Keston E19 Fault Code: Causes, Fixes & Repair Costs
What does the Keston E19 fault code mean?
The E19 fault code on Keston boilers indicates that the temperature of the water returning from the heating circuit back into the boiler has risen above the permitted threshold. In a healthy system, return water should always be noticeably cooler than the flow water leaving the boiler — a high return temperature tells the boiler's controls that heat is not being distributed properly around the system. When this condition is detected, the boiler enters a hard lockout and stops operating until the fault is investigated and the reset button is pressed. Note that a closely related code, B19, is a temporary block (soft warning) triggered when the return temperature exceeds 95°C — if the underlying problem is not resolved, the boiler escalates this to a full E19 lockout. The related code B18 flags the same overtemperature condition but on the flow side rather than the return.
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
- Circulation pump seized or underperforming Common
The pump is responsible for keeping water moving continuously around the heating circuit. If it has seized, lost speed, or is running intermittently, hot water from the heat exchanger cannot be pushed out into the system fast enough. Heat builds up locally and the return temperature rises, triggering the E19 lockout. This is the single most common cause of this fault.
- Sludge or scale blocking the heat exchanger or pipework Common
Over time, corroding radiators and pipework release magnetite (black iron oxide) and rust particles that accumulate as sludge throughout the system. This sludge can partially or fully block the heat exchanger passages or restrict flow in pipes and radiator tails, sharply reducing circulation. The boiler then cannot shed its heat efficiently and the return temperature climbs. Keston's compact heat exchanger design is particularly susceptible to sludge build-up.
- Airlock in the heating circuit Sometimes
Air trapped in the pump, pipework, or heat exchanger creates a partial blockage that restricts water flow without necessarily stopping the pump from running. A classic sign is a gurgling or whistling pump combined with some radiators being cold. As circulation becomes restricted, the return temperature can exceed safe limits and trigger E19.
- Faulty return thermistor (temperature sensor) Sometimes
If the NTC thermistor monitoring the return temperature has drifted out of calibration, developed an internal short, or its wiring connection has corroded, it may report a falsely high temperature to the PCB. The boiler then reacts to a problem that does not physically exist. This can be confirmed by an engineer using a multimeter to check the sensor's resistance against its expected temperature curve.
- Loose or corroded electrical connections Rare
Vibration and thermal cycling over years of operation can loosen terminal block connections or cause corrosion on the wiring to the thermistor or pump. A poor connection may produce erratic sensor readings or intermittent pump power, both of which can provoke an E19 fault without any single component having actually failed.
How to fix it
- Reset the boiler DIY safe
Press and hold the reset button on the boiler's control panel until the display responds, or follow the specific reset procedure in your user manual. If the fault clears and does not return within a day or two, it may have been a temporary temperature spike. Do not attempt more than two or three resets — repeated lockouts on the same fault indicate a genuine problem that needs professional investigation.
- Bleed the radiators to clear any airlock DIY safe
If some radiators are cold or you can hear gurgling from the pump or pipework, trapped air is a likely contributor. Turn the heating off and allow the system to cool slightly, then use a radiator key to open each bleed valve a quarter-turn until water (rather than air) trickles out. Close the valve, move to the next radiator, and work your way around the house. After bleeding, check the system pressure gauge — if it has dropped below 1 bar, top up via the filling loop until the gauge reads between 1 and 1.5 bar, then reset the boiler.
- Check your gas supply is available DIY safe
Confirm that other gas appliances in the property (hob, other heating appliances) are working normally. If there is no gas supply, contact your gas supplier. This is a quick sanity check rather than a likely cause of E19, but worth ruling out before calling an engineer.
- Contact a Gas Safe registered engineer — ideally one with Keston experience Gas Safe engineer
If resetting and bleeding do not resolve the fault, or if E19 returns within a short time, a qualified engineer is needed. Keston boilers use a twin-flue design and a modulating pump arrangement that differs from mainstream boilers, so it is worth specifically asking whether the engineer has worked on Keston appliances before. They will test the circulation pump's speed and output, inspect the heat exchanger for sludge or scale, check the return thermistor's resistance readings, and examine all associated wiring and terminals.
