Andrews E42 Fault Code: Causes, Fixes & Repair Costs
What does the Andrews E42 fault code mean?
The E42 code on Andrews water heaters and boilers indicates that the control board has detected an abnormal temperature reading on the water returning from your heating circuit — the pipework bringing cooled water back to the boiler after it has passed through your radiators or underfloor heating. The return temperature is either too high or too low compared to what the system expects, so the boiler locks out as a safety precaution. Once the underlying problem has been resolved, a manual reset using the boiler's reset button is required to restore normal operation. Note: Andrews Water Heaters (part of the Baxi Group) primarily serves commercial applications, and fault code definitions can vary between models and production years. If the code behaviour on your unit does not match the description here, always cross-reference with your specific Andrews installation and service manual or contact Andrews technical support directly with your model number to hand.
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
- Faulty Return NTC Thermistor Common
The return NTC (negative temperature coefficient) sensor is a small resistor that changes its electrical resistance in response to water temperature. When it develops a fault — through age, moisture ingress, or physical damage — it can feed the control board inaccurate readings, causing a spurious lockout even when the actual water temperature is perfectly normal. This is the single most common root cause of an E42 fault.
- Restricted Water Circulation or Pump Failure Common
If the circulation pump is struggling — due to wear, an airlock, or partial seizure — heated water moves through the system more slowly than designed. The temperature difference between the flow and return pipes then exceeds the safe limit set by the control board. Sludge and debris from the heating system can also coat pump internals and restrict impeller movement, producing the same effect without the pump itself being worn out.
- Sludge or Limescale Blocking the Heat Exchanger Sometimes
In hard water areas across the UK, limescale builds up inside the heat exchanger over time. In older or poorly maintained systems, magnetite sludge — corroded metal particles — accumulates in the same way. Both reduce water flow through the heat exchanger, creating hot spots and causing the return temperature to read outside normal parameters.
- Wiring or Connector Fault on the Return Sensor Circuit Sometimes
The wiring loom and plug connectors linking the return thermistor to the PCB can suffer from corrosion, heat damage, or simply work loose over time. A poor connection introduces electrical resistance that the control board interprets as an abnormal temperature reading, triggering the E42 lockout even though the sensor itself is undamaged.
- PCB (Control Board) Fault Rare
In a minority of cases, the PCB itself develops a fault that causes it to misread or fail to process the return temperature signal correctly. This is typically a diagnosis of exclusion — reached only after the sensor, wiring, pump, and system condition have all been checked and found to be acceptable.
How to fix it
- Reset the boiler using the reset button DIY safe
Press and hold the reset button as described in your Andrews boiler handbook. Allow the boiler a few minutes to attempt restart. If it locks out again immediately and shows E42, do not reset more than two or three times in quick succession — repeated unsuccessful resets without resolving the underlying cause can mask the fault and cause additional wear.
- Check system pressure DIY safe
Look at the pressure gauge on the boiler front panel. For most Andrews heating boilers, normal operating pressure is between 1.0 and 1.5 bar when the system is cold. If it reads below 1 bar, top it up slowly via the filling loop until it reaches approximately 1.2–1.3 bar, then re-attempt a reset. If you are unsure how to use the filling loop safely, refer to your boiler handbook.
- Check that radiator TRVs and zone valves are open DIY safe
If thermostatic radiator valves (TRVs) are closed on most or all radiators, flow through the system is severely restricted, which can cause temperature differentials that trigger an E42 lockout. Open TRVs on several radiators and ensure any manual zone valves serving the heating circuit are also fully open, then try resetting again.
- Bleed radiators to clear trapped air DIY safe
Air pockets in radiators reduce the volume of water circulating and can cause uneven temperature distribution across the system. Use a radiator bleed key to release air from the bleed valve at the top of each radiator until water flows steadily. Re-check and top up system pressure afterwards if it has dropped, then reset the boiler.
- Have a Gas Safe engineer test the return NTC thermistor Gas Safe engineer
An engineer will use a calibrated multimeter to check the thermistor's resistance at a known water temperature and compare it against the manufacturer's resistance-temperature curve. A reading that falls outside the acceptable range confirms the sensor has failed and needs replacing. This is a straightforward repair on most Andrews models and often resolves the E42 fault entirely.
