Andrews E41 Fault Code: Causes, Fixes & Repair Costs
What does the Andrews E41 fault code mean?
The E41 code on Andrews boilers signals that the control board has detected the heating flow water temperature is outside its accepted range — either running too high or too low. The boiler monitors flow temperature continuously via an NTC (Negative Temperature Coefficient) thermistor, a small sensor whose electrical resistance changes in step with temperature. When the PCB receives a reading that falls outside safe parameters, it triggers E41 and locks out to protect the appliance. The fault can stem from a genuine overtemperature or undertemperature condition in the water circuit, or it can be triggered by a faulty sensor or wiring fault that is feeding the PCB incorrect data. Andrews is a commercial and industrial specialist within the Baxi Group, and fault code definitions can vary between models — always cross-reference with the installation manual for your specific unit if you have it 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 NTC flow thermistor Common
The flow temperature sensor degrades over time or fails outright, sending a resistance value to the PCB that corresponds to an impossibly high or low temperature. The boiler cannot distinguish a broken sensor from a genuine overtemperature event, so it locks out. This is the single most frequent cause of E41.
- Loose or damaged sensor wiring Common
A corroded connector, chafed cable, or plug that has worked loose between the thermistor and the PCB produces an intermittent or open-circuit signal. The control board interprets the missing signal as an out-of-range temperature and triggers the fault, sometimes only under vibration or after the boiler has been running for a while.
- Circulation problem causing genuine overtemperature Sometimes
If the system pump is failing, seized, or running at reduced speed, hot water cannot circulate away from the heat exchanger quickly enough. The flow temperature genuinely climbs beyond the safe limit, triggering E41 as a protective lockout. Closed or partially closed zone valves and heavily air-locked circuits can cause the same effect.
- Scale or sludge restricting flow Sometimes
In hard-water areas or older systems without inhibitor, scale inside the heat exchanger and sludge in the pipework narrow the waterways, slowing circulation and concentrating heat. This causes flow temperature to spike, leading to repeated E41 lockouts even if the pump and sensor are both healthy.
- PCB misreading the sensor signal Rare
In a small number of cases the control board itself develops a fault that causes it to misinterpret a perfectly good sensor signal. This is usually only identified after the thermistor and wiring have been tested and found to be within specification.
How to fix it
- Attempt a single manual reset DIY safe
Press and hold the Reset button on the boiler's control panel until the fault clears, following the procedure described in your user guide. Allow the boiler to complete a full startup cycle and monitor whether E41 returns. If the fault reappears within a short time, do not keep resetting — repeated resets without identifying the underlying cause can mask a potentially damaging overtemperature condition.
- Check system pressure DIY safe
Look at the pressure gauge on the boiler fascia. For most Andrews heating systems the cold fill pressure should sit between 1.0 and 1.5 bar. If it is below 1.0 bar, top up via the filling loop in short bursts, checking the gauge as you go, until the reading sits in the normal band. Restore pressure before attempting another reset.
- Bleed radiators to remove trapped air DIY safe
Air pockets in the circuit restrict water flow and can cause localised overtemperature spikes. Use a radiator bleed key to release air from each radiator, starting on the ground floor and working upwards. Re-check system pressure afterwards and top up if needed, as bleeding typically causes a small pressure drop.
- Have a Gas Safe engineer test the NTC thermistor Gas Safe engineer
An engineer will isolate the boiler, remove the flow thermistor, and measure its resistance across a range of temperatures using a multimeter, comparing the readings against the manufacturer's specification table. A sensor that is open-circuit, short-circuit, or drifting outside tolerance will be replaced. Andrews-compatible NTC thermistors are generally inexpensive parts and this is often a quick repair.
- Have the sensor wiring and connectors inspected Gas Safe engineer
If the thermistor tests within spec, the engineer will inspect the full cable run from sensor to PCB, checking for corrosion on the connector pins, cracked insulation, or signs of heat damage near the heat exchanger. A damaged harness or connector block will be repaired or replaced as required.
- Have the circulation pump and system flow checked Gas Safe engineer
The engineer will verify that the pump is running at the correct speed and delivering adequate flow, using flow and temperature measurements across the heat exchanger to confirm. A pump that is seized, noisy, or running at reduced output will need to be replaced. Zone valves and any motorised components in the circuit will also be checked.
- Commission a system flush if sludge or scale is suspected Gas Safe engineer
If the heat exchanger or pipework shows evidence of magnetite sludge or limescale — common on older commercial systems in hard-water regions — a power flush or chemical flush may be recommended. A corrosion inhibitor should be dosed into the circuit afterwards to prevent recurrence. This is particularly worthwhile if multiple radiators are cold at the bottom or the pump is working harder than expected.
- Have the PCB inspected or replaced if all other checks pass Gas Safe engineer
If the thermistor, wiring, pump, and circulation are all confirmed as healthy but E41 persists, the control board is the likely culprit. The engineer will inspect the PCB for visible damage, burning, or failed components and advise on repair or replacement. Andrews PCBs are specialist commercial components; confirm parts availability with the engineer before proceeding.
- Call a Gas Safe registered engineer if the fault does not clear after the DIY steps above Gas Safe engineer
If resetting, restoring pressure, and bleeding radiators do not resolve E41, or if the fault returns repeatedly, contact a Gas Safe registered engineer. For Andrews commercial appliances, an engineer familiar with commercial heating equipment is preferable. You can find registered engineers at GasSafeRegister.co.uk.
Parts you may need
- Flow NTC thermistor (Andrews-compatible) · from £28
- Sensor wiring harness / connector · from £22
- Circulation pump (commercial grade) · from £145
- System inhibitor (e.g. Fernox F1, 1 litre) · from £18
- PCB (Andrews model-specific — confirm compatibility) · from £290
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–£380, depending on the underlying cause.
Frequently asked questions
Can I keep resetting the boiler to clear E41?
You should try one reset to see whether the fault clears after a transient event such as a momentary temperature spike. However, if E41 comes back — especially quickly — do not continue resetting. A persistent E41 indicates an underlying fault that needs diagnosis. Repeatedly forcing the boiler back on when it has a genuine overtemperature problem risks damaging the heat exchanger and other components.
How much does it typically cost to fix an Andrews E41 fault in the UK?
For the most common cause — an NTC thermistor replacement — you can expect to pay roughly £120–£200 all-in, covering the part (typically £15–£40) and an hour or so of a Gas Safe engineer's labour. If the pump needs replacing or a system flush is required, costs tend to fall in the £250–£380 range. A PCB replacement is rarer and would push costs higher — typically £350–£550 depending on the specific Andrews model — but this is only necessary if all other components have been ruled out.
Does E41 mean my Andrews boiler is dangerous to use?
The boiler has locked itself out as a safety measure, so it will not fire while displaying E41. Do not attempt to bypass the lockout. If you can smell gas at any point, leave the property, avoid operating any switches, and call the National Gas Emergency Service on 0800 111 999 immediately. For a non-gas smell E41 lockout, the appliance is simply waiting for the fault to be investigated — it is not operational, which is the safe state to be in.
My Andrews boiler is commercial — does E41 mean the same thing across all models?
Andrews produces a wide range of commercial water heaters and boilers, and fault code definitions can differ between product families and firmware versions. E41 most commonly relates to a flow temperature sensor fault across the range, but always cross-reference with the installation and service manual for your specific model. If you do not have the manual, Andrews Water Heaters' technical support line or a specialist commercial heating engineer can confirm the exact definition for your unit.