Overview
Six red flashes on a Baxi 816/825/830 Heat typically indicates a miscellaneous fault related to communications, parameters or a wiring/connection problem between the boiler's control electronics (PCB/combustion control unit/PU) and other modules. In plain terms the boiler is seeing either corrupted configuration data, a loss of communication between internal units, or a poor/failed electrical connection. This will usually cause the boiler to lock out or refuse to fire until the fault is cleared or repaired. The fault is moderate to high severity in the sense that it will stop heating and hot water and indicates an electrical/controls issue that can affect safe operation if ignored. It is not normally an immediate gas-leak danger, but because it involves control electronics that govern ignition and safety interlocks you should not attempt invasive electrical or gas work yourself. Basic checks and resets can be done by a homeowner, but diagnosis beyond checking connectors, resetting and reading/logging the displayed error requires a qualified Gas Safe engineer with service tools or Recom software, and replacement of the combustion control unit or PCB if faulty.
Possible Cause: Bad connection or wiring fault internal or external to the boiler. Parameter error. Communication error.
Troubleshooting Steps
Safety precautions:
- If you smell gas or suspect a gas leak, leave the property immediately and call your gas emergency number. Do not attempt any checks.
- Before opening the boiler casing or touching internal components, isolate the boiler from the mains electricity at the fuse spur and turn the gas supply off. If you are not competent with electrical work, do not open the unit.
- Only qualified Gas Safe engineers should carry out wiring or PCB/combustion control unit replacements.
Initial homeowner checks you can safely do:
1. Check power and reset: Verify the boiler display has power. Try a simple reset using the boiler reset procedure (hold the reset button or turn the selector to R for 3–5 seconds depending on your model) and see if the 6-flash condition clears. If it clears but returns, do not repeatedly reset — record the behaviour and call an engineer.
2. Check external controls: Ensure room thermostat, programmer/timer and any remote controls have power and correct settings. Replace batteries in wireless controls. Turn external controls to call for heat/hot water to confirm the boiler is being asked to run.
3. Check mains fuse and isolator: Verify the boiler’s fused spur or RCD has not tripped and the fuse in the consumer unit hasn’t blown.
4. Visual external inspection: With the boiler powered off, look for obvious signs of damage to cables entering the boiler (chewed, corroded, water-damaged) and ensure terminal covers are present. Do not remove internal covers if you are not competent or qualified.
Specific diagnostic and safe basic fix steps (only if comfortable and experienced with safe isolation):
1. Power down safely: Turn off mains power to the boiler at the isolator and switch off the gas if you will open the casing. Wait a few minutes for capacitors to discharge.
2. Check and reseat plug-in connectors: Open the outer cover and visually inspect connectors to the PCB and combustion control unit. Look for loose connectors, corrosion, damp, or burn marks. If connectors are visibly loose, you may carefully unplug and reseat them to ensure good contact. Do NOT cut or re-terminate wiring unless you are qualified.
3. Inspect for water ingress or burnt components: If you see corrosion, moisture, or burnt/blackened components on the PCB or wiring, do not attempt to operate the boiler — this indicates a need for professional repair.
4. Check fault logs and parameters: If you have access to the Baxi service tool or Recom software (or an engineer has one), read the action log and parameter settings. A parameter error or corrupted configuration can often be identified by the service tool; some parameter resets or firmware updates can only be performed by a qualified engineer.
5. Test external wiring continuity (qualified persons only): If you are competent with electrical testing, check continuity of control wiring from remote thermostats, programmers and room stats back to the boiler terminals. Faulty or shorted external control wiring can produce communication/parameter faults. Do not perform live tests on mains circuits unless you are trained.
6. Replace suspected removable module only by a Gas Safe engineer: If all wiring and parameters check out but the fault persists, the combustion control unit or PCB (also called CSU or PU) is a common item to fail for this error. Replacement and re-configuration must be carried out by a Gas Safe registered engineer and may require setting correct parameters via service software.
What to record for the engineer:
- Note when the fault appeared and any events before it (power cut, lightning, work on system).
- Take photos of the display showing the 6 flashes and of wiring terminals if safe to do so.
- Note any other symptoms (no flame, intermittent operation, other error lights, water on or around PCB).
When to call a professional:
- If the fault does not clear with simple checks and a reset.
- If you find damp, burnt components, damaged wiring, or cannot identify the source of the communication/parameter fault.
- For any work that involves opening, repairing, or replacing the PCB, combustion control unit, gas valve or internal wiring.
Hire a Gas Safe registered engineer who can run the service tool/Recom software, read the action log, check and correct parameters, and replace faulty control units safely. Provide the engineer with the error detail and any photos/logs you captured to speed diagnosis.
Helpful Resources
Boiler Manual
Download the official PDF manual for the Baxi 816 Heat, 825 Heat, 830 Heat.