- Boilers
- Gas boiler
E.04.08
Baxi E.04.08 on a gas boiler: maximum temperature exceeded
Risk level
High
Professional inspection is recommended.
Urgency
High
Quick answer
E.04.08 on Baxi Neodens Lite gas boilers is a permanent lockout showing that the maximum temperature limit has been reached. Baxi links it either to insufficient circulation (system pressure, trapped air, pump fault or blocked flow) or to the safety thermostat and its wiring.
Safety first
Stop and check this first
- Never bypass or modify the safety thermostat, and always disconnect the electrical supply, close the gas valve and let the heating water cool before any internal maintenance.
- If you smell gas, do not light any flame or touch electrical switches, turn off the gas supply valve, open the windows, evacuate the property and notify a qualified professional.
Safe checks you can perform
- Check cold system pressure on the display and open radiator valves
- Perform a single manual reset once the boiler has cooled down
- Run the manual air purge (DEAIR) and test pump, flow and safety thermostat
Leave these tasks to a qualified professional
- Do not attempt this professional task: Check the safety thermostat connection and operation
- Do not attempt this professional task: Run the manual air purge function and verify pump and system circulation
What a technician must be able to justify
Technician visit
These are the steps they must carry out and show you. Tick them as they work: if steps are skipped, do not accept a part swap.
These are not instructions for opening or handling the equipment yourself. They are the checks that should support a technical conclusion.
Check the safety thermostat connection and operation
What they doWith the boiler isolated from the mains and cooled down, the technician inspects the safety thermostat wiring and tests that the safety thermostat opens and closes properly, replacing it with an original Baxi part if defective.
They must show youthe safety thermostat connector and the electrical test reading across the thermostat.
In writingthe test result on the safety thermostat and its wiring, the part number if replaced and the service date.
Run the manual air purge function and verify pump and system circulation
What they doThe technician checks system water pressure, activates the manual air purge function (DEAIR / AIR in the installer menu) and verifies that the pump works and water circulates freely through the boiler and heating system.
They must show youthe DEAIR / AIR purge cycle running, the pump operating and steady heat transfer along the flow and return pipes.
In writingcode E.04.08, the cold system pressure in bar, the air purge completed and the condition of the pump and heating circulation.
Before accepting a part or repair
Nothing has been shown yet. Do not accept a part swap based on the code alone.
Follow these steps
Note: Follow the documented order. An error code identifies the affected system, but it does not prove by itself which part has failed.
Check cold system pressure on the display and open radiator valves
- Who:
- you, looking at the boiler screen and the radiator valves.
- How:
- when the heating system is cold, check that the pressure reading on the display sits between 1.0 and 1.5 bar (Baxi states to top up if it drops below 0.8 bar) and that radiator valves are open.
- Result that matters:
- making sure the heating circuit has enough water and an open flow path before resetting.
- If cold pressure is below 0.8 bar on a combi model, pull down the light-blue filling knob (A) underneath the boiler, turn it slowly anticlockwise by hand without tools until pressure reaches 1.0 to 1.5 bar, then close it firmly.
- If pressure is already between 1.0 and 1.5 bar and radiator valves are open, wait for the boiler to cool down before trying a single reset.
- Evidence to keep:
- the cold pressure reading in bar before and after checking the filling tap and radiator valves.
Evidence: Baxi Neodens Lite 24 AF, 24/24 F y 28/28 F, manual de usuario, instalación y mantenimiento 7892434
Perform a single manual reset once the boiler has cooled down
- Who:
- you, from the front control panel.
- How:
- after verifying pressure and letting the pipes cool, press the RESET key (F2) for 1 second once to clear the permanent E.04.08 lockout.
- Result that matters:
- checking whether the overheat trip came from a temporary flow restriction that has now cleared.
- If E.04.08 clears and the boiler heats quietly without gurgling or rapid temperature spikes, monitor the pressure reading over the next few days.
- If E.04.08 returns or you hear boiling or air noises, stop resetting and call a qualified technician as Baxi advises for recurring faults.
- Evidence to keep:
- whether E.04.08 returned after pressing RESET (F2) for 1 second and what flow temperature was shown.
