Worcester Bosch D5 Fault Code: Causes, Meaning & How to Fix It

What does the Worcester Bosch D5 fault code mean?

The D5 code on a Worcester Bosch boiler signals that the appliance has shut down to protect itself, but the exact meaning genuinely varies between models. Depending on your specific boiler, D5 can point to a pump or circulation problem (water not moving properly through the system), low water pressure, a condensate drainage issue, high flue gas temperature, or a more general internal sensor/PCB fault. Because there is no single universal definition across the entire Worcester range, the most reliable thing you can do is check the manual for your exact model or have a Gas Safe registered engineer confirm what D5 means on your boiler before any parts are replaced.

Common causes

• Pump or poor circulation fault Common

  A stuck, failing or airlocked pump means water isn't circulating properly through the system. Without adequate flow, the boiler shuts down to avoid overheating, often displaying D5.

• Low system pressure Common

  On some models D5 behaves like a low-pressure warning. If the pressure gauge reads below around 1 bar, there isn't enough water in the system for safe circulation.

• Frozen condensate pipe Common

  In cold weather the external condensate pipe can freeze solid, blocking the acidic water the boiler needs to drain. This is one of the most common winter causes of Worcester boiler lockouts.

• Blocked or sludged system Sometimes

  Years of dirt, debris and magnetite sludge can clog pipes, radiators and the heat exchanger, restricting flow and triggering a circulation-related shutdown.

• Condensate drainage or pump issue Sometimes

  On models with a condensate pump, a faulty pump, blocked pipe, stuck float switch or wiring problem can stop the boiler safely draining and cause it to lock out.

• High flue gas temperature / restricted flue Rare

  On certain models D5 relates to excessive flue gas temperature, often caused by a blocked air intake or flue restricting proper airflow.

• Sensor or PCB fault Rare

  A failed temperature sensor, loose wiring, moisture ingress or a faulty PCB can produce a D5 as a catch-all internal fault that needs professional diagnosis.

How to fix it

1. Check your boiler's pressure gauge DIY safe

   Look at the pressure gauge on the front of the boiler. If it reads below 1.0 bar, the system pressure is too low and could be contributing to the D5 lockout.

2. Top up the system pressure using the filling loop DIY safe

   If pressure is low, use the filling loop to bring it back up to between 1.2 and 1.5 bar. Open both valves slowly, watch the gauge, then close them once you reach the correct level. Refer to your manual for the location of your filling loop.

3. Check for a frozen condensate pipe if it's cold outside DIY safe

   Locate the white or grey plastic condensate pipe outside (typically 21–32mm wide). If frozen, pour warm — never boiling — water slowly along its full length. You may hear gurgling as the blockage clears.

4. Reset the boiler DIY safe

   Once your checks are done, reset the boiler following the on-screen or manual instructions. Do this no more than 2–3 times. If the D5 returns immediately, stop resetting, as repeated resets can damage components.

5. Have a Gas Safe engineer check the pump, flue, sensors or PCB Gas Safe engineer

   If the fault persists, an engineer will need to open the boiler to test the pump, flush the system, check the flue and air intake, or test sensors and the PCB. Never remove the boiler casing yourself — it is dangerous and can invalidate your warranty.

6. Confirm the exact D5 meaning for your model Gas Safe engineer

   Because D5 differs between Worcester models, ask your Gas Safe engineer to confirm the precise meaning against the model-specific documentation before any part is replaced.

Parts you may need

• Central heating pump · from £150
• Condensate pump · from £120
• Temperature sensor (NTC) · from £35
• PCB (printed circuit board) · from £250
• System flush / power flush · from £400

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–£400, depending on the underlying cause.

Frequently asked questions