Overview
This fault (5 flashes green on Baxi 816/825/830 Heat models) is reported as a parameter fault. In short, the boiler’s control electronics have detected a problem with configuration or communication parameters — often described as a parameter error or a faulty PU (power/processing unit) — and the board may not be accepting or reading required settings. That can be caused by corrupted configuration data, a failing PCB/combustion control module, or an intermittent electronics/communication fault. Severity is moderate to high. The fault can put the boiler into lockout and will prevent normal operation of heating and hot water until resolved. Sometimes parameter faults are temporary and can clear with a proper reset, but repeated occurrence usually indicates an underlying hardware or configuration issue that needs professional diagnosis. This is not a straightforward DIY repair: initial checks and resets can be carried out by a competent homeowner, but replacing or reconfiguring the PU/PCB, checking internal wiring, or changing firmware/settings must be left to a Gas Safe–registered engineer or authorised Baxi technician for safety and warranty reasons.
Possible Cause: Parameter error or faulty PU. Configuration error or PCB fault.
Troubleshooting Steps
Safety precautions:
- If you smell gas, leave the property immediately and call the gas emergency number — do not attempt any checks.
- Switch off electrical power to the boiler at the isolator before touching anything behind the front panel.
- If you are not comfortable working around gas or mains electricity, stop and call a Gas Safe engineer.
Initial checks a homeowner can do (no tools or only simple tools):
1. Note the exact fault display and any other lights or codes. Photograph the display for the engineer.
2. Check the boiler’s supply: confirm electricity to the boiler is on and the mains fuse/trip hasn’t operated. Confirm gas supply is on (check another gas appliance or meter and, if you have a prepayment meter, that there is credit).
3. Check system water pressure on the pressure gauge. Aim for roughly 1–1.5 bar when cold. If pressure is very low, top up via the filling loop following the boiler manual, then reset the boiler and see if the fault clears.
4. Try a controlled reset: find the reset button or selector, press and hold as per the manual (typically 3–10 seconds) and allow the boiler to restart. Do not keep repeatedly resetting more than once or twice. If the error returns immediately or repeatedly, stop and call an engineer.
5. Check for obvious external issues: ensure the flue terminal is not blocked, the condensate pipe is not frozen or blocked, and there are no visible leaks or water ingress near the boiler cupboard/electronics.
If initial checks do not clear the fault (diagnostic steps for an engineer / advanced checks):
1. Record fault history and any patterns (e.g. after power cuts, after topping up pressure, intermittent). This helps diagnosis.
2. With power isolated, inspect internal connectors for loose or corroded plugs between the combustion control unit (PU) and the main PCB. Re-seat connectors only if qualified to do so.
3. Check for visible PCB damage: burnt components, blown capacitors, water damage or corrosion. Photographs are useful for later reporting.
4. An authorised engineer will verify communication between modules, check parameter/firmware integrity and configuration settings, and may attempt a diagnostic reset of parameters if supported. If configuration has become corrupted the PU or PCB may require reprogramming or replacement.
5. The manufacturer-suggested fix for persistent parameter/PU faults is replacement of the combustion control unit (PU) or the PCB. This is not a DIY job — it requires a Gas Safe engineer to fit the correct parts, configure parameters, and test safe operation (gas tightness, combustion, flame sensing, safety interlocks).
When to call a professional immediately:
- If the fault does not clear after the simple reset and pressure check.
- If the fault returns repeatedly after reset.
- If you see signs of water damage, burning, or smell burning from the boiler, or if any electrical tripping occurs.
- If there is any gas smell.
Final notes:
- This error can be temporary but is frequently an indication of an electronic/PCB or configuration failure.
- Do not attempt in-depth electrical or gas repairs yourself. A Gas Safe–registered engineer or authorised Baxi technician should diagnose and, if necessary, replace the PU/combustion control unit and restore correct configuration and safe operation.
Helpful Resources
Boiler Manual
Download the official PDF manual for the Baxi 816 Heat, 825 Heat, 830 Heat.