Worcester Bosch D5 fault — condensate trouble, and how to fix it

Can I fix the Worcester Bosch D5 fault myself?

Often, yes. The Worcester Bosch D5 fault is a condensate fault — your boiler can't clear the waste water it makes, so it stops and shows D5 to keep itself safe. In cold weather the usual culprit is a frozen external condensate pipe, and thawing that is a safe homeowner job with no gas work involved.

So before you reach for the phone, the first thing to check in winter is the white plastic pipe running outside from under your boiler. If it's frozen, a few minutes with some warm water often clears D5 for nothing.

If the pipe isn't frozen, or D5 comes straight back after you've thawed it, the cause is usually inside the boiler — most often the condensate pump on a Greenstar Highflow 440CDi or 550CDi. That's where a Gas Safe registered engineer takes over.

One quick note. D5 is a condensate code on the Greenstar CDi range that has a condensate pump (Highflow and floor-standing models). On a few older CDi models a similar code can be a service reminder rather than a fault, so codes do vary a little between Greenstars — check your boiler's manual for the exact wording on your model.

D5 at a glance — the key facts

What does the Worcester Bosch D5 fault mean?

Every condensing boiler makes a small amount of acidic waste water — condensate — as it burns gas efficiently. That water drains away through a white plastic pipe, usually out through an external wall to a drain or soakaway.

When that water can't drain away, it backs up. On the Greenstar models with a built-in condensate pump (the Highflow 440CDi and 550CDi, and the floor-standing CDi Regular), a float switch inside the pump senses the level rising and the boiler locks out with D5 rather than overflow inside.

Worcester Bosch documentation and engineer guidance describe D5 as the condensate pump's float switch reading as defective — pointing the diagnosis at the pump, its wiring and the boiler's control board. In the real world, the thing that most often trips that switch is a blocked or frozen condensate pipe outside, which is why winter is peak D5 season. Codes vary by Greenstar model, so confirm the exact wording against your own boiler's manual.

Worcester Bosch also lists condensate freezing in the external pipe (often alongside the 'EA' code and a slow-flashing blue light) as a common cold-weather cause of a condensate lockout. So in cold weather, frozen pipe first; pump fault second.

D5 by Greenstar model — what it means on yours

What causes the D5 fault?

In cold weather, the dominant cause is a frozen external condensate pipe. The acidic water freezes at the most exposed point — the open end, a bend, or a sag where water pools — the condensate backs up, and the boiler locks out. This is strongly seasonal: D5 spikes during sub-zero spells.

The rest of the time, D5 points inside the boiler at the condensate pump and its wiring. The table below runs through the usual causes, from the one you can sort yourself to the ones that need an engineer.

Why does D5 happen in cold weather?

Because the condensate pipe often runs outside, and the water in it is barely above freezing. When the temperature drops below zero, a slug of ice forms — usually overnight — and blocks the pipe. The boiler can't push its condensate out, so it stops. That's why so many people get D5 (and the related EA code) on the first really cold morning of the year.

Once you've thawed it, lagging the external pipe properly stops it happening again — there's a how-to for that further down.

D5 causes and symptoms

How do I fix the D5 fault? Thawing a frozen condensate pipe

If it's freezing outside and you can hear the boiler gurgling, a frozen condensate pipe is the likely cause — and that's the one job here you can safely do yourself. There's no gas work involved. You're just warming a plastic drain pipe on the outside of your house.

First, find the pipe. It's the white plastic pipe (usually around 22mm, sometimes upgraded to 32mm) running down an external wall from underneath your boiler, often ending over a drain or a soakaway.

The ice usually forms at the most exposed spot — the open end, an elbow, or a low point where water sits. That's where you want to apply the warmth.

Use warm water, never boiling

Warm water at around 60–70°C is perfect — hand-hot, not from the kettle on a rolling boil. Boiling water can crack the cold plastic pipe and leave you worse off. A watering can, a hot-water bottle held against the pipe, or a microwaveable heat pack all work well.

Reset the boiler once after you've thawed it, and it should relight and run. If it does, you're done — and a good lag of the pipe will stop it recurring.

How to thaw a frozen condensate pipe — step by step

Only do this if the pipe is somewhere you can reach safely from the ground. If it runs high up or you can't get to it without a ladder, leave it to an engineer.

• Confirm it's the condensate pipe — the white plastic pipe running outside from under the boiler. In freezing weather, with gurgling from the boiler, a freeze is likely.
• Find the frozen spot. It's usually the exposed end, a bend, or a low sag where water collects. Run your hand along it for the cold, hard section.
• Warm some water to about 60–70°C — hand-hot, not boiling. Boiling water can crack the cold pipe.
• Pour the warm water slowly along the outside of the pipe at the frozen point. A hot-water bottle or heat pack held against it also works and helps stop it re-freezing.
• Repeat until the ice clears — you may hear it start to drain. Give it a few minutes.
• Reset the boiler once, holding the reset button until it relights. If it fires and runs, D5 is cleared.
• If it doesn't clear, or D5 returns, stop resetting and book a Gas Safe registered engineer — the cause may be the condensate pump rather than the pipe.

