Overview
Error 433 on a Baxi EcoBlue means the boiler has detected the heat exchanger temperature has exceeded the safe limit (reported as greater than about 95°C). The boiler trips the fault to protect the appliance and the heating circuit from overheating. Common underlying reasons are poor or no water circulation through the primary heat exchanger (blocked pipework, seized or faulty pump, closed isolation valves, airlocks), restricted heat transfer inside the exchanger (scale, sludge or blockage), or a faulty temperature sensor/thermistor falsely reporting a high temperature. Severity is moderate to high: the boiler is safe in that it will usually lock out to prevent further overheating, but repeated overheating can damage components (heat exchanger, seals, sensors) and there is a scald risk with hot water. Some basic checks can be done by a homeowner, but because this relates to circulation, gas-fired components, and safety limits, most diagnostic and repair work should be carried out by a Gas Safe registered engineer if the fault does not clear after simple checks and safe resets.
Possible Cause: Heat exchanger temperature exceeded (greater than 95 degrees)
Troubleshooting Steps
Safety precautions:
- If you smell gas, do not touch the boiler: turn off gas at the meter if you can do so safely, ventilate the area, leave the property and call the gas emergency number immediately.
- Do not keep repeatedly resetting the boiler; repeated resets can cause more damage and may hide a developing fault.
- Turn off electrical supply to the boiler before removing any covers. Do not open sealed or gas-carrying components. If you are not competent, stop and call a Gas Safe engineer.
Initial homeowner checks you can safely do:
1. Let the boiler cool for at least 20–30 minutes after it has locked out before doing anything.
2. Note any symptoms: are radiators cold, is hot water affected, are there unusual noises, any recent plumbing work? This helps narrow cause (circulation vs sensor).
3. Check the system pressure on the boiler gauge. Normal is generally around 1–1.5 bar when cold. If pressure is very low (<0.5 bar) or very high (>2.5–3 bar), top up or isolate and seek professional help as appropriate.
4. Check that radiator thermostatic valves or lockshield valves are open and that the boiler flow/return isolation valves (if fitted) are open.
5. Bleed one or two radiators (start with those nearest the boiler) to remove airlocks — trapped air can stop circulation and cause overheating.
6. Check if the central heating pump is running: with the power on you may feel slight vibration through the pump body or hear it. Do not touch moving parts. If the pump is silent when the boiler is calling for heat, circulation may be the problem.
7. Try a single reset of the boiler using the reset button according to the manual after the unit has cooled. If the fault returns immediately or after a short run, do not keep resetting.
Specific diagnostic and next-step actions (if you are safe and comfortable performing basic checks):
1. After cooling and one safe reset, run the heating and watch radiator temperatures. If radiators remain cold but the boiler fires and the exchanger overheats, suspect circulation fault (pump, blocked pipework, closed valves, magnetic filter full or sludge in heat exchanger).
2. If the pump appears not to run, do not dismantle it unless qualified. Isolate electrical supply and call a Gas Safe engineer to inspect/replace the pump or wiring.
3. If you have a magnetic filter or system filter fitted, check any visible isolation valves are open. Do not open the filter housing unless you are competent; a full/blocked filter requires cleaning and may need an engineer.
4. If the boiler stops overheating when you open a radiator fully or adjust a bypass, that indicates a circulation restriction you will need an engineer to clean/flushing the system, remove sludge, or replace the pump/valves.
5. If circulation appears fine but the boiler still reports over-temperature, the temperature sensor (NTC) or PCB could be faulty and falsely reporting high temperature. Faulty sensors or wiring require an engineer to test and replace.
6. If the fault is linked to domestic hot water only or intermittent, the diverter valve or flow sensor may be sticking or failing — again, a Gas Safe engineer should diagnose and replace these parts.
When to call a professional:
- If the error does not clear after the simple checks above (bleeding radiators, checking pressure, one safe reset).
- If you suspect the pump is seized, the magnetic filter is blocked, there is sludge/scale in the heat exchanger, or any electrical/component fault (sensor, wiring, PCB, diverter valve).
- Immediately if you smell gas or if the boiler repeatedly locks out or makes unusual noises.
What the engineer may do (for information only):
- Check flow and return temperatures, measure circulation flow, test pump operation and electrical supply, inspect and clean/replace magnetic filter, check and test temperature sensors, inspect heat exchanger for scale/blockage, test diverter valve operation, and replace faulty components. In some cases a powerflush or chemical clean of the system is recommended if sludge is present.
Final note:
This fault protects the boiler but indicates a real circulation or sensor problem. Homeowner checks are limited to non-invasive tasks (bleeding radiators, checking pressure and valves, one reset). For diagnosis beyond that and for any repairs involving gas, the correct action is to contact a qualified Gas Safe engineer.
Helpful Resources
Boiler Manual
Download the official PDF manual for the Baxi EcoBlue.