Andrews E31 Fault Code: Causes, Fixes & Repair Costs
What does the Andrews E31 fault code mean?
The E31 code on Andrews boilers and water heaters (including ECOflo and MAXXflo series) indicates that the control board has detected a problem with the heating circuit flow sensor or flow switch. In plain terms, the boiler's electronics are unable to get a reliable reading of how much water is circulating through the system. This could be because the sensor itself has failed, because of a wiring fault between the sensor and the PCB, or because water is not flowing adequately for the sensor to register correctly. The fault is manually resettable once the underlying cause has cleared, but if it keeps returning the boiler should not continue to be operated — persistent faults can cause damage to internal controls. Note: Andrews fault codes can vary between product ranges and control board generations, so always cross-reference with your specific model's documentation or contact Andrews Water Heaters directly if you are unsure.
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 or failed flow sensor / NTC thermistor Common
The flow sensor degrades over time and can give out-of-range resistance readings or stop responding altogether. This is the most frequently diagnosed cause of E31. A Gas Safe engineer will test the sensor's resistance with a multimeter against the manufacturer's specification and replace it if it has failed.
- Corroded or loose wiring terminals Common
Even when the sensor itself is functional, corroded pins or loose connectors in the wiring loom between the sensor and the PCB can produce exactly the same fault. Engineers often find this during the same inspection as the sensor check, particularly on older or damp-exposed installations.
- Restricted water flow — pump, blockage or air lock Sometimes
If circulation is poor, the flow sensor may not register adequate movement and can trigger E31 even though it is physically intact. Causes include a failing or seized circulation pump, a clogged inlet filter, sediment build-up in the system, or an air lock preventing proper flow. Low system pressure can also contribute.
- PCB (printed circuit board) failure Rare
In cases where both the sensor and the wiring have been confirmed as good, the fault can point to the control board itself no longer reading the sensor's resistance correctly. PCB failure is relatively uncommon but should be considered once other components have been ruled out.
How to fix it
- Check system pressure and top up if needed DIY safe
Look at the pressure gauge on the boiler front panel. For most heating systems, normal operating pressure is between 1 and 1.5 bar when cold. If it has dropped below 1 bar, use the filling loop to top it up slowly. Low pressure can contribute to poor flow and trigger flow-related faults.
- Attempt a manual reset DIY safe
Press the RESET button on the boiler. E31 is designed to be manually resettable once the cause has gone away. If the boiler fires up and runs normally, monitor it closely over the next few hours. Do not reset more than two or three times — repeated resets without the fault clearing can mask an underlying problem and risk damage to the controls.
- If the fault returns, stop using the boiler and call a Gas Safe registered engineer Gas Safe engineer
A recurring or non-resettable E31 means the root cause has not been resolved. At this point internal investigation is required and must only be carried out by a Gas Safe registered engineer. Continuing to operate the boiler risks damage to the control electronics. You can verify an engineer's Gas Safe registration at gassaferegister.co.uk.
- Engineer: test flow sensor resistance and inspect the wiring loom Gas Safe engineer
Using a multimeter, measure the resistance of the flow sensor or NTC thermistor and compare it against the Andrews specification for that model. At the same time, inspect all connectors and terminals in the wiring loom between the sensor and the PCB for corrosion, looseness or damage. Replace the sensor if it is out of specification; clean or replace corroded terminals as needed.
- Engineer: check circulation pump and system for blockages or air Gas Safe engineer
If the sensor tests within specification, investigate the flow side. Check the pump is running correctly, test its output, and look for a blocked inlet filter or strainer, sediment in the system, or air locks. Flush and re-pressurise as required to restore adequate circulation.
- Engineer: test and replace the PCB if all other components are confirmed good Gas Safe engineer
Where the sensor, wiring, pump and flow path have all been verified as serviceable and the E31 fault persists, the PCB is likely at fault. The engineer should test the board's ability to read the sensor signal correctly before authorising a replacement, as PCBs are a significant cost.
Parts you may need
- Heating flow sensor / NTC thermistor · from £45
- Wiring loom / connector harness · from £35
- Circulation pump · from £120
- PCB (control board) · 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 £150–£400, depending on the underlying cause.
Frequently asked questions
Can I reset an Andrews E31 fault myself?
Yes — the E31 is designed to be manually resettable using the boiler's RESET button once the cause has gone. If the boiler runs normally after a reset and the fault does not return, keep an eye on it. However, if the fault comes back, do not keep resetting it. Repeated resets without resolving the cause can damage the control electronics, and at that point you need a Gas Safe engineer to investigate.
How much does it cost to fix an Andrews E31 fault?
For the most common repairs — replacing the flow sensor or NTC thermistor and rectifying any wiring issues — most homeowners pay in the region of £150 to £400 including parts and labour. If the circulation pump also needs replacing, costs typically sit toward the top of that range. A PCB replacement is a less common outcome but can cost £300–£500 or more depending on the specific Andrews model and part availability, especially on older commercial units where components can be harder to source. Always get a written quote before authorising work.
Why does my Andrews boiler keep showing E31 even after a new flow sensor was fitted?
If the E31 returns after a sensor replacement, the next things to check are the wiring terminals — corroded or loose connectors between the new sensor and the PCB can produce identical symptoms. If the wiring checks out, the PCB itself may no longer be reading the sensor's resistance signal correctly. An engineer should test the board's signal input before recommending a PCB replacement.
Is the Andrews E31 code the same across all Andrews models?
Not necessarily. Andrews produces a wide range of commercial and semi-commercial products including the ECOflo and MAXXflo ranges, and fault code definitions can vary between models and control board generations. E31 as a flow sensor or flow switch fault is consistent with the control board platforms used across a number of Andrews units, but you should always cross-reference with your specific model's manual. If you are unsure, contact Andrews Water Heaters directly via andrewswaterheaters.co.uk or speak to your installer with your model number to hand.