How do I stop D5 happening again? Lagging the condensate pipe

If you get D5 every cold snap, the fix is to stop the pipe freezing in the first place. The simple version is insulation; the permanent version is re-routing.

Wrap the external run with proper weatherproof condensate-pipe lagging — DIY stores sell pre-sized foam that clips on and is rated for outdoor use. Make sure the open end and any bends are covered, as those freeze first.

Worcester Bosch's own guidance is to keep external condensate runs as short as possible (ideally under 3 metres) and to increase the pipe to 32mm outside, which is much less prone to freezing. A 32mm run under 3 metres on some Greenstar models doesn't strictly need insulating, but lagging it anyway is cheap peace of mind.

The most permanent fix is re-routing the condensate to drain internally, into a soil stack inside the warm house, so there's barely any pipe left outside to freeze. That's an engineer job — and a good one to ask about if you're tired of the same fault every winter.

As a rough guide, condensate-pipe insulation is an inexpensive job — often around £50–£150 if a tradesperson fits it, less if you do it yourself. That's a typical industry range, not our price.

Is the D5 fault dangerous?

No. D5 is a condensate lockout, not a safety emergency. It isn't a gas leak, it isn't a carbon monoxide risk, and it isn't a 999 emergency. The boiler is doing the safe thing — it stops rather than let waste water overflow inside.

Because of that, the National Gas Emergency line — 0800 111 999 — is not the number for D5. That line is only for a smell of gas or a carbon monoxide alarm. If you ever smell gas, put everything else aside, call 0800 111 999 straight away, open the windows, turn the gas off at the meter, don't touch any electrical switches, and leave the house.

For D5 itself, the worst that happens is no heating or hot water until the condensate can drain again. It's safe to thaw the pipe and carry on. If it won't clear, that's an engineer job, not an evacuation.

What can I do safely, and when must I call a Gas Safe registered engineer?

Here's the honest line between the two. Thawing and lagging the outside pipe is yours to do. Anything inside the boiler — the pump, the float switch, the wiring or the board — is an engineer's job.

Safe to do yourself

Thawing a frozen external condensate pipe with warm (not boiling) water, as long as you can reach it safely from the ground. Lagging the external pipe to stop it re-freezing. Clearing a simple blockage at the open end. And a single reset once you've cleared the cause.

Call a Gas Safe registered engineer when

The pipe isn't frozen but D5 is still showing, D5 returns after you've thawed and reset, the pump light won't go solid green, or you want the condensate pipe re-routed internally. Replacing or repairing the condensate pump, the non-return valve, the float switch, the wiring or the control board, and any work that needs the boiler casing off, all need a registered engineer. Never attempt gas work yourself.

You can check any engineer is properly registered on the Gas Safe Register. Worcester Bosch also runs its own customer line on 0330 123 9559 if you'd rather talk to the manufacturer.

How much does it cost to repair a D5 fault?

If it's a frozen pipe, it costs you nothing — you thaw it yourself, and a few pounds of foam lagging stops it coming back. The cost only comes in when the condensate pump or its wiring is at fault and an engineer is needed.

As a rough guide using typical UK industry ranges (not our prices, and worth confirming with a local quote): a gas engineer's call-out and diagnosis is often around £50–£60 an hour plus parts. A new condensate or circulation pump is commonly in the £150–£400 region fitted, depending on the model and how easy the part is to source. At the cheap end, a stuck or perished non-return valve in the pump can be under £20 for the part plus labour — well worth an engineer checking before condemning the whole pump.

If it does turn out to be the control board (PCB), that's the dearest of the common condensate repairs, often £350–£500 fitted. Repairs add up, especially on an older boiler where one failed part tends to be followed by another — which is where ongoing cover earns its keep.

D5 keeps coming back? Here's how Smart Plan helps

A one-off frozen pipe is easy — thaw it, lag it, done. But if D5 keeps returning when there's no frost, or the pump is on its way out, that's a real fault behind it, and that's not something to keep papering over with resets.

Smart Plan is a service plan, not insurance. When the simple fix runs out, you've got two easy options.

Need it fixed now?

Book a one-off repair and we'll come and fix it for you. A Gas Safe registered engineer traces why the condensate won't clear — pump, float switch, wiring or pipework — and gets your heating back on.

Want cover for next time?

Take out a Smart Plan boiler module and you're sorted when the next fault appears. Parts and labour are included up to your cover limit, and an annual service helps catch a tired condensate pump before it turns into a no-heat breakdown on the coldest day of the year.

Cover is modular, so you pick only what you want — don't pay for what you don't use. Boiler cover runs up to £500 per year if your boiler's under 7 years old, or up to £200 if it's older. A £95 call-out fee applies. We've looked after over 15,000 customers, we've been trading since 2014, and we're rated on Trustpilot. When D5 won't clear, we'll come and fix it for you.

Worcester Bosch D5 fault FAQs

D5 won't clear? We'll come and fix it for you.

Book a one-off repair or set up an ongoing Smart Plan boiler module. A service plan, not insurance — parts and labour included up to your cover limit.

Helpful links

• All Worcester Bosch fault codes
• EA / 227 fault — no flame detected
• H07 fault — low pressure fix
• No heating? Book a repair
• Boiler & central heating cover
• Check an engineer on the Gas Safe Register