Potterton E385 Fault Code: Causes, Fixes & Repair Costs
What does the Potterton E385 fault code mean?
The E385 fault code indicates that the electrical supply voltage reaching your Potterton boiler has fallen below 195V — the minimum threshold the boiler needs to operate safely. UK mains voltage is nominally 230V, so a reading this low suggests either a genuine supply problem from the grid, a fault in your home's wiring, or an issue with the boiler's internal power circuitry. The boiler locks itself out automatically to prevent damage to sensitive components. Once the underlying cause is resolved and the voltage returns to an acceptable level, the boiler should be able to resume normal operation.
General guidance only — not a substitute for professional advice, and not certified by a Gas Safe engineer. Always have your specific appliance assessed by a qualified professional. Any gas work must be carried out by a Gas Safe registered engineer. If you smell gas or suspect carbon monoxide, leave the property and call the National Gas Emergency line on 0800 111 999.
Common causes
- Genuine low mains supply voltage Common
The most straightforward explanation is that the electricity supply to your home — or specifically to the boiler's circuit — has dipped below 195V. This can happen during periods of high local demand, a fault on the distribution network, or if your property has an ageing or undersized supply cable. If neighbouring appliances are also behaving oddly, a grid supply issue is likely.
- Loose or deteriorated wiring connections Common
Wiring inside the boiler or at the fused spur can work loose over time, especially if the property has experienced vibration or if original installation quality was poor. A loose connection creates resistance, which effectively reduces the voltage seen by the boiler's PCB and can trigger E385 even when the mains supply itself is fine.
- Faulty PCB or internal power circuitry Sometimes
If the mains supply voltage tests as correct at the socket or fused spur but the E385 code persists, the boiler's own power supply circuitry or PCB may be misreading or mishandling the incoming voltage. The PCB monitors supply voltage and if its sensing circuitry has failed it will incorrectly flag an undervoltage condition.
- Damage from a power surge Sometimes
A sudden voltage spike — from a lightning strike, a large appliance switching on, or an unstable local grid — can damage the boiler's PCB in a way that causes it to misinterpret subsequent voltage readings and repeatedly trigger E385 even when supply voltage is normal.
- Faulty voltage-sensing component Rare
Occasionally the specific sensor or component on the PCB responsible for monitoring incoming voltage fails independently of the rest of the board. This is relatively uncommon but can produce an E385 code that seems intermittent or that appears without any obvious electrical event.
How to fix it
- Check whether other electrical appliances in your home are working normally DIY safe
If lights are dim, other appliances are struggling, or your smart meter is showing an unusually low reading, the issue may be with the mains supply to your property. Contact your electricity distribution network operator (DNO) — they are responsible for the supply up to your meter and will investigate free of charge.
- Verify the boiler's fused spur is switched on and the fuse is intact DIY safe
Locate the dedicated fused spur switch that supplies the boiler — it is usually on the wall nearby. Ensure it is in the on position and, if you are comfortable doing so, check that the fuse inside the spur has not blown. A blown fuse is a sign of an underlying electrical problem and should prompt you to call an engineer rather than simply replacing the fuse repeatedly.
- Attempt a boiler reset DIY safe
Press and hold the reset button on your Potterton boiler for around three seconds, then release and allow the boiler up to two minutes to restart. A single reset is worth trying in case the E385 was triggered by a brief, one-off voltage dip. Do not reset more than two or three times — repeated resets without success will not fix the fault and may mask a more serious problem.
- Have a Gas Safe engineer test the incoming voltage at the boiler terminals Gas Safe engineer
A registered engineer will use a calibrated multimeter to measure the actual voltage at the boiler's electrical terminals. This confirms whether the fault is in the supply or within the boiler itself, and guides what needs to be repaired. This step requires opening the boiler casing and should not be attempted by a homeowner.
- Have a Gas Safe engineer inspect and test all internal wiring Gas Safe engineer
The engineer will check wiring connections throughout the boiler, including at the PCB, transformer, and any junction points, looking for loose terminals, corrosion, heat damage, or deteriorated insulation. Faulty wiring found at this stage can often be repaired at relatively low cost compared with component replacement.
- PCB inspection, repair, or replacement if required Gas Safe engineer
If the supply voltage is confirmed as correct but E385 persists, the engineer will assess the PCB. Some boards can be sent away for specialist repair at lower cost than outright replacement; others will need a new board. Only a Gas Safe registered engineer should handle this work, as it involves internal boiler electrics.
- Call a Gas Safe registered engineer if the fault has not resolved Gas Safe engineer
If the steps above have not identified or fixed the cause, contact a Gas Safe registered engineer to carry out a full electrical and boiler diagnosis. You can verify an engineer's registration at GasSafeRegister.co.uk. Do not continue to run the boiler if it is locking out repeatedly on E385.
Parts you may need
- Potterton boiler PCB (model-specific) · from £180
- Fused spur switch (13A) · from £15
- Boiler wiring harness · from £45
- Mains voltage sensing component / relay · from £25
The exact spare depends on your boiler's GC number (on the data badge). Check this against the part before buying.
Typical repair cost
Expect to pay roughly £120–£380, depending on the underlying cause.
Frequently asked questions
Can I keep using my Potterton boiler while it is showing E385?
No — the boiler has locked out specifically to protect itself from operating below safe voltage. Attempting to force repeated resets and run the boiler in this condition risks damaging the PCB or other electrical components. Leave it switched off at the fused spur until the cause has been identified and fixed.
Could the E385 fault be caused by my electricity supplier rather than the boiler?
Yes, and it is worth checking this first. UK mains voltage should sit at 230V ±10%, meaning anything below about 207V is already low. If your supply is consistently below 195V, contact your electricity distribution network operator (DNO) — they are legally obliged to maintain an adequate supply and will investigate without charge. If the supply is fine, the fault lies within the boiler or its wiring.
How much does it typically cost to fix a Potterton E385 fault?
Most people with this fault pay somewhere between £120 and £380 all in, depending on what is causing it. A wiring repair at the lower end of that range is the most common outcome. PCB replacement is more expensive — a new Potterton PCB typically costs £150–£220 for the part, plus £100–£150 labour, which can push the total toward £350–£400. In rare cases where the PCB cannot be sourced or the boiler is elderly, replacement of the boiler may be more economical — worth discussing with your engineer.
Is E385 a gas-related fault that needs a Gas Safe engineer, or can an electrician fix it?
Although E385 is an electrical fault, the work involves opening the boiler casing and working on internal components, which must be carried out by a Gas Safe registered engineer — not a general electrician. If the problem turns out to be with your home's mains supply upstream of the boiler, a Part P-registered electrician may also need to be involved, but the boiler itself must be assessed by a Gas Safe professional.