Overview
H.01 – .05 on a Baxi 200 / 400 Combi is a temporary fault code that means the boiler has detected an excessive difference between the flow (outgoing) and return (incoming) water temperatures, or it has detected poor or no circulation, or a problem with one of the temperature sensors. In practice this usually happens when the pump or a zone valve isn’t allowing water to circulate through the heat exchanger, when radiators or pipework are airlocked, when a filter or blockage restricts flow, or when a flow/return sensor is faulty or has a bad connection. Severity ranges from low to medium depending on cause. If it is intermittent you may only experience temporary lockouts or loss of heating/hot water until the fault clears or the boiler is reset. If left unresolved, persistent lack of circulation or a sensor fault can cause repeated lockouts, overheating of the heat exchanger or trigger permanent safety faults. Basic checks and resets can be done by a homeowner, but diagnosis and repairs involving the pump, sensors, gas valve, internal wiring or the gas/air unit must be carried out by a Gas Safe registered engineer.
Possible Cause: Difference between the flow and return temperature is too high, sensor error or insufficient circulation.
Troubleshooting Steps
Safety precautions:
- If you are not a qualified heating engineer do not remove the boiler cover or attempt to work on gas or live electrical components. Risk of gas leaks, electric shock and injury.
- Before any visual checks switch the boiler to standby or switch off the electrical supply at the isolator/fusebox. Do not isolate the gas supply unless instructed by a Gas Safe engineer.
- Wear gloves and eye protection for any work on radiator bleed points or the system.
Initial checks a homeowner can do:
1. Note the exact error code and when it appears (heating, hot water or both). Try a normal reset: press and hold the reset button for 3–5 seconds and observe if the code clears and if the boiler runs. If it clears and does not return, monitor performance.
2. Check system pressure on the boiler gauge. Pressure typically should be around 1.0–1.5 bar when cold. If pressure is very low (<0.5 bar) top up via the filling loop following the boiler handbook. Low pressure can prevent correct circulation and cause faults.
3. Confirm the heating demand: check room thermostat/programmer and that any zone valves or TRVs are calling for heat (set to a temperature higher than room temperature).
4. Check radiators: feel the flow and return pipes on the radiator farthest from the boiler once the system is calling for heat. If one side is cold while the other is hot, air in the system may be restricting flow — bleed radiators starting with the highest ones.
5. Listen for the pump: when the boiler is running you should hear or feel a faint hum from the pump and pipes will be warm. If the pump is completely silent and pipes remain cold while boiler runs, circulation may be absent.
Specific diagnostic and simple fix steps:
1. Bleed radiators: release trapped air from radiators (highest first). After bleeding, check and restore system pressure if required. Reset the boiler and see if the code clears.
2. Check that all thermostatic radiator valves (TRVs) and isolation valves are open on the heating circuit and that zone valves (if present) are not stuck closed. Manually open any locked isolation valves where accessible.
3. Check condensate and filters (if you have a magnetic or debris filter on the return): if you have experience and the filter has an isolation valve, you can isolate, collect a small drain and clean the filter. If you are unsure, stop and call an engineer — incorrect draining/isolating can cause leaks.
4. Visual wiring check: with the boiler electrical supply off, you may inspect visible sensor wiring and connectors for obvious damage or loose plugs. Do not disconnect or probe internal PCB connections unless qualified.
5. Try a controlled reset again after the above steps. If the fault is intermittent and does not reoccur, continue to monitor. If it returns, note the exact pattern (e.g. when DHW or CH calls are made).
When to call a professional:
- If the pump appears faulty (no hum or no temperature rise on pipework) or you cannot restore circulation by bleeding/valving, call a Gas Safe engineer.
- If you find or suspect a faulty sensor, wiring fault, blocked heat exchanger, persistent large flow/return temperature differential, or repeated lockouts, do not attempt internal repairs — these require specialist tools and Gas Safe certification.
- If the boiler shows related codes (E.04, E.01, E.04.xx etc.), or there are any signs of leaks, gas smell, repeated overheating, or inability to light, stop using the boiler and call a Gas Safe engineer immediately.
What to tell the engineer:
- Provide the exact H.01.x code and times it occurred, what you tried (reset, bleed, pressure check), current system pressure, whether pump was audible and whether radiators heat, and any other error codes shown.
Summary:
- H.01 – .05 often means poor circulation or a sensor issue. Homeowners can perform safe initial checks: reset boiler, check pressure, bleed radiators, ensure valves are open, and listen for the pump. If the problem persists, involves internal wiring, the pump, sensors or gas/air unit, or if you are unsure at any stage, contact a Gas Safe registered engineer to diagnose and repair.
Helpful Resources
Boiler Manual
Download the official PDF manual for the Baxi 200 / 400 Combi Gas Boiler.