Overview
Error code 356 on a Worcester Bosch Greenstar i indicates the boiler has detected low or unstable mains (electrical) voltage. The boiler’s control electronics monitor the incoming supply and if the voltage falls below a safe operating threshold or becomes inconsistent, the boiler can display this code and may lock out or stop operating to protect sensitive components and prevent unsafe behaviour. This is not a gas or water fault — it is an electrical supply issue. Severity depends on whether the voltage problem is transient or persistent. Short, rare drops in voltage (blips) may only cause occasional lockouts and can sometimes be cleared with a reset. Persistent low or fluctuating voltage is more serious: it can prevent the boiler from running, cause repeated lockouts, and over time could damage electronic control parts. Because this fault involves the mains supply, diagnosing and fixing it safely usually requires a qualified electrician and, for any boiler repairs after a power issue, a Gas Safe registered heating engineer. Homeowners can do basic checks, but they must not work on live mains or inside the boiler unless competent and qualified.
Possible Cause: The voltage may be interrupted or inconsistent.
Troubleshooting Steps
Safety precautions
- Do not open the boiler casing or work on internal electrical components unless you are a qualified engineer. Working on mains electricity and gas appliances is dangerous.
- If the boiler is repeatedly locking out, consider turning it off at the isolator switch until you can investigate or get professional help.
- If you suspect any sparking, burning smell, or visible damage to wiring or the consumer unit, isolate the supply and call a qualified electrician immediately.
Initial checks a homeowner can do
1. Check other electrical devices in the house for signs of low voltage (dimming lights, slow or flickering appliances). Note if the issue affects the whole house or only the boiler.
2. Check the boiler’s external fused isolator (spur) and any wall switch to make sure the boiler is switched on and the fuse has not blown. Replace the isolator fuse only with the correct type if you are confident and the supply is isolated.
3. Try a simple reset of the boiler: press and hold the reset or tick button for about 8 seconds until the boiler reboots. If the fault was a transient blip this may clear the code. Do not repeatedly reset the boiler more than a couple of times if the fault returns immediately.
4. Note the pattern: does the code appear at certain times (e.g. when high-demand appliances run), only during storms, or continuously?
Specific diagnostic and fix steps (do not attempt live electrical work unless qualified)
1. If the problem is intermittent: record when it happens and what else is running at that time (washing machine, cooker, electric shower, etc.). Interference from high-load appliances or poor household wiring can cause voltage dips. Share this information with an electrician.
2. If the problem is persistent after reset: do not keep resetting. Contact the electricity supplier to ask if there are known supply issues or planned works; they can check the incoming supply to your property.
3. Get a qualified electrician to measure the incoming mains voltage at your consumer unit and at the boiler supply point during a fault condition. The electrician should check for undervoltage, voltage imbalance, and any loose or corroded connections on the supply cable, consumer unit, and the boiler isolator.
4. Ask the electrician to inspect the boiler’s dedicated supply cable and isolator for loose terminals or damage. Loose connections can cause local voltage drops and should be corrected by the electrician.
5. If the electrician confirms the mains supply is within acceptable limits and wiring is sound, have a Gas Safe heating engineer inspect the boiler. Voltage faults can damage the boiler’s control electronics; the engineer will check the boiler’s control box, power supply board, and sensors and will carry out safe restoration and any necessary repairs.
6. If the boiler displays the code but an external voltage check appears normal, note and provide the engineer with the exact times and circumstances of occurrences — intermittent faults can be the hardest to trace and benefit from detailed reporting.
When to call a professional
- Call an electrician if you suspect low or unstable mains supply, if lights/appliances are affected across the house, or if you find loose or damaged wiring. An electrician must test and, if necessary, repair the incoming supply and household wiring.
- Call a Gas Safe registered heating engineer if the mains supply is confirmed OK but the boiler still shows error 356, or after a verified mains fault to check the boiler’s electronics and safe operation.
Additional advice
- Do not keep operating or repeatedly resetting the boiler if the 356 error persists — this risks further damage.
- Keep a written log of the fault occurrences, times, any resets performed and any other appliances running at the time; this will help the electrician and heating engineer diagnose intermittent issues.
- If the electricity supplier or electrician confirms a supply problem outside your property (local network issue), follow their advice and do not attempt in-house repairs.
Summary
Low mains voltage errors indicate an electrical supply problem that should be investigated by a qualified electrician. A Gas Safe engineer should inspect the boiler if the supply is corrected or if the boiler exhibits internal faults after a supply problem. Do basic checks and a single reset at home, but avoid any live work or opening the boiler—call professionals for safe diagnosis and repair.
Helpful Resources
Boiler Manual
Download the official PDF manual for the Worcester Bosch Greenstar I System / Combi Boiler.