or if your model uses D5 for high flue gas temperature or an internal sensor fault, stop attempting DIY fixes and contact a Gas Safe registered engineer. Pump failures, flue blockages, sensor replacements, and any PCB or gas component work must only be carried out by a qualified professional. Never remove the boiler casing or attempt internal repairs yourself — doing so is dangerous and will invalidate your Worcester Bosch guarantee.
Parts you may need
- Condensate trap and siphon assembly · from £25
- Circulating pump (e.g. Grundfos UPS or equivalent) · from £85
- Flue gas thermistor / sensor · from £35
- Pressure relief valve · from £20
- Filling loop assembly · from £15
The exact spare depends on your boiler's GC number (on the data badge). Check this against the part before buying.
Typical repair cost
Expect to pay roughly £120–£380, depending on the underlying cause.
Frequently asked questions
Is D5 the same as EA 229 on a Worcester Bosch boiler?
They are closely related. On many Worcester Bosch Greenstar models, D5 and EA 229 both point to a condensate drainage problem — most often a frozen external condensate pipe in cold weather. If your display shows EA 229 alongside or instead of D5, the same thawing and reset procedure applies. The EA prefix codes are the more detailed diagnostic codes the boiler uses internally, while D5 is the simplified display code a user sees.
Why does my Worcester Bosch keep showing D5 after I reset it?
A D5 that keeps returning after resetting usually means the underlying problem has not been resolved. If you've thawed the condensate pipe, the freeze may have recurred — consider lagging the external section of pipe to prevent repeat freezing. If the code is related to low pressure, a persistent drop suggests a slow leak somewhere in the system that needs a Gas Safe engineer to find and fix. If neither of those applies to your model, an internal fault such as a failing pump or sensor is likely and requires professional diagnosis.
Can I fix D5 myself or do I need a Gas Safe engineer?
It depends on the cause. Thawing a frozen condensate pipe and topping up system pressure are both tasks a homeowner can safely carry out. However, if the fault relates to a seized pump, a flue blockage, high flue gas temperature, or an internal sensor or PCB issue, you must call a Gas Safe registered engineer. Never open the boiler casing or touch gas components yourself — it is illegal and dangerous for anyone who is not Gas Safe registered to work on gas appliances.
How much does it cost to repair a Worcester Bosch D5 fault?
For the most common causes, expect to pay roughly £120–£380 in total including labour and parts. Thawing and insulating a condensate pipe yourself costs very little; having an engineer do it typically costs a callout fee of £60–£100. A pump replacement including labour and parts usually falls in the £200–£350 range. If the fault turns out to be a faulty PCB or heat exchanger, costs can rise significantly — in those cases, on a boiler over 10–12 years old, it is worth comparing the repair quote against the cost of a new boiler. Prices in London and the South East tend to run 15–25% higher than elsewhere in the UK.