- Engineer to test and replace the circulation pump if necessary Gas Safe engineer
The engineer will measure pump performance against the manufacturer's specification. A seized or weak pump will need replacing. Pump replacement on a Keston boiler requires draining down the relevant part of the circuit, fitting a new pump (ensuring the correct head and flow rating), and refilling and testing the system.
- Engineer to clean or powerflush the system if sludge is found Gas Safe engineer
If the heat exchanger, pipework, or radiators show evidence of magnetite contamination, a chemical flush or full powerflush will be recommended. A powerflush connects a high-flow machine to the system and forces cleaning chemicals around every radiator and pipe run to dislodge and remove debris. This is particularly important on Keston boilers, whose compact heat exchanger blocks easily. A magnetic filter should be fitted at the same time to protect the system going forward.
- Engineer to test and replace the return thermistor if it is reading incorrectly Gas Safe engineer
Using a multimeter, the engineer will measure the sensor's resistance and compare it to the expected value at the current water temperature. A thermistor that is significantly out of tolerance will need replacing. This is generally a straightforward repair involving disconnecting the old sensor, fitting a new one, and verifying accurate readings before sign-off.
Parts you may need
- Circulation pump (suitable replacement for Keston models) · from £85
- Return NTC thermistor / temperature sensor · from £25
- Magnetic system filter (e.g. Magnaclean or equivalent) · from £60
- Central heating inhibitor / sludge remover chemical · from £20
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 £150–£450, depending on the underlying cause.
Frequently asked questions
What is the difference between Keston B19 and E19?
B19 is a soft block — a temporary warning that the return temperature has exceeded 95°C. The boiler pauses but may attempt to restart automatically once the temperature drops. If the underlying cause is not resolved, or if the temperature continues to rise, the boiler escalates to E19, which is a hard lockout. With E19 the boiler will not restart at all until you manually press the reset button. Think of B19 as the boiler's first warning and E19 as its final refusal to operate until the problem is fixed.
Can I fix a Keston E19 fault myself?
There are a couple of safe checks you can carry out yourself: reset the boiler (up to two or three times), bleed the radiators to remove trapped air, and top up the system pressure if it has dropped. Beyond that, the fault almost always requires a Gas Safe registered engineer. Work on the pump, heat exchanger, thermistors, wiring, or PCB must not be attempted by an unqualified person, both for safety reasons and to avoid invalidating any remaining warranty.
How much does it cost to fix a Keston E19 fault?
For most households the repair falls between £150 and £450. A pump replacement typically costs £150–£350 including parts and labour, while a thermistor swap is usually at the lower end, around £100–£200. If the whole system is heavily sludged and needs a powerflush, expect to pay £350–£850 depending on the number of radiators — though this cost also protects the boiler from future faults. PCB replacement, which is occasionally needed if the fault has escalated, can cost £400–£600 or more and may make an older boiler uneconomical to repair.
Why does my Keston boiler keep showing E19 after I reset it?
Repeated E19 faults after resetting almost always mean the root cause has not been addressed. The most likely culprits are a failing circulation pump that is losing performance progressively, or a system that is becoming increasingly blocked with sludge. Each time the boiler fires, circulation is insufficient, the return temperature climbs, and the lockout repeats. An engineer needs to inspect the pump output and check for sludge or blockages — repeated resets without fixing the underlying issue will not resolve the problem and may cause additional wear to the heat exchanger.
Does fitting a magnetic filter help prevent Keston E19 faults?
Yes, fitting a magnetic filter (such as a Magnaclean or equivalent) is one of the most effective preventive measures for this specific fault. Keston's compact heat exchanger is particularly vulnerable to magnetite sludge, and a filter captures iron oxide particles before they can accumulate in the boiler. Keston have historically suggested their design does not require a filter, but engineers who work regularly on these boilers strongly recommend fitting one — ideally at the same time as any sludge-related repair, and cleaned at every annual service.