Warmflow E15 Fault Code: Causes, Fixes & Repair Costs
What does the Warmflow E15 fault code mean?
The E15 fault code on Warmflow Agentis combination boilers indicates a problem with the central heating return NTC temperature sensor. This thermistor sits on the return pipework and continuously monitors the temperature of water flowing back into the boiler. It works by changing its electrical resistance as water temperature rises or falls, allowing the PCB to modulate the burner accurately and prevent overheating. When the boiler's control board detects that the sensor is giving an implausible reading — or no reading at all — it triggers E15 and shuts the boiler down as a safety precaution. The root cause is almost always a short circuit, an open circuit (disconnected wiring), or a physically displaced sensor probe rather than anything more serious.
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
- Short circuit or open circuit on the CH return NTC sensor Common
The most frequently confirmed cause according to Warmflow's own documentation. A short circuit drives the resistance reading to near zero, making the PCB think water temperature is extremely high. An open circuit (disconnected wire or broken sensor element) gives no signal at all. Either condition causes an immediate lockout.
- Sensor probe physically dislodged from the return pipe Common
The NTC probe is held in contact with the return pipework by a clip or pocket. Vibration over time, or previous servicing work, can cause it to work loose. Even a small air gap between the probe and the pipe will cause erratic or wildly inaccurate temperature readings, eventually triggering E15.
- Corroded or damaged wiring and connector Sometimes
The low-voltage wiring harness running from the sensor to the PCB can suffer from moisture ingress, especially in condensing boilers where internal condensate is present. Corroded connector pins or chafed cable insulation can intermittently break the circuit, producing sporadic E15 faults that may clear on reset before returning.
- Failed NTC sensor at end of life Sometimes
Like any electrical component, NTC thermistors degrade over time. An ageing sensor may drift out of its expected resistance range without showing obvious physical damage, causing the PCB to flag a fault. This is more likely on older Warmflow Agentis units or boilers that have not been serviced regularly.
- PCB misreading the sensor signal Rare
If a new, correctly connected sensor still produces an E15 fault, the problem may lie with the PCB itself failing to interpret the sensor's resistance values correctly. This is comparatively rare and is usually only confirmed after a sensor replacement has been ruled out.
How to fix it
- Check that the gas supply is on and the boiler has adequate system pressure DIY safe
Before anything else, confirm the gas supply to your property is active (check other gas appliances) and look at the boiler's pressure gauge — it should read between 1 and 1.5 bar when cold. If pressure is below 1 bar, top up via the filling loop following your Warmflow Agentis manual. Low pressure or no gas supply can sometimes contribute to nuisance faults.
- Reset the boiler once or twice following the manufacturer's procedure DIY safe
Locate the reset button on your Warmflow Agentis control panel — typically held for 3–5 seconds. Attempt a reset no more than two or three times in total. If E15 returns immediately or within a short time, do not keep resetting; repeated resets will not fix a genuine sensor fault and may mask a more serious underlying issue.
- Have a qualified engineer inspect the CH return NTC sensor connection and position Gas Safe engineer
A Gas Safe registered engineer (or OFTEC-registered engineer for oil-fired models) will open the boiler casing and check that the return NTC sensor probe is correctly seated in its clip or pocket against the return pipework, and that the wiring connector is fully pushed home and free of corrosion or damage. This is work on live components inside the control panel and must not be attempted by a homeowner.
- Have the engineer test the sensor's resistance values Gas Safe engineer
Using a multimeter, the engineer will measure the sensor's resistance at a known water temperature and compare this against Warmflow's specification. A reading that is shorted (near zero ohms), open circuit (infinite ohms), or outside the expected range at the measured temperature confirms the sensor needs replacing.
- Replace the CH return NTC temperature sensor if faulty Gas Safe engineer
NTC sensors are relatively inexpensive parts and a straightforward swap for a competent engineer. The engineer will source the correct Warmflow-compatible sensor, fit it securely to the return pipework, reconnect the wiring harness, and verify the boiler operates through a full heating cycle without the E15 code returning.
- If E15 persists after sensor replacement, have the PCB inspected Gas Safe engineer
A continuing fault after a confirmed sensor replacement points toward the PCB failing to correctly process the sensor signal. Before condemning the PCB, a thorough engineer will also inspect the pump, heat exchanger, and associated wiring to rule out other causes. PCB replacement is a more costly repair and should only be undertaken once other causes have been eliminated.
- Call a Gas Safe registered engineer if the fault has not already been resolved Gas Safe engineer
If your reset attempts have not cleared E15, arrange an appointment with a Gas Safe registered engineer (or an OFTEC engineer for oil models). Do not continue trying to operate the boiler while the fault is active. Ensure your boiler receives an annual service to maintain warranty validity and reduce the likelihood of sensor-related faults recurring.
Parts you may need
- Warmflow CH Return NTC Temperature Sensor · from £25
- Sensor wiring harness / connector · from £20
- Warmflow Agentis PCB (if required) · 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–£200, depending on the underlying cause.
Frequently asked questions
Can I fix a Warmflow E15 fault myself?
No. While you can safely try a boiler reset once or twice and check system pressure, diagnosing and replacing the CH return NTC sensor requires opening the boiler casing and working near live electrical terminals. Warmflow's own documentation states that fault-finding must be carried out by a competent and suitably qualified engineer. For gas Warmflow boilers, that means a Gas Safe registered engineer; for oil-fired models, an OFTEC-registered engineer. Attempting this work yourself is unsafe and could invalidate your warranty.
How much does it cost to fix a Warmflow E15 fault in the UK?
For the most common cause — a faulty or displaced CH return NTC sensor — expect to pay roughly £100 to £200 all-in on a standard weekday appointment, covering the call-out, an hour or so of labour, the part itself (typically £15–£30), and VAT. If the PCB turns out to be the cause, the total can rise considerably; a PCB for a Warmflow Agentis can cost £180–£400 for the part alone, pushing the all-in repair to £300–£500 or more. Emergency or out-of-hours call-outs carry significant premium charges on top of these figures.
Why does my Warmflow Agentis keep showing E15 even after I reset it?
A fault that clears briefly on reset but returns within minutes or hours almost always indicates a genuine hardware problem rather than a transient glitch. The CH return NTC sensor is likely short-circuited, disconnected, or has drifted out of its calibrated resistance range. Repeated resetting will not cure the underlying issue. Book an engineer promptly — running the boiler repeatedly through lockout cycles is not recommended.
Will an annual service help prevent the E15 fault from coming back?
Yes. During a Warmflow service, an engineer will check all sensors including the CH return NTC, inspect wiring connectors for corrosion, and ensure sensor probes are correctly seated. Catching a deteriorating sensor or a corroded connector early prevents an unexpected breakdown. An annual service is also a condition of most Warmflow warranties — skipping it can render your warranty invalid.