Baxi 600 Combi 2 E.04.18 fault code: low flow temperature or sensor not connected

Baxi lists E.04.18 on the Baxi 600 Combi 2 with two possible meanings: the flow temperature is below the minimum temperature, or the flow-temperature sensor is not connected. The display does not tell a homeowner which of those two conditions applies, and it does not prove that the sensor itself needs replacing.

The boiler may stop or restrict operation because it cannot rely on the flow-temperature information it is receiving. Heating, hot water or both may be affected. This is a fault for professional investigation; the useful homeowner task is to record what happened and avoid opening the boiler or testing electrical parts.

What “flow temperature” means

When the boiler heats water for the central-heating circuit, that heated water leaves the boiler through the flow pipe and travels towards the radiators. The flow-temperature sensor gives the boiler information about how hot that water is. The controls use this information to manage operation safely.

E.04.18 means the boiler has detected either a flow temperature below its minimum expected value or a missing connection to the flow-temperature sensor. Those alternatives can look similar from the front of the appliance because both affect the information available to the boiler. The code alone cannot separate them.

Why this is not a definite sensor diagnosis

The manufacturer’s wording includes “sensor is not connected”, but that describes the condition detected by the control system. It does not confirm that the sensor body is the faulty part. A suitably qualified engineer may need to consider the sensor, its connection, associated wiring, the control board and the actual temperature behaviour of the appliance.

Homeowners should not attempt continuity, resistance or voltage tests. Those checks require the boiler casing to be opened and involve components that must be assessed safely and in context. Replacing a part because its name appears in the code can waste money and leave the real problem unresolved.

What you may notice at home

The Baxi 600 Combi 2 may remain stopped with E.04.18 on its display. Radiators may not warm, a hot tap may stay cold, or the boiler may appear to begin operating before the code returns. The exact symptom depends on when the reading becomes unacceptable and what demand the boiler was trying to meet.

You may also notice that the boiler is unusually quiet when heating is requested, or that the usual heating symbol appears briefly and then disappears. These observations are worth recording, but they do not reveal whether the temperature was genuinely low or the signal was missing.

Safe information to collect

Record the exact code

Write down E.04.18 exactly, including the full stops. Similar-looking Baxi codes have different meanings. A photograph of the display can help if it can be taken without touching or opening the appliance.

Note which service is affected

Check whether the problem affects central heating, hot water or both. Try only the normal user controls and one ordinary demand; do not repeatedly turn services on and off to force the code to reappear.

Check the controls at user level

Make sure the thermostat or programmer is asking for heat and that the boiler has electrical power. If other electrical equipment is also behaving abnormally, tell the relevant professional. Do not remove covers, open a fused connection unit or investigate the boiler’s wiring.

Look and listen without dismantling

From the normal user position, note whether there are unusual noises, warning symbols or visible water around the appliance. Do not touch hot pipework, remove the front panel or reach underneath the boiler to inspect internal components.

Is a reset appropriate?

Follow the Baxi 600 Combi 2 user guide for normal controls. If the guide permits a reset for this condition, carry it out once. If E.04.18 remains or returns, stop resetting and arrange a repair. A reset cannot restore a disconnected signal or correct a genuine temperature condition.

Repeated resets also make it harder to describe the original pattern. Leave the code displayed where practical and give the engineer the observations you recorded.

What not to do

• Do not remove the boiler casing or look for the flow-temperature sensor.
• Do not unplug, reconnect or move wires and connectors.
• Do not use a multimeter or attempt resistance, continuity or voltage checks.
• Do not replace the sensor, circuit board or wiring based on the display alone.
• Do not alter installer settings or enter service menus.
• Do not try to heat the sensor, cool pipework or manipulate the boiler into a different reading.

These actions can create electrical, gas or hot-water risks and may obscure the actual fault. Work inside the boiler must be completed by a suitably qualified Gas Safe registered engineer.

When repair is needed

Book a repair when E.04.18 persists, returns after one permitted reset, or makes heating or hot water unreliable. You should not wait for both household services to fail: the boiler has already reported that an important temperature condition or signal is outside what it expects.

If the property is very cold, protect vulnerable occupants with safe temporary warmth while the repair is arranged. Never use an outdoor appliance, barbecue or unflued improvised heater indoors.

Gas and carbon-monoxide safety

E.04.18 is a temperature or sensor-connection code, not a declaration that gas is leaking. Normal gas-safety rules still apply. If you smell gas, suspect carbon monoxide, hear a carbon-monoxide alarm, or someone develops headache, dizziness, nausea or unusual weakness, stop using the appliance, move into fresh air and follow UK gas-emergency guidance.

Do not remove the case to investigate. A display code cannot confirm that a separate smell, alarm or symptom is harmless.

What the engineer will need to know

Provide the exact Baxi 600 Combi 2 model and code, when it first appeared, which services were affected, and whether the display is constant or intermittent. Mention any unusual noise, visible water or wider electrical problem. If the code followed a power interruption or recent work, report that fact without assuming it caused the fault.

The engineer can then compare the actual flow temperature with the signal received by the boiler and safely check the relevant connections and controls. That evidence is what distinguishes the two manufacturer-listed possibilities.

Quick Answers

What does Baxi 600 Combi 2 E.04.18 mean?

It means the flow temperature is below the minimum temperature or the flow-temperature sensor is not connected.

Does E.04.18 prove the sensor has failed?

No. The code includes two possibilities and does not identify one definite failed component.

Can I reconnect the sensor myself?

No. It is inside the boiler and requires safe professional checks. Do not open the casing or touch wiring.

Will resetting clear the problem?

A single reset may be allowed by the user guide, but a returning code needs repair. Repeated resets do not correct the temperature condition or missing signal.