Warmflow E42 Fault Code: Causes, Fixes & Repair Costs
What does the Warmflow E42 fault code mean?
The E42 code appears on certain Warmflow Agentis combination boiler models and, based on the fault numbering pattern used across the Warmflow range, most likely indicates a domestic hot water (DHW) temperature sensor or thermistor fault. The boiler's PCB has detected an abnormal reading — or no reading at all — from one of the sensors monitoring the hot water circuit, and has shut down or locked out to prevent damage or unsafe operation. It is worth noting that E42 does not appear in all publicly available Warmflow manuals; the code is most relevant to Agentis combi variants. If you cannot find E42 in your model's documentation, contact Warmflow directly on 028 9262 1515 with your exact model number to confirm the definition before attempting any remedial action.
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 failing NTC thermistor (temperature sensor) Common
The NTC thermistor is a small resistive component that continuously feeds temperature data from the DHW circuit to the PCB. Over time these sensors degrade, give out-of-range resistance readings, or fail completely — triggering the E42 lockout. This is the single most common root cause of sensor-type fault codes across the Warmflow Agentis range.
- Loose or corroded wiring connection to the sensor Common
Vibration during normal boiler operation, or simply the passage of time in a damp plant-room environment, can loosen the connector between the sensor and the PCB wiring harness, or cause corrosion that breaks the signal circuit. The sensor itself may be perfectly healthy but return an intermittent or absent signal.
- Limescale build-up affecting sensor accuracy Sometimes
In hard-water areas of the UK, limescale can coat heat exchanger surfaces and the sensors embedded in them, insulating the sensor from the water temperature it is trying to read. Warmflow's own guidance notes that inline water softeners may be necessary in hard-water regions. The fault may be intermittent at first, becoming permanent as scale accumulates.
- PCB fault Sometimes
If the PCB itself has developed a fault — due to a power surge, moisture ingress, or component failure — it may misinterpret correct sensor signals or fail to process them entirely, generating a spurious E42. A Gas Safe or OFTEC engineer will typically rule out the sensor and wiring before condemning the PCB.
How to fix it
- Check the boiler's LCD display and note the exact code shown DIY safe
Warmflow Agentis boilers display fault codes in blue (warning) or red (error/lockout) on the LCD. Confirm you are seeing E42 and not a similar code such as E02, E05, or E22, as these have different meanings. Take a photo for reference before resetting.
- Attempt a single boiler reset DIY safe
Press the reset button (located under the right-hand side of the control panel on Agentis models) once and observe whether the fault clears. If the boiler fires up and runs normally, monitor it over the next 24 hours. If E42 returns immediately or after a short time, do not keep resetting — repeated resets without addressing the underlying fault can mask worsening problems.
- Check system water pressure DIY safe
The pressure gauge or LCD pressure display should read between 1.0 and 1.5 bar when the system is cold. If it reads below 1.0 bar, top up via the filling loop until pressure returns to around 1.2 bar, then attempt one further reset. Low pressure alone is unlikely to cause E42 but can contribute to abnormal sensor readings.
- Consult your model-specific manual and contact Warmflow if E42 is not listed DIY safe
Because E42 is not confirmed in all publicly available Warmflow documentation, it is worth downloading the correct manual for your exact model from Warmflow's website or ringing Warmflow's service department on 028 9262 1515. Quote your model number and serial number so they can confirm the fault definition before an engineer visits.
- Arrange a visit from a Gas Safe registered (gas) or OFTEC registered (oil) engineer Gas Safe engineer
Warmflow Agentis boilers may be gas or oil-fired. Gas models must be worked on by a Gas Safe registered engineer; oil models require an OFTEC registered technician. The engineer will use diagnostic equipment to test the DHW thermistor's resistance against the manufacturer's specification, inspect the wiring loom and connectors for corrosion or damage, and check the PCB's sensor-input circuits.
- Engineer to test and replace the DHW thermistor if faulty Gas Safe engineer
NTC thermistors are relatively inexpensive parts. The engineer will isolate the boiler, disconnect the sensor, measure its resistance across the specified temperature range, and replace it if it falls outside tolerance. This is the most common fix for sensor-type fault codes and often resolves the fault in a single visit.
- Engineer to inspect and reseat all wiring connections Gas Safe engineer
Even if the thermistor tests within specification, loose or corroded connectors can produce identical symptoms. The engineer will check every connector between the sensor and the PCB, clean any corroded terminals, and reseat connections to ensure a reliable signal path.
- Engineer to assess for limescale and recommend a powerflush if appropriate Gas Safe engineer
If the boiler is installed in a hard-water area and there are signs of scaling, a powerflush may be recommended to clear the heat exchanger and restore correct sensor contact with the water. If scaling has caused heat exchanger damage, the engineer will advise on repair or replacement options.
- Engineer to test or replace the PCB if all other components check out Gas Safe engineer
If the thermistor and wiring are both confirmed healthy but E42 persists, the PCB is the remaining suspect. PCB replacement is a specialist task involving isolation of gas or oil supplies, electrical isolation, and reconfiguration of boiler parameters. The engineer will inspect the heat exchanger, pump, fan, and gas valve before condemning the PCB to ensure no secondary fault is present.
Parts you may need
- NTC thermistor / DHW temperature sensor (Warmflow Agentis compatible) · from £25
- Sensor wiring harness / connector loom · from £35
- PCB (printed circuit board) for Warmflow Agentis · from £290
- Plate heat exchanger (Warmflow Agentis combi) · from £180
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 £120–£350, depending on the underlying cause.
Frequently asked questions
Is E42 definitely a sensor fault on my Warmflow boiler?
Based on the fault-code numbering pattern used across the Warmflow Agentis combination boiler range, E42 is most consistent with a domestic hot water (DHW) temperature sensor or thermistor fault. However, because this code does not appear in all publicly available Warmflow manuals, you should verify it with Warmflow directly (028 9262 1515) or check your specific model's manual before arranging a repair. Giving your model number and serial number to Warmflow or to your engineer will allow them to confirm the exact definition.
Can I fix a Warmflow E42 fault myself?
Homeowners can safely try one boiler reset, check the system pressure is between 1.0–1.5 bar and top up if needed, and note whether the fault is intermittent or permanent. Beyond that, the repair involves testing and replacing electrical components (thermistor, wiring, or PCB), which must be carried out by a Gas Safe registered engineer for gas models or an OFTEC registered technician for oil-fired models. Attempting to open the boiler casing or work on sensors without the appropriate registration is both illegal and potentially dangerous.
How much will it cost to fix a Warmflow E42 fault?
For the most common repair — replacing the DHW thermistor and checking the wiring — most homeowners pay somewhere in the region of £120–£250 including callout, labour, and the part. If the PCB needs replacement, expect to pay £250–£450. A heat exchanger replacement is significantly more expensive (typically £400–£750 or more including labour) and at that cost it is worth getting a boiler replacement quote alongside the repair quote, particularly if the boiler is out of warranty.
Will my Warmflow warranty cover an E42 repair?
Warmflow's standard warranty covers defective parts and workmanship for 24 months from manufacture, extendable to 5 years on registration via the Warmflow website. To keep the warranty valid, the boiler must receive an annual service from an OFTEC registered technician (oil models) or a Gas Safe registered engineer (gas models). If this has lapsed, the warranty may be void. Contact Warmflow's service department before arranging an independent repair, as a valid warranty claim could mean no cost to you for parts or labour.