Evidence: Baxi Neodens Lite 24 AF, 24/24 F y 28/28 F, manual de usuario, instalación y mantenimiento 7892434
Run the manual air purge (DEAIR) and test pump, flow and safety thermostat
- Who:
- qualified installer or authorised Baxi service technician.
- How:
- run the manual air purge function (DEAIR / AIR) from the installer menu, verify that the pump runs and water circulates through the boiler and system, and check the connection and operation of the safety thermostat.
- Result that matters:
- clearing airlocks or identifying whether the pump, circuit circulation or safety thermostat is at fault.
- If trapped air or a stuck pump is found, complete the DEAIR cycle, restore circulation and bleed the radiators as set out in the manual.
- If water circulation is normal but the safety thermostat or its wiring is faulty, repair the connection or replace the safety thermostat with an original spare part.
- Evidence to keep:
- the outcome of the DEAIR purge cycle, the pump operation check and the continuity test on the safety thermostat.
Always isolate the electrical supply, close the gas valve and wait for the heating water to cool down before working inside the boiler to prevent electric shock and scalding.
Evidence: Baxi Neodens Lite 24 AF, 24/24 F y 28/28 F, manual de usuario, instalación y mantenimiento 7892434
Still not solved
If the error persists, stop here. See what the technician must do and show you.
Where this code applies
A code is meaningful only inside the right product and version context.
Primary scope
- System
- Boilers
- Brand
- Baxi
- Product type
- Gas boiler
Known code variants
- Baxi E.04.08
- Baxi error E.04.08
- E 04 08
- E.04 .08
- E.04.08
- E04.08
- E0408
- Error E.04.08 Baxi
- Neodens Lite E.04.08
What it means
In Table 39 (permanent faults requiring a reset) of the Baxi Neodens Lite 24 AF, 24/24 F and 28/28 F manual, code E.04.08 means the maximum temperature limit has been reached, shutting the boiler down until a manual reset is performed. Baxi divides the checks into two groups: insufficient circulation (check system pressure, activate the manual air purge function, check that the pump works and check circulation through the boiler and heating circuit) and other possible causes (check the safety thermostat connection and test that the safety thermostat operates properly). Seeing E.04.08 alone does not prove the pump or safety thermostat has failed.
Warnings and stop conditions
- Highsource verified
Never bypass or modify the safety thermostat, and always disconnect the electrical supply, close the gas valve and let the heating water cool before any internal maintenance.
Evidence: Baxi Neodens Lite 24 AF, 24/24 F y 28/28 F, manual de usuario, instalación y mantenimiento 7892434
- Criticalsource verified
If you smell gas, do not light any flame or touch electrical switches, turn off the gas supply valve, open the windows, evacuate the property and notify a qualified professional.
Evidence: Baxi Neodens Lite 24 AF, 24/24 F y 28/28 F, manual de usuario, instalación y mantenimiento 7892434
Symptoms and causes
Probable causes · in order
- Low system water pressure or air trapped in the heating loopCommon
Cold system pressure below the recommended 1.0 to 1.5 bar range or trapped air pockets prevent heat from leaving the primary heat exchanger and trip E.04.08.
Evidence: Baxi Neodens Lite 24 AF, 24/24 F y 28/28 F, manual de usuario, instalación y mantenimiento 7892434
- Insufficient water circulation through the boiler or heating systemCommon
Closed radiator valves, a blocked heating pipe or a stalled circulation pump stops water flow through the boiler and drives temperature up to the maximum limit.
Evidence: Baxi Neodens Lite 24 AF, 24/24 F y 28/28 F, manual de usuario, instalación y mantenimiento 7892434
- Faulty safety thermostat connection or safety thermostat operationPossible
A loose or damaged connector on the safety thermostat wiring or a faulty safety thermostat opening prematurely triggers the permanent E.04.08 lockout.
Evidence: Baxi Neodens Lite 24 AF, 24/24 F y 28/28 F, manual de usuario, instalación y mantenimiento 7892434
Virtual technician
Does E.04.08 still appear after these checks?
Answer a few questions to narrow down the likely cause and the safest next action. Guidance only - it never replaces a professional.
Sources and technical references
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