Overview
Error 2971 B on a Worcester Bosch Greenstar 8000 means the boiler has detected low system pressure and has gone into a protective lockout or warning state. A sealed heating system needs a certain water pressure (typically around 1.0–1.5 bar at rest) to circulate water correctly; if the pressure falls too low the boiler will not run to avoid pump damage, overheating or poor performance. The common, non‑serious reasons are air in the system, or simply that water has been lost and the system needs topping up. Severity is generally moderate: it is not an immediate gas-safety emergency but it will stop central heating and hot water until corrected. Many homeowners can safely follow basic checks and repressurise the system themselves, but if the fault returns, the pressure gauge shows normal pressure while the error persists, or you find leaks or PRV discharge, you should call a Gas Safe registered heating engineer. Persistent or repeating low pressure can indicate a leak, a failed pressure sensor, a faulty cable to the sensor, a leaking pressure relief valve, or a failed expansion vessel, all of which usually need a professional diagnosis and repair.
Possible Cause: System pressure too low.
Troubleshooting Steps
Safety precautions:
- Before doing anything, read the boiler user manual for model-specific instructions. If unsure, turn off the boiler and isolate electrical power at the fused spur. Do not attempt gas or combi boiler internal repairs yourself. Wear protective gloves/eye protection when bleeding radiators.
- If you see water actively leaking or the boiler or pipework are very hot, turn the boiler off and call a qualified engineer.
Initial checks a homeowner can do:
- Check the boiler pressure gauge/indicator. At rest the needle should usually be around 1.0–1.5 bar (check the manual for exact recommended range). If it is below ~1.0 bar or in the red, that confirms low pressure.
- Look for obvious signs of leakage: visible drips around the boiler, radiators, valves, underfloor pipework or the external pressure relief discharge pipe (pipe that usually runs from the boiler to an outside wall). If you find a leak, isolate the system if possible and call a Gas Safe engineer.
- Check radiators for cold spots (top cold, bottom warm) that suggest air trapped in the system.
How to vent/bleed radiators (to remove trapped air):
- Turn the heating on and allow radiators to warm up, then turn the system off and give radiators time to cool slightly.
- Starting at the highest radiator in the house, use a radiator key or screwdriver to open the bleed valve a small amount until air hisses out; when water starts to flow steadily, close the valve. Repeat for other radiators.
- Recheck the boiler pressure gauge after bleeding — bleeding will often reduce system pressure further, so you may need to top up afterwards.
How to top up (repressurise) the system:
- Identify your filling loop type (external flexible hose with two valves, an internal keyed filling link under a flap, or a keyless filling lever). Consult the boiler manual or the markings under the boiler if unsure.
- Turn the boiler off and allow it to cool slightly before opening the filling loop. Open the mains side valve first (or follow the boiler manual steps) then open the filling valve slowly. Watch the pressure gauge and stop filling once the needle reaches the recommended resting pressure (typically just over 1 bar). Close the filling valve, then the mains valve, and remove an external hose if fitted. Wipe any drips and ensure valves are fully closed.
- Turn the boiler back on and check whether the 2971 B error clears. Some boilers reset automatically when pressure is restored; others may need a manual reset — follow the manufacturer's reset procedure in the manual.
If the error persists after topping up and bleeding:
- Verify the pressure gauge reading is stable. If the gauge shows normal pressure but the boiler still reports 2971 B, the pressure sensor or its wiring may be faulty or disconnected. Do NOT attempt to repair or replace wiring or sensors yourself unless you are qualified.
- If pressure drops again soon after repressurising, look for leaks or signs the pressure relief valve (PRV) is weeping. If water is present on the PRV discharge pipe outside, this indicates the PRV or a related component may be faulty.
- Repeated pressure loss can also be caused by a failed expansion vessel or an internal leak; these require a professional diagnosis.
When to call a professional:
- Call a Gas Safe registered heating engineer if you cannot find or stop a leak, if pressure keeps falling after repressurising, if the boiler still shows 2971 B despite correct pressure, or if you are not comfortable performing the steps above.
- Ask the engineer to check for leaks, the pressure relief valve, expansion vessel, filling loop integrity, the pressure sensor and its cable. Tell them you have already bled radiators and tried repressurising and whether the error cleared briefly.
- Any replacement of pressure sensors, wiring, or internal boiler components must be carried out by a qualified engineer.
Final notes:
- Do not tamper with gas valves, burners, or internal boiler wiring. Keep records of what you checked and when, and provide those details to the engineer to speed diagnosis. Restoring correct pressure often fixes 2971 B, but persistent or recurrent faults need professional attention to prevent future breakdowns or water damage.
Helpful Resources
Boiler Manual
Download the official PDF manual for the Worcester Bosch Greenstar 8000 Boiler.