- Have the circulation pump inspected and serviced or replaced Gas Safe engineer
An engineer will check pump operation, look for signs of seizure or reduced output, and assess whether debris from the system has restricted the impeller. A pump service (freeing a seized shaft) may suffice on minor cases, but a failed pump will need replacement. The engineer will also check for air in the pump head and ensure the pump speed setting is correct for the system.
- Commission a system flush if sludge or scale is suspected Gas Safe engineer
If internal inspection or a water sample reveals significant magnetite sludge or limescale, an engineer will carry out a power flush — using pressurised water and a chemical cleaning agent circulated through the system — to dislodge and expel debris from pipework, radiators, and the heat exchanger. A magnetic system filter should be installed or cleaned at the same time to prevent recurrence.
- Have wiring and connector integrity checked Gas Safe engineer
An engineer will inspect the wiring loom from the return thermistor back to the PCB, checking for corrosion on connector pins, chafed insulation, or loose terminals. Damaged sections of wiring or corroded connectors can be repaired or replaced without needing to touch any gas components.
- Call a Gas Safe registered engineer if the fault persists Gas Safe engineer
If none of the above steps resolve the E42 code, the PCB may need professional diagnosis and replacement. Andrews Water Heaters also has a dedicated technical support line — having your model number and serial number to hand will allow them or your engineer to confirm the exact fault code definition for your specific unit and advise on any model-specific procedures.
Parts you may need
- Return NTC thermistor (temperature sensor) · from £25
- Circulation pump (compatible replacement) · from £85
- PCB / control board · from £220
- Inline magnetic system filter · from £55
- Wiring loom / connector repair kit · from £15
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
Can I fix an Andrews E42 fault myself without calling an engineer?
There are a few safe checks you can carry out yourself: resetting the boiler (up to two or three times), topping up system pressure via the filling loop, bleeding radiators to release trapped air, and making sure TRVs and zone valves are open. However, any internal work — testing or replacing the thermistor, inspecting the pump, flushing the system, or diagnosing the PCB — must be carried out by a Gas Safe registered engineer. Andrews water heaters are predominantly commercial products, so finding an engineer with experience on the brand is worth the effort.
How much does it typically cost to repair an Andrews E42 fault in the UK?
Most people with an E42 fault end up paying somewhere between £150 and £450 all-in, covering the engineer's call-out, diagnosis, and the most common repairs such as a thermistor replacement or pump service. A full pump replacement tends to sit in the £200–£400 range, while a power flush for sludge removal typically costs £300–£500 depending on system size and how heavily contaminated it is. If the PCB turns out to be the root cause, expect to add £300–£600 for parts and labour on top of the diagnostic fee. Heat exchanger replacement is rarer but can reach £450–£750 or more — at that level on an older boiler, replacement may be the more economical route.
Why does my Andrews boiler keep returning to E42 after I reset it?
If the E42 code reappears within a short time of resetting, the underlying fault has not been cleared. A one-off lockout that resolves with a single reset is often a momentary circulation hiccup or a minor air bubble passing through the sensor circuit. Repeated lockouts point to a persistent fault — most likely a genuinely failing return thermistor giving consistently wrong readings, chronic poor circulation from a degraded pump, or significant sludge build-up reducing water flow. Continued resetting without investigation will not fix these issues and may eventually cause additional damage, so an engineer visit is the appropriate next step.
Is Andrews the same as Baxi — will a standard Baxi engineer be able to work on it?
Andrews Water Heaters is indeed part of the Baxi Group, and many Andrews commercial water heaters share control platform technology with Baxi and Potterton products. In practice, most experienced Gas Safe engineers familiar with the Baxi product family will be able to work on Andrews units. That said, Andrews focuses on commercial and semi-commercial applications, so it is worth confirming the engineer has experience with commercial water heater systems before booking. For model-specific fault code confirmation and parts support, Andrews has its own dedicated technical helpline.