Overview
Boiler model: Baxi 816 Heat / 825 Heat / 830 Heat. Error: 5 red flashes. This indicates a parameter fault — a communication or configuration error between the boiler PCB and the combustion/control unit (often referred to as the PU or power/combustion control unit). In plain terms the boiler has detected incorrect or missing parameters or a failure in the control electronics and will lock out or refuse to run until the fault is cured. Why it occurs and severity: Causes are typically a bad or loose electrical connection between the PCB and the PU, a corrupt or incorrect parameter set, water or heat damage to the PCB/wiring loom, or a failed PU/combustion control module. This is a relatively serious fault because it affects core electronics that control ignition and safety functions; the boiler may repeatedly lock out and will not reliably operate until repaired. Because the fault relates to internal electronics and gas-safe components, diagnosis beyond basic checks should be done by a Gas Safe registered engineer. DIY vs professional: You can safely carry out basic, non-invasive checks (reset, visual inspection of accessible wiring, check system pressure and for obvious leaks). Do not attempt to replace or repair the PCB, PU, or internal wiring unless you are a qualified heating engineer. Electronic module replacement and any work involving the gas or sealed combustion parts must be carried out by a qualified professional.
Possible Cause: Bad connection to PU. Parameter error or faulty PU.
Troubleshooting Steps
Safety first
1) If you smell gas: stop immediately, do not operate electrical switches, ventilate the area, turn off the gas supply at the meter if you can do so safely, and call the emergency gas number and a Gas Safe engineer.
2) If the boiler is displaying the 5-red-flash fault repeatedly, do not keep trying to run it. Repeated resets may cause further lockouts or stress to components.
3) If you are not competent with electrical or gas appliances, do not open the boiler or touch internal wiring. Call a Gas Safe registered engineer.
Initial homeowner checks (non-invasive)
1) Note down model and serial number and when the fault started, including any other symptoms (loss of hot water/heat, recent power cuts, water leaks). This information helps the engineer.
2) Try a controlled reset: follow the boiler manual reset procedure (press and hold the reset or selector R for 5–10 seconds). If the fault clears and the boiler runs, monitor for recurrence. If it returns or recurs frequently, call an engineer.
3) Power-cycle the boiler: switch off the electrical supply at the isolator or fuse box, wait 5–10 minutes, then switch back on and try starting. If a parameter fault is transient this may clear it once; if it doesn't, do not keep power-cycling repeatedly.
4) Check system pressure: look at the pressure gauge (aim for approximately 1–1.5 bar when cold on most domestic Baxi systems). If pressure is very low, top up using the filling loop following the manual. Low pressure can cause separate faults but will not usually fix a parameter/PCB communication fault — still worth checking as a basic step.
5) Visual external checks: look for obvious signs of water leaks around the boiler and pipework, visible corrosion, or scorch/burn marks on the cabinet. Any water ingress can damage electronics and justify turning the boiler off until an engineer attends.
Limited safe diagnostic checks you can do if competent with basic DIY and the boiler is isolated from mains
1) Isolate electrical power at the mains before opening the outer casing. Only proceed if you are confident in doing this safely.
2) With power isolated and the cover removed, do a visual inspection of the PCB, PU connector and wiring loom for loose connectors, disconnected plugs, corrosion, burnt components, or water residue. Do not touch components with power applied.
3) If you find a clearly loose plug or connector that is simply unseated, you may reseat it carefully (ensure power is off, push the connector fully home). Do not force connectors or repair wiring yourself beyond reseating.
4) If you see damaged or burnt wiring, melted connector housings, or water-damaged components: do not attempt repair. Reassemble the boiler, restore power, and turn the boiler off. Call a Gas Safe engineer.
When to call a professional and what to tell them
1) Call a Gas Safe registered engineer if the 5-red-flash fault persists after a reset and basic checks, if you find any damage, or if you are not confident performing the limited checks above.
2) Tell the engineer: boiler model and serial, that the fault is 5 red flashes (parameter fault), what you have already tried (reset, power-cycle, pressure check), and any visible findings (loose connector, water ingress, burnt smell, repeated lockouts). This helps them bring appropriate parts (PCB/PU) and diagnostic equipment.
3) Likely professional work: the engineer will perform live diagnostic checks, read internal service logs/fault counters, check communications between PCB and PU, inspect and test the wiring loom, and if required replace the combustion control unit (PU) or PCB. They will also ensure gas and safety settings are correct and run ignition and flue safety checks.
Final notes
- This fault is normally not a simple homeowner repair. While you can perform safe, external checks and a reset, the root cause is often internal electronics, a failed control module, or damaged wiring and requires a qualified engineer.
- Do not attempt to operate the boiler when the fault persists. If in doubt, isolate power and call a Gas Safe engineer for a correct, safe repair.
Helpful Resources
Boiler Manual
Download the official PDF manual for the Baxi 816 Heat, 825 Heat, 830 Heat.