Overview
E385 on a Baxi 600 combi commonly signals an under‑voltage or unstable supply condition detected by the boiler PCB. The boiler's electronic control board contains a switched‑mode power supply (SMPS) that creates the voltages the rest of the electronics need. If the SMPS cannot produce stable voltages because of a mains supply dip, failing components in the SMPS, or internal board faults, the PCB will throw E385 and lock out to protect the boiler and the gas components. The fault can be temporary (a short mains dip or transient) or permanent. A temporary fault may clear with a single reset; a persistent or repeating E385 indicates an internal PCB power fault, most often caused by aging or failed input capacitors, a failed power IC (examples cited in repairs include the VIPer22A) or other degraded SMPS components. Severity ranges from medium to high because the boiler will be taken out of service and repeated undervoltage or a failing SMPS can cause further PCB or ignition instability. Basic checks and a single reset are reasonable for a homeowner, but component‑level diagnosis and PCB repairs require qualified electrical and gas personnel and should not be attempted by an untrained person.
Possible Cause: Under voltage.
Troubleshooting Steps
Safety first
- If you smell gas: do not operate electrical switches, leave the building immediately, call your gas emergency number and a Gas Safe registered engineer. Do not attempt any checks if a gas leak is suspected.
- Before opening the boiler or touching the PCB isolate the mains power at the house consumer unit and switch the boiler to OFF. Be aware that PCB capacitors can hold lethal charge even after power is removed; do not touch the board unless you are trained and competent.
- If you are not comfortable with electrical work, stop and call a Gas Safe registered engineer or Baxi service.
Initial homeowner checks you can safely do
1. Note when the code appears and any recent events. Did you have a recent power cut, lightning strike, or intermittent supply? Temporary mains dips commonly trigger under‑voltage faults.
2. Check the consumer unit (fusebox). Ensure no MCB or RCD has tripped. Reset any tripped switches and try the boiler reset once.
3. Check the boiler display and other fault codes. If other faults (low pressure, frozen condensate, ignition failure) appear, deal with those first using normal safe procedures.
4. Check system water pressure on the boiler gauge. Baxi boilers normally need about 1 to 1.5 bar. Top up the system using the filling loop if pressure is low, then try a single boiler reset.
5. Perform one reset only: find the reset button or turn the selector to R and hold for about 5 seconds. Do not keep repeatedly resetting; if the fault returns immediately or often, stop and call a professional.
If E385 persists after basic checks
- Record how often the error appears and under what conditions (after power cuts, during ignition attempts, randomly). This information helps the engineer.
What a qualified engineer or electronics technician will do (do not attempt unless you are trained)
1. Verify mains supply at the boiler terminals. Measure incoming AC voltage and stability. Persistent undervoltage or wiring issues at the consumer unit can trigger E385 and should be rectified by a qualified electrician.
2. With proper isolation and safety procedures the engineer will measure SMPS output rails on the PCB to confirm undervoltage on secondary supplies.
3. Visually inspect the PCB for common signs of failure: swollen or leaking electrolytic capacitors, burn marks, cracked solder joints, or a burnt smell. On Baxi 600/System boards E385 commonly links to degraded SMPS caps and the power IC.
4. Component‑level repair or replacement: if SMPS parts are failing, a competent electronics technician may replace the input electrolytic capacitors, the power IC (reported replacement example VIPer22A), the low voltage supply capacitors and large post‑transformer electrolytics. Some repair videos and guides reference replacing specific caps (for example a 400V input cap and various 35V caps) and an additional 400V cap near the ignition transformer. Only experienced technicians should do this; capacitors hold charge and the board must be tested under load afterwards.
5. If board repair is not viable or safe, the engineer may replace the entire PCB with an approved Baxi replacement. After any repair or replacement the installer should carry out full boiler commissioning, gas tightness checks, ignition and combustion checks, and run the boiler under realistic demands to confirm stability.
Final notes and when to call a professional
- If the E385 fault does not clear with a single reset, reappears often, follows power interruptions, or you see signs of PCB damage, call a Gas Safe registered engineer and, if electrical supply issues are suspected, a qualified electrician.
- Do not attempt gas or high voltage electrical repairs yourself. PCB repair requires electronics experience and safe discharge of capacitors.
- A qualified engineer can diagnose whether the problem is a simple supply issue, a repairable PCB fault, or needs full PCB replacement and will complete safety and combustion testing after any work.
Helpful Resources
Boiler Manual
Download the official PDF manual for the Baxi 600 Combi Gas Boiler.