The E78 fault code appears on 3 brands — Andrews, Baxi and Potterton — and it does not mean the same thing on each. The same two or three characters can signal a pressure problem on one boiler and an ignition fault on another, so identify your brand first and use the table below to jump to the right page.
The brand is printed on the front casing. Tap your brand for the full causes, fixes and repair costs.
| Brand | What E78 means | Severity | DIY? |
|---|---|---|---|
A Andrews | The E78 fault code on Andrews water heaters (Baxi GA Range) indicates that the boiler's control board has detected the electrical current reaching the gas valve has dropped below the… Full guide → | Lockout — engineer likely | No |
B Baxi | The E78 code means your Baxi boiler has detected a problem with its water pressure sensor — the component responsible for monitoring system pressure and feeding that information to the… Full guide → | Lockout — engineer likely | No |
P Potterton | The E78 fault code on a Potterton boiler indicates that the boiler's control board is unhappy with the signal it is receiving from the water pressure sensor. Full guide → | Engineer may be needed | No |
The E78 fault code on Andrews water heaters (Baxi GA Range) indicates that the boiler's control board has detected the electrical current reaching the gas valve has dropped below the minimum acceptable threshold. As a safety precaution, the appliance locks out to prevent unreliable or unsafe combustion. This is a gas-circuit fault — not a temperature or pressure sensor issue — and almost always requires a Gas Safe registered engineer to diagnose and resolve.
The E78 code means your Baxi boiler has detected a problem with its water pressure sensor — the component responsible for monitoring system pressure and feeding that information to the control board. Rather than simply reading low pressure, E78 indicates the control board cannot trust the data it is receiving: the signal may be absent, unstable, or clearly outside the expected range. Because the boiler needs reliable pressure data before it can fire safely, it locks out entirely until the issue is resolved. Note that topping up the system pressure may not clear this fault if the sensor itself is at fault — a normal low-pressure condition typically produces a different error on Baxi appliances.
The E78 fault code on a Potterton boiler indicates that the boiler's control board is unhappy with the signal it is receiving from the water pressure sensor. This sensor continuously monitors the pressure inside the sealed heating circuit and feeds that data back to the PCB. When the signal looks absent, unstable, or outside what the board considers plausible, it raises E78 and shuts the boiler down as a precaution. Importantly, this is not always a straightforward low-pressure situation — the pressure gauge on the front of the boiler may look perfectly normal, yet E78 can still appear if the sensor itself is sending a corrupted or intermittent reading. The fault can affect both heating and hot water and should be investigated promptly, though it does not necessarily represent an immediate safety emergency.
Causes, fixes & costs for Potterton E78 → · All Potterton codes
No. E78 means different things on different brands — see the table above. Always identify your boiler's brand (it is printed on the front casing) before acting on any advice about the code.
Not usually. On every brand listed, E78 points to a fault that needs a suitably registered engineer to diagnose and repair. A single reset to rule out a transient glitch is reasonable; repeated resets are not.
For gas boilers, a Gas Safe registered engineer — it is a legal requirement for any work on the appliance. If you smell gas at any point, leave the property and call the National Gas Emergency line on 0800 111 999.
General guidance only — not a substitute for professional advice, and not certified by a registered engineer. Any work on a gas appliance must be carried out by a Gas Safe registered engineer. If you smell gas, call 0800 111 999.
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