Error T4

Safety precautions:

1. If you smell gas or suspect a leak, stop, ventilate the area, do not operate electrical switches, evacuate the property and contact your gas emergency number or supplier immediately. Do not attempt repairs.

2. If you suspect carbon monoxide (unusual smells, headache, fumes), turn the boiler off, ventilate, evacuate and arrange an immediate Gas Safe visit.

3. When working near the boiler, switch the electrical supply off at the isolator before removing panels. Only qualified engineers should remove internal covers and work on gas or high voltage wiring.

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

1. Note the display: if the T4 is flashing with a triangle or alongside a lock symbol the boiler has locked out. Make a note of any static cause code visible.

2. Try a reset: press the spanner/return or reset button as per your boiler control. On many Worcester models press the spanner/return to enter diagnostics and follow prompts for the valve test, or perform a normal reset. Wait for the boiler to attempt the test again and watch the display.

3. Check system pressure on the front gauge. Ensure pressure is around 1–1.5 bar when cold. Low pressure can cause circulation issues that might trip systems; repressurise only if you know how and there are no leaks.

4. Create a demand and listen: turn on a hot tap (DHW demand) and then request CH (radiators) from the programmer. Listen for a clicking or whirring noise from the boiler/valve area which indicates the actuator is moving. Note if the valve moves in either demand.

5. Check external controls: ensure timers/room stats and thermostats are calling correctly. A false external signal could make the control run tests.

Specific diagnostic steps and safe checks (do not open the boiler unless competent):

1. Use the spanner/return diagnostics: follow the boiler’s diagnostic mode to run the three‑way valve test. The boiler will normally try to move the valve and report status; if it reports blockage or failure, proceed to the next checks.

2. Observe and listen during the test: if you hear the actuator running/making noise and the boiler still reports T4, the valve may be moving but the position feedback switch or sensor is failing. If you hear nothing, the actuator may have failed or be without power.

3. Visual external inspection: with the electrical supply off at the isolator, inspect any visible wiring and connectors to the valve/actuator for obvious damage or water ingress. Re‑secure any loose connectors. Do not touch internal components with the power on.

4. Attempt a power cycle: switch the boiler off at the isolator for 30 seconds and switch back on, then run the valve test again. This can clear transient faults and allow the actuator to re‑calibrate.

5. Accessible actuator spindle test (only if you are confident and the valve actuator is accessible externally): with the boiler isolated from power, some actuators allow you to remove the actuator and turn the valve spindle by hand to check if it is seized. If the spindle is very stiff or will not move, the valve may be mechanically seized and likely needs replacement. If it turns freely, refit the actuator and retest.

6. Check for water damage: signs of moisture around the valve motor or control wiring can cause failures. If you see corrosion or wetness, switch the boiler off and call an engineer—do not attempt to dry or repair electrical components yourself.

7. If actuator runs but no position change: this suggests the valve head or internal valve mechanism is faulty or the feedback sensor inside the valve is faulty. The valve or actuator assembly is normally a replaceable component but requires an engineer to fit and commission.

When to call a professional:

1. If the valve does not move during the test, if you cannot identify or safely fix an obvious loose connector, or if visual inspection finds corrosion or water ingress, call a Gas Safe registered engineer.

2. Any electrical, mechanical replacement of the valve actuator, motor or internal valve body, or wiring repairs must be completed by a qualified engineer. The three‑way valve and its actuator are common replacement items for engineers.

3. If the boiler shows a lockout triangle or other locking symbols after the fault is addressed, a professional should verify the repair and reset/clear the fault and confirm safe operation.

Additional notes:

- Pressing the spanner/return or performing an engineer reset can allow the boiler to run its diagnostic and sometimes clear transient faults, but this will not permanently fix a mechanically seized or electrically failed valve.

- Do not attempt to repair gas valves, the burner, or internal electronic PCBs yourself. Always use a Gas Safe registered engineer for repairs and commissioning.

- After any repair the engineer should test the boiler through full heating and hot water cycles to confirm correct diverter valve operation and restore normal function.