Error H.01 – .08

Safety precautions:

1) If you smell gas, turn off the gas supply at the meter, do not operate electrical switches, ventilate the area and call the gas emergency line immediately. Do not attempt internal repairs.

2) If there is visible water leakage or extreme overheating, isolate electrical power to the boiler and the water supply if safe to do so and call a professional.

3) Never bypass safety devices or attempt to work on gas, ignition or PCB circuitry unless you are a qualified Gas Safe engineer.

Initial homeowner checks you can do safely:

1) Note the exact error code and subcode and the time it appeared. This information is useful for the engineer.

2) Try a simple reset: press and hold the boiler reset button for 3–5 seconds and allow it to restart. If the fault returns immediately or within a short time, do not repeatedly reset more than a couple of times.

3) Check the system water pressure on the boiler gauge. It should generally be around 1.0–1.5 bar when cold. If pressure is very low (below 0.5 bar) the boiler may not run correctly; top up the system using the filling loop according to the boiler handbook, then recheck.

4) Check that TRVs and radiator thermostats are open and that the programmer/room thermostat is calling for heat. Close any radiator valves that are fully closed and ensure the motorized valves (if present) are in the correct position.

5) Bleed radiators to remove airlocks. Air in the system can prevent circulation and cause rapid temperature changes at the flow sensor.

6) Listen to the pump: with the boiler running and safe to approach, you should hear or feel the pump running (a quiet hum). If the pump is silent but the boiler tries to run, circulation may be absent.

Specific diagnostic and fix steps (what to try and when to stop and call a pro):

1) If radiators are cold or partial and you suspect poor circulation: bleed radiators, check magnetic filter/strainer (if you can access it safely) for obvious debris, and ensure all zone valves and TRVs are open. After bleeding, check system pressure and top up if needed.

2) After restoring pressure and bleeding, restart the boiler. Monitor flow and return piping (careful—these can be hot) to see if the temperature difference looks normal and whether the flow rises gradually rather than instantly spiking. If normal, the fault may clear.

3) If the pump appears seized or stuck (no humming, no flow), do not forcibly disassemble electric components. Switch off the boiler and isolation power and call a Gas Safe engineer to inspect/replace the pump. Repeated operation while pump is not circulating can damage the heat exchanger.

4) If the fault reappears immediately and you have confirmed circulation is present, suspect a sensor (flow thermistor) or wiring fault. These faults often require testing resistance values and wiring checks with appropriate tools and knowledge—call a Gas Safe engineer or authorized Baxi technician.

5) If the error was preceded by electrical disturbances, intermittent communication problems with the gas valve or PCB may be indicated (.00 subcode). In this case note any recent power cuts or flickering and call a qualified engineer to check connections and the air/gas unit.

6) If you see other related warnings (overheat, high flue temps, repeated lockouts) or the boiler displays permanent E.04 / E.00 style sensor errors, stop attempts to restart and call a professional right away.

When to call a professional immediately:

1) Any smell of gas, visible water leaks, severe overheat, or if the boiler will not reset and the error persists. 2) If you suspect the pump, heat exchanger or gas/air unit is faulty, or if the fault points to a sensor or PCB/communication issue. 3) If you are not comfortable performing the safe checks listed above.

What to tell the engineer when you call:

1) Exact boiler model and the full error code/subcode and any LED/flash patterns. 2) What you tried (reset, pressure checked, radiators bled, pump sound) and whether that changed behavior. 3) Any other symptoms (no hot water, boiler locking out, noises, leaks).

Final note: H.01 – .08 is typically a temporary protective lockout related to rapid flow temperature rise or lack of circulation. Simple circulation issues (air in system, low pressure, closed valves) are frequently DIY-fixable. Sensor faults, pump failures, wiring or gas/air unit faults must be diagnosed and repaired by a Gas Safe registered engineer to ensure safe and correct repairs.