Baxi E78 Fault Code: Causes, Fixes & Repair Costs
What does the Baxi E78 fault code mean?
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.
General guidance only — not a substitute for professional advice. 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 water pressure sensor Common
The pressure sensor has failed internally and is no longer sending a reliable signal to the PCB. This is the most common root cause when system pressure looks normal on the gauge but E78 still appears. The sensor is a relatively inexpensive component but must be replaced by a Gas Safe engineer.
- Low system pressure Common
If the heating system pressure has dropped below roughly 1.0 bar, the boiler will lock out as a precaution. Check the pressure gauge on the front of the boiler — if it reads below 1 bar, repressurising via the filling loop may resolve the fault. However, if pressure drops repeatedly within a day or two, there is likely a leak that needs professional attention.
- Loose or corroded wiring connections Sometimes
The wiring loom between the pressure sensor and the PCB can develop loose terminals or corrosion, particularly in older appliances or those in damp locations. A poor connection causes intermittent or missing signals that the board interprets as a sensor fault.
- Air trapped in the pressure sensor port Sometimes
Air locked in the pipe or port leading to the pressure sensor can prevent an accurate pressure reading, causing the boiler to detect an anomalous value and lock out. This can occur after system work or if the boiler has been drained and refilled incorrectly.
- Leak in the heating system or expansion vessel failure Sometimes
A slow leak from pipework, radiator valves, or the filling loop, or a waterlogged expansion vessel, will cause repeated pressure loss. The boiler may lock on E78 as pressure falls. If the pressure gauge drops noticeably between checks, a leak or vessel fault is the likely culprit.
- PCB fault Rare
In a small number of cases the pressure sensor is functioning correctly but the control board itself cannot process the incoming signal. This presents identically to a sensor failure and is only confirmed once the sensor and wiring have been ruled out. PCB replacement is one of the more expensive outcomes.
How to fix it
- Check the system pressure gauge DIY safe
Look at the pressure gauge on the boiler's front panel. For most Baxi boilers the correct cold-system pressure is between 1.0 and 1.5 bar. If the needle sits below 1 bar, the low pressure may be contributing to the lockout.
- Top up system pressure via the filling loop if pressure is low DIY safe
Locate the filling loop — usually a braided flexible hose with one or two valves beneath the boiler. Slowly open the valve(s) until the gauge rises to around 1.2–1.5 bar, then close the valve(s) firmly. Never overfill beyond 2 bar. If you are unsure how to do this, your Baxi manual has model-specific instructions.
- Inspect for visible leaks DIY safe
Walk around all visible radiators, towel rails, pipework, and the area beneath the boiler. Look for damp patches, rust staining, or drips. Even a pin-hole leak can cause gradual pressure loss. If you find a leak, do not attempt to repair it yourself — call a Gas Safe engineer.
- Reset the boiler DIY safe
Press and hold the reset button on the boiler's control panel for around 3–5 seconds (refer to your model's manual for the exact method). Wait a minute or two to see whether the boiler fires up normally. If the fault clears but returns within a short time, do not keep resetting — repeated lockouts on the same fault indicate a component problem that needs diagnosis.
- Monitor pressure over the next 24–48 hours DIY safe
If the boiler runs after a reset, check the pressure gauge the following day. If it has dropped again, there is a slow leak or expansion vessel fault that requires a professional. Continuing to top up without finding the root cause is not a solution.
- Call a Gas Safe registered engineer to diagnose and repair the fault Gas Safe engineer
If the fault persists after the steps above, or if system pressure is normal but E78 still shows, the pressure sensor, its wiring, or the PCB will need testing with appropriate equipment. The engineer will check the sensor output, inspect the wiring loom for corrosion or loose connections, clear any air from the sensor port, and replace whichever component has failed. Do not attempt to open the boiler casing or handle internal components yourself.
Parts you may need
- Water pressure sensor · from £25
- Pressure sensor wiring harness · from £30
- Expansion vessel · from £55
- Filling loop assembly · 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 £120–£320, depending on the underlying cause.
Frequently asked questions
Can I fix the Baxi E78 fault myself by just topping up the pressure?
You can top up the system pressure yourself if the gauge reads below 1 bar, and this is worth trying as a first step. However, E78 is specifically a sensor signal fault, not simply a low-pressure warning, so if pressure is already within the normal 1.0–1.5 bar range and the code still appears, the sensor or its wiring is the problem — and that requires a Gas Safe engineer to fix. Even when low pressure is the immediate trigger, if pressure keeps dropping you have a leak or expansion vessel fault that also needs professional attention.
How much does it cost to fix a Baxi E78 fault in the UK?
For most homeowners the bill falls between £120 and £320 depending on the cause. A pressure sensor replacement including labour typically costs around £120–£180. Wiring repairs are in a similar range. If the expansion vessel needs replacing, expect £150–£250 all in. A PCB replacement is the expensive end of the spectrum and can reach £350–£500 or more, but this is an uncommon outcome — the vast majority of E78 faults are resolved by replacing the sensor or correcting a wiring issue.
Why does my Baxi boiler keep showing E78 even after I reset it?
If E78 returns immediately or within a few hours of a reset, the underlying component fault has not been resolved. Persistent lockouts usually mean the pressure sensor has failed, or there is an ongoing wiring fault between the sensor and the PCB. Repeatedly resetting the boiler will not fix the problem and is not recommended — each reset is simply masking the fault. Arrange a visit from a Gas Safe engineer who can test the sensor output and replace what is needed.
Is it safe to use my Baxi boiler with the E78 code showing?
No. When E78 is active the boiler has locked itself out and will not fire. This is a deliberate safety measure to prevent the appliance operating without reliable pressure data. Do not attempt to bypass or override the lockout — this could cause damage to the boiler or create an unsafe situation. The correct action is to follow the basic homeowner checks described above and, if the fault persists, to book a Gas Safe registered engineer.