Baxi 800 System 2 H.00.42 code: pressure sensor open or faulty

Baxi’s current error-code table lists H.00.42 as “Pressure sensor open/faulty” for the Combi & System 2 group, which includes the Baxi 800 System 2.

The wording describes the signal the boiler is seeing. It does not prove the pressure sensor itself must be replaced. “Open” is technical shorthand for a signal or circuit condition; it is not an instruction to open the boiler, loosen a fitting or operate an internal valve.

What the pressure sensor does

A sealed heating system needs an appropriate water pressure for safe, reliable operation. The boiler uses pressure information as part of its monitoring and protection.

The pressure sensor converts the condition it measures into information the boiler’s controls can use. If the expected signal is missing, implausible or outside the accepted state, H.00.42 may be displayed.

Baxi describes the entry as pressure sensor open or faulty. The display cannot tell a homeowner why that state exists. The issue could involve the sensor, its connection or another part of the signal path, but naming a component without testing would be guesswork.

What you may notice

The boiler may restrict operation, stop a heating demand or continue to show the warning while the controls monitor the condition. You may find that:

• the heating does not start when requested;
• radiators stay cold;
• the separate hot-water cylinder does not reheat as expected;
• the display shows H.00.42 continuously or intermittently;
• the external pressure gauge appears normal even though the code remains;
• another pressure-related message appears at a different time.

An external gauge and the electronic pressure signal are related to the same heating system, but they are not the same component. A normal-looking gauge does not disprove H.00.42, and an unusual reading does not confirm the sensor is faulty.

Safe observations to record

The Baxi 800 System 2 is listed by Baxi with an external analogue pressure gauge. You can look at that gauge without removing the casing. Photograph or note the position of the needle and whether it changes between a cold system and a normal heating demand.

Do not rely on a generic “correct” number from another boiler. The appropriate operating information should come from the user guide for your installation, and the engineer needs to interpret the reading alongside the fault code.

Also record:

• whether H.00.42 is constant or comes and goes;
• whether heating and stored hot water are both affected;
• whether the code appeared after maintenance or other work;
• whether there is visible water beneath the boiler or around accessible pipework;
• whether the pressure-gauge needle has changed noticeably;
• whether the boiler makes any unusual noise before stopping.

Keep these as observations, not conclusions. They can shorten the conversation with the engineer without encouraging unsafe investigation.

Should you add water to the system?

Do not top up solely because H.00.42 is displayed. The code concerns the pressure-sensor signal, and adding water will not repair a broken signal path.

If the external gauge also appears low, consult the user guide for the exact boiler and installation. Only use a normal homeowner filling method if it is clearly identified, you understand the instructions and there are no visible leaks. Stop if you are uncertain.

Never keep adding water to compensate for repeated pressure loss. Repeated topping-up can mask a separate problem and is information an engineer needs to know.

If the gauge appears unusually high, do not drain water, operate relief devices or loosen fittings. Switch off demand using the normal controls and arrange professional advice.

Can you reset H.00.42?

Follow the Baxi user guide for your exact model. If it allows a normal user reset for the displayed condition, carry that out once and note the result.

Do not repeat resets or cycle the power over and over. If the code returns, remains on the display or the boiler does not work normally, book a repair. Repeated resets do not repair a sensor or signal fault and can make an intermittent pattern harder to explain.

Do not enter installer menus, change parameters or follow online instructions for testing the sensor. Electrical measurements and internal access belong to a suitably qualified engineer.

Why professional repair is appropriate

H.00.42 refers to information used by the boiler to judge the heating system’s condition. A reliable repair requires the engineer to determine why that information is not being received or accepted.

That can involve checks behind the closed casing and interpretation of model-specific information. It may require work around electrical connections, water-bearing components and boiler controls. Those are not safe DIY jobs.

A Gas Safe registered engineer can assess the boiler and installation, use the manufacturer’s technical information and identify the actual remedy. The aim is to correct the fault found, not simply to replace the part named in the short display description.

When to stop and seek urgent help

H.00.42 does not, by itself, mean there is a gas leak or carbon-monoxide escape. Treat actual warning signs separately.

If you smell gas, avoid electrical switches and naked flames, open doors and windows if safe, leave the building and call the National Gas Emergency Service on 0800 111 999 from outside.

If a carbon-monoxide alarm sounds or anyone has headache, dizziness, nausea, weakness or confusion, move into fresh air, seek medical attention and arrange an urgent Gas Safe inspection. Do not reuse the boiler until it has been declared safe.

If water is leaking near the boiler or electrical equipment, keep away from the wet area and obtain urgent professional help. Do not touch wet switches, plugs or appliances.

What not to do

Do not:

• remove the boiler casing;
• disconnect or test the pressure sensor;
• probe wiring, terminals or the circuit board;
• measure resistance, continuity or voltage;
• loosen water connections;
• drain the system in response to the code;
• repeatedly refill or repeatedly reset the boiler;
• buy a replacement sensor before the fault has been confirmed.

The phrase “open or faulty” is a fault description, not a safe homeowner test procedure.

Quick Answers

What does Baxi H.00.42 mean?

Baxi lists H.00.42 as “Pressure sensor open/faulty” for its Combi & System 2 group, including the Baxi 800 System 2.

Does the code prove the pressure sensor has failed?

No. It describes the detected condition, but the cause must be confirmed. The sensor, a connection or another part of the signal path may need assessment.

Can the external gauge look normal?

Yes. The external analogue gauge and the electronic signal used by the boiler are not identical. Record the gauge position, but do not use it to dismiss the code.

Should I top up the boiler?

Not because of H.00.42 alone. Follow only the exact user guide if the external gauge is low and the normal filling method is clear. Never keep topping up a system that loses pressure.

Can I continue using the boiler?

If the warning persists, operation is unreliable or heating has stopped, arrange repair. Stop immediately if there is a leak, gas smell, carbon-monoxide alarm, smoke or scorching.

What information will help the engineer?

Share a photo of the code, the external gauge position, whether the code is constant or intermittent, which services are affected and whether recent work or pressure changes preceded it.