Warmflow E05 Fault Code: Causes, Fixes & Repair Costs
What does the Warmflow E05 fault code mean?
The E05 code on a Warmflow boiler signals a problem with the boiler flow NTC (Negative Temperature Coefficient) thermistor — the sensor responsible for monitoring the temperature of water circulating through the heating circuit. On the Agentis Series this is described as a Boiler NTC Error, while on the G-Series (such as the GS25A) it may appear labelled as a Heating Sensor Fault, but both refer to the same underlying issue. An NTC thermistor works by changing its electrical resistance as temperature rises or falls; the boiler's PCB reads that resistance to determine how hot the water is and adjusts the burner accordingly. When the PCB receives a signal that is absent, wildly out of range, or simply implausible, it treats the situation as potentially unsafe and locks the boiler out, displaying E05. The fault can stem from the sensor itself failing (either going open-circuit or drifting out of calibration), from wiring faults between the sensor and the PCB, or — less commonly — from a problem with the PCB itself.
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
- Failed or degraded NTC thermistor Common
The sensor itself is the most frequent culprit. Over time the thermistor element can fail open-circuit — producing an infinite resistance reading that the PCB immediately flags as a fault — or it can drift so that its resistance-temperature curve no longer matches the expected values, causing erratic or implausible temperature readings. Either mode typically triggers an immediate lockout.
- Loose, damaged, or corroded wiring connections Common
The low-voltage signal cable running between the flow sensor and the PCB is vulnerable to vibration, heat cycling, and moisture. A loose spade connector, a corroded pin, or a chafed wire can mimic a failed sensor entirely. Engineers will check the wiring harness and connectors at both ends before condemning the sensor itself.
- Limescale build-up in the sensor pocket Sometimes
In hard-water areas of the UK, scale can accumulate around the sensor pocket on the heat exchanger, effectively insulating the thermistor from the water it is supposed to be measuring. This causes the sensor to report temperatures that lag behind reality, leading to erratic burner behaviour and eventually an E05 lockout.
- Faulty PCB Rare
If the sensor and its wiring both test correctly with a multimeter, the fault may lie on the PCB itself — specifically the input circuitry that reads and interprets the thermistor's resistance. A damaged or component-failed PCB can generate false E05 codes even when the sensor is perfectly healthy. This is the least common cause and is usually confirmed only after everything else has been ruled out.
How to fix it
- Attempt a single boiler reset DIY safe
Locate the reset button on your Warmflow boiler's control panel (refer to your user guide if needed) and hold it until the boiler attempts to restart. If the E05 code disappears and the boiler runs normally, monitor it over the next hour. If the fault returns promptly, do not keep resetting — repeated lockouts with the same code mean something needs physical inspection, and further resets will not help.
- Check that your gas supply is working DIY safe
Confirm that other gas appliances in your home (hob, gas fire) are working normally. If there is no gas reaching the property, contact your gas supplier rather than an engineer. A gas interruption can sometimes cause unusual fault codes as the boiler tries and fails to fire.
- Call a Gas Safe registered engineer to test the NTC sensor Gas Safe engineer
An engineer will disconnect the sensor's wiring and measure its resistance with a multimeter at a known ambient temperature, comparing the reading against the manufacturer's resistance-temperature table. A reading that is open-circuit, short-circuit, or significantly off the expected curve confirms sensor failure. For oil-fired Warmflow boilers, you should use an OFTEC registered technician rather than a Gas Safe engineer.
- Have the engineer inspect and test the sensor wiring and connectors Gas Safe engineer
Even if the sensor tests within tolerance on the bench, intermittent faults can be caused by a poor connection at the plug or a hairline break in the cable insulation that only shows under operating conditions. The engineer will check continuity along the full length of the harness and clean or replace corroded connectors as needed.
- Have the engineer clean any limescale from the sensor pocket Gas Safe engineer
If scale is found around the sensor pocket on the heat exchanger, the engineer will remove the build-up carefully. In hard-water properties it is also worth discussing whether a scale reducer or system inhibitor should be added to reduce future accumulation.
- Replace the faulty NTC sensor if confirmed defective Gas Safe engineer
NTC thermistors are relatively inexpensive parts. Once the engineer has confirmed the sensor is outside specification, they will fit a replacement, reconnect the wiring, and test the boiler through a full heating cycle to verify the E05 code does not return.
- If the sensor and wiring are both healthy, have the PCB investigated Gas Safe engineer
A Gas Safe engineer will carry out live electrical checks inside the control panel — work that must only be performed by a qualified person. If the PCB input circuit is found to be at fault, the board will need to be replaced. Given the cost of a new PCB (typically £180–£400 for the part alone), the engineer may also advise whether the boiler's age makes full replacement a more economical option.
Parts you may need
- Boiler flow NTC thermistor (sensor) · from £25
- Sensor wiring harness / loom · from £35
- Warmflow PCB (control board) · from £280
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–£280, depending on the underlying cause.
Frequently asked questions
Is it safe to keep using my Warmflow boiler while E05 is showing?
No. The E05 lockout means the boiler has shut itself down because it cannot reliably monitor the water temperature in the heating circuit. Running it in this condition could allow the water to overheat without the boiler detecting it. Leave the boiler in lockout and arrange for an engineer to inspect it promptly, especially during cold weather.
How much does it cost to fix a Warmflow E05 fault?
In the majority of cases the repair involves replacing the flow NTC sensor, which is a straightforward job. Including the engineer's callout, labour, and the part itself, most homeowners pay somewhere between £100 and £280 on a standard weekday appointment. If the PCB turns out to be the root cause rather than the sensor, the cost of the board alone is typically £180–£400, so the total bill could rise to £350–£500 or more. Prices in London and the South East tend to run around 15–25% higher than the national average, and evening or weekend callouts usually attract a 30–50% uplift.
My Warmflow boiler keeps coming back with E05 after I reset it — what should I do?
A fault code that returns immediately or very quickly after a reset almost always means there is a genuine component problem that needs hands-on investigation. Repeated resets will not fix the underlying issue and in some cases can mask how frequently the fault is occurring. Reset the boiler no more than two or three times in total, and if E05 keeps reappearing, call a Gas Safe registered engineer (or OFTEC registered technician if you have an oil-fired Warmflow).
Could a hard-water area make E05 more likely on my Warmflow boiler?
Yes. Limescale can build up around the sensor pocket on the heat exchanger, insulating the thermistor from the water it is meant to be reading. This causes the sensor to report temperatures that do not reflect actual water conditions, which can trigger the E05 lockout. If you live in a hard-water area of the UK (much of the South and East of England, for example), ask your engineer to check for scale during the repair visit and consider adding a scale inhibitor or magnetic filter to the system to slow future build-up.