Potterton E78 Fault Code: Causes, Fixes & Repair Costs
What does the Potterton E78 fault code mean?
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.
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
- Faulty or worn water pressure sensor Common
The pressure sensor itself is the most frequent culprit. Over time the sensor can drift out of calibration or fail outright, sending readings to the PCB that do not reflect actual circuit pressure. This explains why the gauge can look normal while E78 is still displayed — the sensor and the mechanical gauge are separate components.
- Damaged or loose sensor wiring Common
The wiring loom between the pressure sensor and the PCB can suffer from loose connectors, corrosion on terminals, or chafed insulation. Any interruption to that electrical path produces an unreliable signal and can trigger E78 just as readily as a failed sensor.
- Actual pressure outside normal range Sometimes
If the system pressure has dropped below roughly 1 bar or climbed unusually high, the sensor may be reporting a genuine out-of-range reading that the boiler interprets as a fault. A leak, a failing expansion vessel, or a weeping pressure relief valve can all cause pressure to drift and prompt this code.
- Sludge or debris partially blocking the sensor port Sometimes
Magnetite sludge and scale can accumulate in the small port through which the sensor reads circuit pressure. A partial blockage dampens or distorts the reading, causing erratic signals that the PCB flags as an error.
- PCB fault Rare
In a small number of cases the control board itself misinterprets or fails to process a perfectly healthy sensor signal. This is typically only suspected once the sensor and wiring have been ruled out.
How to fix it
- Check the pressure gauge on the boiler DIY safe
Look at the pressure gauge on the boiler's front panel. For most Potterton boilers the cold system pressure should sit between 1 and 1.5 bar. If it reads below 1 bar, low pressure could be contributing to the fault. If it reads above 2.5 bar, the system may be over-pressurised. Note the reading before doing anything else.
- Top up the system pressure if it is low DIY safe
If the gauge shows below 1 bar, locate the filling loop — usually a braided hose with one or two valves beneath the boiler. Open the valve(s) slowly and watch the gauge rise. Stop as soon as it reaches around 1.2–1.5 bar. Close the valve(s) fully. Never overfill: exceeding 2 bar can cause the pressure relief valve to discharge.
- Attempt a single boiler reset DIY safe
Once you have checked or corrected the pressure, press and hold the reset button (typically marked with a flame symbol or the word Reset) for around 3 seconds until the boiler attempts to fire. If E78 clears and the boiler runs normally, monitor it over the next day or two. If the fault returns, do not keep resetting — repeated resets will not fix a failed sensor or wiring fault and may mask an ongoing issue.
- Check whether the pressure drops repeatedly DIY safe
If you find yourself topping up the system pressure more than once every few months, there is likely a leak somewhere in the circuit — from a radiator valve, a joint, the expansion vessel, or the pressure relief valve. Note how quickly the pressure falls and report this to your engineer, as it may be related to the E78 fault.
- Call a Gas Safe registered engineer to diagnose and repair the fault Gas Safe engineer
If E78 returns after a single reset, or if the pressure gauge looked normal all along, the fault requires professional diagnosis. An engineer will use suitable test equipment to check the pressure sensor's output signal, inspect the wiring loom and connectors for corrosion or damage, assess whether sludge is affecting the sensor port, and test the PCB's response. They will then replace whichever component is at fault — most commonly the pressure sensor itself or the associated wiring — and verify the repair before leaving.
Parts you may need
- Water pressure sensor · from £25
- Sensor wiring loom / harness · from £30
- Expansion vessel · from £55
- Pressure relief valve · from £20
- PCB (printed circuit board) · from £220
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 £100–£280, depending on the underlying cause.
Frequently asked questions
Can I fix the Potterton E78 fault myself?
The only safe DIY steps are checking the pressure gauge, topping up the system if pressure is low, and attempting one reset. Everything beyond that — testing the sensor, inspecting wiring, or replacing any internal components — must be carried out by a Gas Safe registered engineer. Opening the boiler casing to work on electrical or gas parts is not safe for a homeowner and is a legal requirement in the UK to have done by a qualified professional.
My pressure gauge looks normal but I still have E78 — why?
This is a common point of confusion. The pressure gauge on the boiler's fascia is a simple mechanical device, while the water pressure sensor is an electronic component that feeds data to the PCB. The two can disagree if the sensor has failed or its wiring is faulty. E78 reflects what the PCB is seeing from the sensor, not what the mechanical gauge shows, so a normal-looking gauge does not rule out a sensor or wiring problem.
How much does it cost to fix a Potterton E78 fault?
Most E78 repairs fall in the range of £100–£280 including parts and labour. A straightforward sensor swap with standard call-out and labour typically sits towards the lower end, while faults involving wiring repairs or multiple components push costs higher. If the PCB itself turns out to be the root cause, replacement boards can cost £180–£350 or more for the part alone, which would take the total bill beyond the typical range — your engineer should advise whether repair or boiler replacement makes more economic sense in that scenario.
Why does E78 keep coming back after I reset the boiler?
A fault that returns after resetting almost always means there is an underlying hardware problem that the reset cannot cure — a sensor that has genuinely failed, a connector that is intermittently losing contact, or a pressure issue caused by a leak or failing expansion vessel. Each time the boiler restarts it encounters the same faulty signal and locks out again. Repeated resetting without a repair can also mask the true severity of the fault, so book an engineer as soon as E78 reappears following your first reset attempt.