Protherm EP Fault Code: Causes, Fixes & Repair Costs
What does the Protherm EP fault code mean?
The EP fault code on a Protherm boiler indicates a heating circuit error. The boiler's control unit continuously monitors pressure, water flow, and pump operation across the heating circuit. When one or more of these readings falls outside the acceptable range — or a sensor signal disappears entirely — the boiler shuts down and displays EP as a safety measure. In plain terms, something is wrong with the pressure or circulation in your central heating system, and the boiler has locked out to prevent damage.
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
The single most common cause of the EP code. Protherm boilers typically require between 1 and 1.5 bar to operate safely. If pressure has dropped below the minimum threshold — usually due to a small leak, recently bled radiators, or natural seepage over time — the control unit will shut the boiler down and display EP.
- Faulty or blocked pressure sensor Sometimes
The pressure sensor feeds live readings to the control board. If its electrical connections become loose or corroded, or the sensor itself fails, the board receives a signal that is missing or out of range and interprets this as a circuit fault. This can trigger EP even when the actual system pressure is perfectly normal.
- Circulation pump failure or blockage Sometimes
If the pump impeller becomes seized, clogged with debris, or air-locked, water cannot circulate properly around the heating circuit. The boiler detects the lack of flow and shuts down. Leaking seals, corrosion in older systems, or the pump simply running dry can all lead to this failure mode.
- Blocked or failed automatic air vent (de-aerator) Sometimes
The automatic air vent is designed to bleed trapped air from the system continuously. If it becomes blocked or fails, air accumulates in the circuit, disrupting pressure readings and circulation. This can produce intermittent EP errors that are difficult to diagnose without a proper inspection.
- Electrical supply instability Rare
Voltage surges or dips on the mains supply can cause sensors to return erratic readings. The control board may interpret these transient signals as genuine circuit faults and trigger EP. This is more common in properties with older wiring or where the boiler is not protected by a voltage stabiliser.
How to fix it
- Check the pressure gauge on your boiler DIY safe
Look at the pressure gauge on the front of the boiler. For most Protherm models the normal operating range is 1 to 1.5 bar when the system is cold. If the needle is below 1 bar, low pressure is the most likely cause of the EP code.
- Top up system pressure using the filling loop DIY safe
Locate the filling loop — usually a flexible braided hose with one or two valves beneath the boiler. Slowly open the valve(s) and watch the pressure gauge rise. Stop when the needle reaches approximately 1.2 to 1.5 bar, then close the valve(s) firmly. Never exceed 2 bar.
- Bleed your radiators to remove trapped air DIY safe
Starting with the radiators furthest from the boiler, use a radiator key to open each bleed valve a quarter turn until water (not air) trickles out, then close it. After bleeding all radiators, recheck the system pressure and top up again if it has dropped below 1 bar.
- Reset the boiler DIY safe
Once pressure is correct, press and hold the reset button on your Protherm boiler for a few seconds (refer to your model's controls — it is often labelled with a flame or reset symbol). Allow the boiler to restart and monitor whether the EP code clears. Do not reset more than two or three times in total.
- Check for visible leaks if pressure keeps dropping DIY safe
If you find yourself topping up the pressure repeatedly, there is likely a leak somewhere in the system. Walk around your radiators, pipework, and underneath the boiler and look for damp patches, staining, or corrosion. Do not attempt to repair any leaking components yourself — note what you find and report it to an engineer.
- Have a Gas Safe engineer inspect the pressure sensor and wiring Gas Safe engineer
If the system pressure is correct but the EP code persists, a registered engineer needs to test the pressure sensor and check its wiring connections to the PCB. A faulty sensor will typically need replacing, and the engineer can determine whether the fault lies with the sensor itself or the control board.
- Have a Gas Safe engineer service or replace the circulation pump Gas Safe engineer
A registered engineer can remove the pump, inspect the impeller for blockages, check for seized bearings, and test whether the pump is running at the correct speed. If the pump has failed, it will need to be replaced with a compatible unit. This work involves isolating the boiler and draining part of the system.
- Have a Gas Safe engineer check the expansion vessel and automatic air vent Gas Safe engineer
The engineer should verify that the expansion vessel is correctly charged (typically around 0.75 to 1 bar pre-charge, depending on system design) and that the automatic air vent is functioning. A failed auto air vent may need cleaning or replacement to allow the system to purge air correctly.
- Call a Gas Safe registered engineer if the fault persists Gas Safe engineer
If none of the homeowner checks have resolved the EP code, or if it returns repeatedly, contact a Gas Safe registered engineer to carry out a full diagnostic. Attempting to bypass or ignore this fault could damage the boiler or leave your home without heating and hot water.
Parts you may need
- Heating circuit pressure sensor · from £25
- Circulation pump (compatible replacement) · from £85
- Automatic air vent / de-aerator · from £15
- Expansion vessel · from £45
- Filling loop assembly · from £20
- PCB / control 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 £80–£350, depending on the underlying cause.
Frequently asked questions
Can I fix the Protherm EP fault code myself?
Partially. You can safely check and top up the system pressure to 1–1.5 bar using the filling loop, bleed trapped air from your radiators, and reset the boiler. If those steps clear the fault, no engineer visit is needed. However, if the EP code returns or the pressure is already correct, the cause is likely a faulty pressure sensor, a failing circulation pump, or a blocked auto air vent — all of which require a Gas Safe registered engineer to diagnose and repair safely.
How much does it cost to fix a Protherm EP fault in the UK?
For most households, the repair falls between £80 and £350. A simple pressure top-up or re-pressurisation during an engineer visit costs roughly £80–£120 (mainly a call-out fee). Replacing a pressure sensor typically costs £100–£180 including labour. A circulation pump replacement usually runs £200–£350 all in, depending on the pump specification and how accessible it is. In rare cases where the control board (PCB) is at fault, costs can reach £400 or more — if you are facing that level of expenditure on an older boiler, it is worth getting a replacement boiler quote for comparison.
Why does my Protherm boiler keep losing pressure and showing EP?
Repeatedly dropping pressure almost always points to a leak somewhere in the system. Common locations include radiator valves, compression fittings on pipework, the boiler's internal heat exchanger, the pressure relief valve (which may be weeping to the outside drain), or pump seals. A Gas Safe engineer can pressure-test the system to pinpoint the source. Topping up the pressure repeatedly without finding and fixing the leak is not a long-term solution and can mask a worsening problem.
Could an electrical problem cause the Protherm EP fault?
Yes, though it is less common. Voltage instability on the mains supply — such as surges or significant dips — can cause the boiler's sensors to send erratic signals to the control board, which then logs it as a heating circuit fault. If your EP code appears after a power cut or during stormy weather and clears after a single reset, electrical interference is a plausible explanation. Some engineers and Protherm specialists recommend connecting the boiler via a voltage stabiliser or UPS (uninterruptible power supply) in areas prone to supply fluctuations, as this can reduce the frequency of nuisance faults.