Baxi’s current error-code table defines E.04.18 as: “The flow temperature is below the minimum temperature or the flow temperature sensor is not connected.” On a Baxi 800 Combi 2, the code therefore presents two separate possibilities.
It does not confirm whether the water temperature is actually below the expected minimum or whether the boiler is missing the required sensor signal. It also does not prove the sensor itself has failed. Keep the casing closed and arrange repair if the fault remains or returns.
What E.04.18 means
“Flow temperature” is the temperature of heating water as it leaves the boiler for the heating system. The boiler monitors that temperature as part of normal and safe operation. E.04.18 appears when the monitored condition matches either a below-minimum reading or a flow-temperature sensor that is not connected.
These possibilities must remain distinct. A low reported temperature is not the same as an absent sensor signal. The display alone cannot tell a homeowner which applies, whether the reading reflects the actual water temperature or what work is required.
Baxi lists E.04.18 for the current Combi & System 2 group, including the Baxi 800 Combi 2. Photograph every digit because neighbouring codes describe different flow-temperature conditions.
What you may notice
The boiler may stop heating, fail to respond normally to the thermostat or show the code while remaining powered. As a combi boiler, the Baxi 800 Combi 2 also provides hot water at taps, so both services can be interrupted.
Radiators may stay cool even though the boiler display is on. Tap water may be cold or inconsistent. Do not use those symptoms to decide between the official possibilities; both can lead to similar household effects.
Safe observations to record
- Photograph the full E.04.18 display and any accompanying symbol.
- Note whether heating, tap hot water or both were requested.
- Record whether the fault appeared immediately or after the boiler had been running.
- Note whether radiators stayed cool without touching boiler pipework.
- Record any recent professional work near the heating system.
Do not touch pipes or the boiler casing to judge the flow temperature. Parts can be hot even when a displayed condition includes the words “below minimum”.
Why a low reading is not proof of cold pipework
The code reports what the boiler’s monitoring has detected. It does not provide a safe surface-temperature guide and does not mean every part of the heating circuit is cold. Water and metal can retain heat after operation stops.
Keep children and pets away from the appliance and nearby pipework. Do not loosen fittings, drain water or open radiator valves in an attempt to check temperature or circulation.
Why “not connected” does not prove a loose plug
In fault-code language, “not connected” describes the signal state seen by the boiler. It does not authorise a homeowner to look for a connector, and it does not prove that an accessible plug has come loose.
Do not remove the casing, trace wires, test resistance or continuity, or buy a sensor based on the code. A qualified engineer must assess the appliance and distinguish the two official possibilities.
Can you reset E.04.18?
Use the user guide for the exact Baxi 800 Combi 2. If it expressly permits a homeowner reset for this displayed fault, one reset may be attempted. Stop if the code remains, returns or the boiler behaves unexpectedly.
Do not reset repeatedly. A temporary return to heating or hot water does not show that the monitored condition has been resolved.
When repair is needed
Book a suitably qualified Gas Safe registered engineer when E.04.18 persists, comes back, or leaves heating or hot water unreliable. The engineer will need the exact code and the circumstances in which it appeared, not a homeowner guess about a particular part.
Share the display photograph and note whether the fault occurred on heating or hot-water demand. Mention recent servicing or heating-system work without opening the appliance to investigate.
If there is water or an electrical hazard
If water is near the boiler or electrical fittings, do not touch the appliance, switches or wet equipment. Keep people away and seek urgent professional help. Isolate power only if that can be done safely from a dry, undamaged location.
If you see smoke, scorching or exposed wiring, leave the area and contact the appropriate emergency service.
Gas and carbon-monoxide safety
E.04.18 is a flow-temperature code, not a carbon-monoxide alarm. If you smell gas, a carbon-monoxide alarm sounds, or anyone develops headache, dizziness, nausea, confusion or unusual weakness, stop ordinary fault-code checks.
Move everyone into fresh air, leave the property and follow current emergency advice from the Gas Emergency Service and Gas Safe Register. Do not return to attempt another reset.
Why one normal heating cycle is not proof of repair
The code may clear temporarily or the boiler may respond after a reset. That does not establish which official possibility occurred or show that the sensor signal is reliable. If E.04.18 returns, stop testing and retain the timeline for the engineer.
Similarly, a cool radiator is not a measurement of boiler flow temperature. Room controls, timing and retained system heat can change what you feel at a radiator. Use household symptoms only as context, not as evidence of the cause.
Keeping comfortable while waiting
Use only suitable portable electric heating according to its instructions, keep it clear of fabrics and avoid overloaded sockets. Never use an oven, barbecue or outdoor heater indoors. Make prompt arrangements for anyone vulnerable to cold.
Quick answers
What does Baxi E.04.18 mean?
It means the flow temperature is below the minimum or the flow-temperature sensor is not connected.
Does E.04.18 prove the sensor has failed?
No. It presents two possibilities and does not identify the underlying reason.
Can I check the sensor connection?
No. The sensor and its connection are inside the closed appliance and require qualified assessment.
Why can hot water be affected?
The Baxi 800 Combi 2 supplies heating and tap hot water, so stopped boiler operation can interrupt both.
Should I keep resetting it?
No. Use only one reset if the exact user guide permits it, then arrange repair if the code remains or returns.
Need the boiler repaired?
Your boiler fault is fixed for the agreed price, subject to eligibility and terms.