On a Worcester Bosch Greenstar Highflow 440CDi or 550CDi ErP boiler, D5 means a condensate pump fault: the boiler cannot pump away the waste water it makes while running, so it shuts down. This Worcester Bosch D5 error code guide explains what the code means, which models list it, the one safe check you can make in cold weather, and when to call a Gas Safe registered engineer.
Quick answer: D5 on a Worcester Bosch Greenstar Highflow boiler means a condensate pump fault. In freezing weather, check the condensate pipe outside at ground level, thaw it with warm water and reset once. If D5 returns or the weather is mild, call a Gas Safe registered engineer, because the pump is not a part homeowners should touch.
What Does the Worcester Bosch D5 Error Code Mean?
D5 means “condensate pump fault”. Worcester lists the code, written as d5 on the display, in the user instructions for the Greenstar Highflow 440CDi and 550CDi ErP floor standing combination boilers.
Every condensing boiler produces condensate, water that forms when the flue gases cool. According to Wikipedia’s condensing boiler article, this water is acidic, with a pH between 3 and 4, and where it cannot drain away by gravity a small condensate pump lifts it to a drain. When the boiler detects that its condensate pump is not clearing the water, it stops firing and shows D5 rather than letting water back up.
D5 is not in every Worcester manual. We did not find it in the user instructions for the Greenstar CDi Classic range or in an earlier 2015 edition of the Highflow instructions. Some websites describe D5 more loosely as a blocked condensate pipe. If your boiler is not a Highflow model and shows D5, check the fault code table in your own manual before acting, and give the full code to your engineer.
What Causes the D5 Error?
Anything that stops condensate leaving the boiler can trigger D5, and freezing weather is a well known trigger for condensate problems.
- A frozen condensate pipe. Worcester warns that condensate pipework outside or in unheated areas can freeze, even when well insulated. With nowhere to go, the water backs up to the pump.
- A blocked condensate pipe or trap. Debris or sludge in the trap or pipe has the same effect as ice.
- A failed condensate pump. The pump itself can wear out or stop working.
- An electrical fault. Wiring or a connection between the pump and the boiler controls can fail.
How Do You Fix the D5 Error Yourself?
The only homeowner fix Worcester describes is thawing a frozen condensate pipe and resetting the boiler. Only try this if it is freezing or has been below zero recently.
- Find the condensate pipe. It is the plastic pipe that runs from the boiler to a drain. Worcester says the frozen section is likely to be at the most exposed point outside.
- Check it is reachable at ground level. Worcester says never attempt to thaw a condensate pipe that is at height.
- Thaw it with warm heat. Hold a hot water bottle, a microwaveable heat pack or a cloth soaked in hot water against the pipe. Do not use boiling water.
- Reset the boiler. After thawing a condensate pipe, the Highflow manual says to press the reset button for five seconds. For other faults it gives three seconds.
- Check the pressure while you are there. Worcester says the system needs recharging if the gauge reads below 1 bar, filling up to 1.5 bar. Our guide on how to repressurise a combi boiler explains the filling loop.
Safety: Stop after one thaw and one reset. Worcester’s fault code table tells users not to try to rectify the listed faults themselves and to contact a Worcester or other Gas Safe registered engineer. Do not open the boiler casing, unplug or test the condensate pump, or climb a ladder to reach a pipe at height.
When Should You Call an Engineer?
Call a Gas Safe registered engineer if D5 appears in mild weather, comes back after you have thawed the pipe, or appears with any water near the boiler. The HSE domestic gas safety FAQs say anyone employed to work on gas appliances in homes must be Gas Safe registered, and you can check an engineer’s ID card before they start.
- The weather is above freezing, so ice is unlikely to be the cause.
- The condensate pipe is at height or hard to reach.
- You see water leaking from the boiler, the pump or the pipe joints.
- D5 keeps returning, even if a reset clears it for a while.
- The display shows another code alongside D5.
Worcester does not publish a set price for condensate pump repairs, so ask your engineer for a quote once they have found the cause.
Which Other Codes Does the Highflow Manual List?
| Code | Worcester meaning (Highflow 440CDi and 550CDi ErP user instructions) |
|---|---|
| d5 | Condensate pump fault |
| E9 | Safety temperature limiter in CH flow has tripped |
| EA | Flame not detected |
| E2 | CH flow NTC sensor defective |
| A7 | Hot water NTC sensor defective |
| C6 | Fan speed too low |
| SE | Service required |
| U1 to U3 | Boiler flow temperature too high; the boiler shuts down until it has cooled |
These meanings come from the December 2015 edition of the Highflow ErP user instructions. Other ranges use different tables. If your display shows EA, see our Worcester Bosch EA error code guide.
How Can You Stop the D5 Error Coming Back?
- Have the boiler serviced every year by a competent registered gas engineer, as the Highflow manual requires. Ask them to check the condensate trap and pump.
- Ask your engineer whether any outside condensate pipe can be insulated, enlarged or rerouted, which Wikipedia lists as ways to reduce freezing.
- Keep the outside drain where the pipe ends clear of leaves and debris.
- Keep the pressure between 1 and 1.5 bar so the boiler is not also locking out for low pressure.
- Size your heating sensibly. Our boiler size calculator is useful if you are planning a replacement.
Worcester Bosch D5 Error Code FAQs
Can I reset a Worcester boiler showing D5?
Yes, once. If it is freezing, thaw the condensate pipe at ground level first, then press the reset button for five seconds as the Highflow manual describes. If D5 comes back, stop resetting and book a Gas Safe registered engineer.
What is a condensate pump on a boiler?
A condensate pump is a small pump that lifts the acidic water a condensing boiler produces up to a drain when it cannot flow away by gravity. If the pump fails or its outlet is blocked, the boiler shuts down to stop the water backing up.
Is D5 the same as a frozen condensate pipe?
Not exactly. Worcester’s Highflow manual defines D5 as a condensate pump fault. A frozen or blocked pipe is one thing that can stop the pump clearing water, but the pump or its wiring can also fail, which needs an engineer.
How do I thaw a frozen boiler condensate pipe safely?
Hold a hot water bottle, a microwaveable heat pack or a cloth soaked in hot water against the frozen section, usually the most exposed point outside. Never use boiling water and never try to thaw a pipe at height, as Worcester warns.
Which Worcester boilers show the D5 code?
We confirmed D5 in the user instructions for the Greenstar Highflow 440CDi and 550CDi ErP. We did not find it in the CDi Classic manuals. If another model shows D5, check the fault table in its own manual and quote the full code to your engineer.
Will I have heating and hot water with a D5 fault?
Usually not. The boiler stops firing while the fault is active, so both heating and hot water stop until the cause is cleared and the boiler is reset. An electric shower or immersion heater, if fitted, will still work.
Checked October 2026. Sources: Worcester Greenstar Highflow 440CDi and 550CDi ErP user instructions, Worcester Greenstar CDi Classic combination boiler user instructions, Wikipedia, Condensing boiler, HSE domestic gas safety FAQs.
