Error T3

Safety precautions:

- If you smell gas, hear a hissing gas sound, or suspect carbon monoxide exposure, evacuate the property immediately and contact the gas emergency number and emergency services. Do not attempt any checks.

- Before opening any boiler covers or touching electrical parts isolate the mains electrical supply to the boiler. If you are unfamiliar with isolating or safe isolation procedures do not proceed.

- Do not attempt internal boiler repairs, gas valve or burner work unless you are a Gas Safe registered engineer.

Initial homeowner checks (simple, safe checks you can do first):

1) Note the exact display: is T3 flashing only during start-up or is it a persistent fault with a lockout triangle? Photograph the display and record any other codes.

2) Press the spanner/return (diagnose/return) or reset button once as per the user manual. Some faults clear after a reset if they were transient. If the code returns immediately or shortly after, continue checks.

3) Check system water pressure on the boiler gauge - it should typically be about 1.0–1.5 bar cold. If pressure is low (below ~0.7–1.0 bar), top up the system to the recommended pressure and re-test. Low pressure can cause pump dry-run detection.

4) Ensure external controls (room thermostat, programmer, TRVs) are calling for heat. If no demand is present the pump may not run as expected during tests.

5) Listen at the boiler: when the boiler starts do you hear the pump running (a steady hum or vibration)? Carefully feel the flow and return pipes (without touching hot parts) or use the back of your hand briefly—if one pipe warms and the other stays cold that indicates poor circulation.

6) Check the pump isolating/service valves either side of the pump (if accessible) are open. Many installations have small handles or slotted screws—these must be in the open position for circulation.

7) Bleed radiators to remove trapped air. Airlocks can stop circulation and cause a pump test failure. Re-pressurise the system after bleeding if pressure drops.

8) If you have a magnetic debris filter/strainer in the system, check visually for obvious blockages or heavy sludge around the filter if accessible and only clean from the isolating valves after isolating power and closing inlet/outlet valves. (If unsure, call an engineer.)

Specific diagnostic and limited remedial steps (only if competent and safe):

1) After isolating mains power, remove the boiler front cover only if you are comfortable and have isolated power. Do not touch gas or burner components. Check the pump body for obvious signs of seizure (corrosion, heavy sludge) or loose wiring. Re-seat any obvious plug connections that are safe to access.

2) With power still isolated and if you are confident, you can try to manually turn the pump impeller (small shaft) by hand to free a seized pump. This must only be done with the pump isolated from electrical supply and with any valves closed to avoid water loss. If the impeller moves freely when spun, reassemble, restore power and test the boiler. If you are uncertain at any point stop and call an engineer.

3) If the pump hums but doesn’t circulate, or if the pump spins slowly or makes unusual noises, the pump is likely failing and needs replacement or testing with proper instruments by an engineer.

4) If the pump appears to operate correctly yet T3 persists, the fault may be wiring to the pump, the pump speed setting, a flow/safety sensor, or the control board. These require a qualified engineer to diagnose with test equipment.

When to call a professional:

- If the reset does not clear the code permanently or the fault returns repeatedly.

- If the pump will not run, is noisy, or is seized and you are not a qualified engineer.

- If you need to remove the pump, replace seals, perform electrical tests on wiring or control boards, or touch any gas-bearing components.

- Any time you are unsure about a step, or see other error codes, signs of leaks, overheating, or electrical burning.

Prepare for the engineer: have the boiler model and serial number, the exact error code(s), photos of the display and pump area, and a brief summary of checks you performed. After the engineer repairs the cause you will normally need to reset the boiler to clear the stored code.

Final note: T3 indicates a circulation/pump issue; some simple homeowner checks can resolve airlocks, closed valves, or low pressure, but internal pump, wiring or control faults must be dealt with by a Gas Safe registered engineer for safety and to maintain warranty and compliance.