Baxi E164 Fault Code: Causes, Fixes & Repair Costs
What does the Baxi E164 fault code mean?
The E164 fault code on a Baxi boiler signals that the boiler's control system has detected an open or absent signal from the hydraulic differential pressure switch (also referred to as the flow switch). In plain terms, the boiler expects confirmation that water is circulating at the correct pressure and flow before it will fire, and that confirmation signal is missing. As a safety measure, the boiler locks out completely and will not restart until the underlying problem is resolved. The issue can stem from genuinely low water pressure, a faulty or disconnected switch, wiring problems, or — less commonly — a PCB fault.
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
- Low system water pressure Common
Baxi boilers typically require the system to be between 1.0 and 1.5 bar when cold. If the pressure gauge reads below 1 bar, the differential pressure switch will not close its circuit and the boiler locks out. This is the most frequently seen root cause of E164 and can often be resolved by the homeowner using the filling loop.
- Faulty or stuck hydraulic differential pressure switch Common
The pressure switch itself can fail mechanically — the internal diaphragm can become stiff, the switch contacts can corrode, or debris in the system can block the sensing port. In any of these cases the switch stays open even when pressure is adequate, sending an incorrect fault signal to the PCB.
- Wiring fault or loose connection Sometimes
The wiring loom connecting the pressure switch to the PCB can develop a loose connector, broken wire, or corroded terminal — particularly in older boilers. The PCB then reads a permanently open circuit, which is indistinguishable from a failed switch. This is worth an engineer checking before replacing the switch itself.
- Faulty diverter valve or diverter valve motor Sometimes
A diverter valve that is stuck or whose motor has developed a water ingress fault can restrict or misdirect flow within the boiler, preventing sufficient hydraulic differential pressure from being generated. This can cause the switch to remain open during normal operation.
- PCB failure Rare
If the PCB itself has developed a fault — whether due to a power surge, component failure, or water damage — it may misread the pressure switch signal or fail to process it altogether. PCB faults are a less common but more expensive cause of E164 and are usually only confirmed after all other causes have been eliminated.
How to fix it
- Check the system pressure gauge DIY safe
Look at the pressure gauge on the front of your boiler. It should read between 1.0 and 1.5 bar when the system is cold. If it reads below 1 bar, low pressure is almost certainly the reason for E164. If the gauge reads 1–1.5 bar already, move on to the next step.
- Repressurise the system via the filling loop DIY safe
Locate the filling loop — usually a braided or plastic-sleeved hose underneath the boiler with one or two valves. Slowly open the valve(s) until the pressure gauge climbs to around 1.2–1.3 bar, then close the valve(s) firmly. Do not exceed 1.5 bar. If pressure drops again within a day or two, there is a leak somewhere in the system; stop topping it up and call an engineer instead.
- Check that your gas supply is active DIY safe
Confirm that other gas appliances in your home (cooker, gas fire) are working normally. If nothing gas-powered is working, there may be a supply interruption — contact your gas supplier. If only the boiler is affected, the gas isolation valve next to the boiler should be in line with the pipe (open position).
- Reset the boiler DIY safe
Once you have checked or corrected the pressure, reset the boiler using the reset button (refer to your manual if unsure of its location). Hold it for the time specified — usually 3–5 seconds. Allow the boiler to attempt a full start-up cycle. If E164 returns immediately or after only one or two attempts, stop resetting and progress to calling an engineer. Repeated resets without fixing the fault can damage components.
- Have a Gas Safe engineer inspect the pressure switch and wiring Gas Safe engineer
If the fault persists after the steps above, a registered engineer needs to check the hydraulic differential pressure switch for continuity, inspect the sensing port for blockage, and trace the wiring loom back to the PCB for any damaged or loose connections. The switch itself is a relatively inexpensive component to replace if found faulty.
- Have a Gas Safe engineer inspect the diverter valve Gas Safe engineer
If the pressure switch and wiring check out, the engineer should test the diverter valve and its motor for correct operation. A diverter valve that is restricting flow can prevent the pressure switch from registering adequate hydraulic differential, and replacement is straightforward for an experienced engineer.
- Have a Gas Safe engineer assess the PCB Gas Safe engineer
If all components downstream of the PCB have been tested and found serviceable, the PCB itself may need to be examined or replaced. PCB diagnostics require specialist knowledge and equipment; this is not a homeowner task under any circumstances.
Parts you may need
- Hydraulic differential pressure switch · from £35
- Diverter valve motor · from £45
- Diverter valve cartridge · from £60
- Wiring harness / connector loom · from £30
- PCB (printed circuit board) · from £250
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–£350, depending on the underlying cause.
Frequently asked questions
Can I fix Baxi E164 myself?
You can safely carry out a few checks: verifying the system pressure is between 1.0 and 1.5 bar, topping up via the filling loop if needed, confirming the gas supply is on, and resetting the boiler up to two or three times. If any of those steps clear the fault, great. If not, the cause is almost certainly a faulty pressure switch, wiring fault, diverter valve issue, or PCB problem — all of which must be diagnosed and repaired by a Gas Safe registered engineer.
Why does my Baxi keep losing pressure and showing E164?
If you find yourself topping up the system pressure regularly to clear E164, the pressure drop is a symptom of a water leak elsewhere in the system — it could be a leaking radiator valve, a weeping pipe joint, a faulty pressure relief valve, or an internal boiler seal. Repeatedly repressurising without finding the source of the leak is not a fix and can mask a worsening problem. An engineer should carry out a pressure test to locate the leak.
How much does it cost to repair a Baxi E164 fault?
For most homeowners, repairs cost somewhere in the £100–£350 range. A straightforward pressure switch replacement including an engineer's call-out and labour typically comes in at around £150–£200. Diverter valve work usually falls in the £150–£250 bracket. If the PCB turns out to be the culprit, the part alone can be £200–£300, so a full PCB repair including labour can push well above £400 — but a PCB diagnosis is only reached after all less-expensive causes have been ruled out, so most people never need to go that far.
Will E164 clear itself, or do I always need to reset the boiler?
E164 is a lockout fault, meaning the boiler will not attempt to restart on its own — it needs a manual reset. Even if the underlying cause was temporary (for example, pressure that dipped briefly and then recovered), the boiler will sit locked out until you press the reset button. If the fault code reappears shortly after resetting, the root cause has not been resolved and further investigation by an engineer is needed.