Andrews A24 Fault Code: Causes, Fixes & Repair Costs
What does the Andrews A24 fault code mean?
On the Andrews MAXXflo, fault codes are displayed as a letter followed by a number. The letter indicates the fault type, and the number identifies which burner module is affected. The letter 'A' denotes a lockout fault — meaning the burner module has been shut down by the control system and cannot attempt to restart without a manual reset. The number '24' breaks down as follows: '2' identifies the fault type (gas valve relay error) and '4' identifies burner module 4 as the one that has locked out. In plain terms, the control system sent a command to open or close the gas valve relay on module 4 but received no confirmation that the relay responded correctly. As a safety measure, the system immediately locked that module out to prevent any unsafe condition. Note that the same relay fault occurring on other modules would appear as A21 (module 1), A22 (module 2), or A23 (module 3) — the diagnostic approach for each is identical, only the affected module differs.
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 gas valve relay contacts Common
The relay contacts inside the burner control assembly can burn out, pit, or stick over time — particularly on a heavily used commercial or semi-commercial water heater like the MAXXflo. When the contacts fail to open or close cleanly, the control system cannot confirm the relay has operated and immediately triggers the lockout. This is the single most common root cause of an A24 code.
- Wiring fault or loose connection between the control board and gas valve Common
Vibration, heat cycling, and age can cause terminals to loosen or wiring insulation to crack and short. If the signal from the control board to the gas valve relay cannot travel cleanly — or if the feedback signal back to the board is interrupted — the controller interprets this as a relay failure and locks out module 4. A visual inspection and continuity check of the harness will usually reveal this quickly.
- Defective gas valve Sometimes
The gas valve itself contains internal electrical components. If the solenoid coil or internal wiring within the valve has failed, the relay may be working correctly but the valve will not respond as expected, causing the control system to register a relay error. A resistance check across the valve coil terminals will help distinguish this from a pure relay or wiring fault.
- Failed burner control board (PCB) Sometimes
If the PCB on module 4 has developed a fault — for example a failed relay driver circuit or a damaged output stage — it may be unable to switch the relay correctly even though the gas valve and wiring are both in good condition. PCB failure is less common than wiring or relay faults but becomes more likely on older units or where there has been a history of electrical surges.
How to fix it
- Check the display and note the exact fault code shown DIY safe
Before doing anything else, confirm the display is alternating between 'A' and '24'. This distinguishes a module 4 relay lockout from other 'A' codes on different modules. Take a photo of the display if possible — your engineer will find it useful.
- Attempt a single manual reset from the control panel DIY safe
The MAXXflo requires a manual reset from the control panel GUI to clear a lockout. Follow the reset procedure described in your unit's quick-reference guide (usually pressing and holding the reset button for the affected module). Do not attempt more than one or two resets. If the code returns immediately or within a short time, the underlying fault is still present and further resets will not help — and could mask important diagnostic information for your engineer.
- Do not attempt to open or interfere with the gas valve, relay, or control board Gas Safe engineer
The gas valve relay and burner control PCB are gas-side and high-voltage electrical components. Any work on these parts must be carried out by a Gas Safe registered engineer with the appropriate commercial heating competencies. Unauthorised intervention could invalidate your warranty, create a dangerous condition, and is contrary to Andrews' own installation and service requirements.
- Have a Gas Safe engineer inspect the wiring harness and terminals on module 4 Gas Safe engineer
The engineer will visually inspect the wiring loom between the burner control board and the gas valve for signs of burning, chafing, corrosion, or loose push-fit terminals. They will then perform continuity and resistance checks to confirm the harness is intact and the gas valve solenoid coil is within specification.
- Have the engineer test the gas valve relay operation Gas Safe engineer
Using a multimeter and the appropriate service mode (if available on the MAXXflo controller), the engineer will command the relay to open and close while monitoring whether it responds correctly. A relay that fails to switch, switches intermittently, or shows high contact resistance will need to be replaced — either as a standalone component if available, or as part of the gas valve assembly.
- Replace the gas valve or relay assembly if found to be faulty Gas Safe engineer
If the relay contacts or gas valve solenoid are confirmed defective, the engineer will fit a manufacturer-approved replacement part. On the MAXXflo this is a module-level repair — only the affected module 4 assembly needs attention, and the remaining modules can continue to operate in the interim if the system allows partial operation.
- Replace the burner module control board (PCB) if the relay and valve test satisfactory Gas Safe engineer
If the gas valve and wiring are both confirmed good but the A24 fault persists, the fault lies within the PCB's relay-driving circuitry. The engineer will source and fit a replacement control board for module 4. After fitting, the module will need to be recommissioned and function-tested before the unit is returned to full service.
- Request a full commissioning check and test of all modules after repair Gas Safe engineer
Once the fault has been rectified, ask the engineer to run a full operational test across all four burner modules and confirm correct gas valve operation, ignition, and flame detection on each. This is particularly important on a multi-module unit where one module's issues can sometimes mask developing faults on others.
Parts you may need
- Gas valve assembly (module-specific, MAXXflo) · from £180
- Burner module control PCB · from £220
- Gas valve relay (if separately serviceable) · from £35
- Wiring harness / connector kit for burner module · from £40
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 £250–£550, depending on the underlying cause.
Frequently asked questions
Can I keep using hot water while the A24 fault is showing?
The MAXXflo is a multi-burner unit, so if only module 4 has locked out, the remaining modules (1, 2, and 3) may continue to fire and provide some hot water output — though at reduced capacity. However, you should still arrange a repair promptly. Running a commercial water heater with a known fault for an extended period puts additional strain on the working modules and can lead to further breakdowns. If the unit has shut down completely, do not attempt repeated resets — call an engineer.
Why does my display show A21, A22, or A23 instead of A24 — is it the same problem?
Yes, essentially the same fault on a different module. On the Andrews MAXXflo, the number after the 'A' identifies both the fault type and the affected burner module. A21 means a gas valve relay error on module 1, A22 on module 2, A23 on module 3, and A24 on module 4. The diagnostic process and likely causes are identical whichever module is affected — only the location of the fault within the unit differs.
How much does it cost to fix an Andrews MAXXflo A24 fault?
Most people with this fault pay somewhere between £250 and £550 all-in, covering the engineer's time and the most commonly needed parts (gas valve assembly or relay). If the fault turns out to be a simple wiring or terminal issue, the bill will be towards the lower end. If a full PCB replacement is needed alongside the gas valve work, costs can push towards £600–£700. The MAXXflo is a commercial-grade unit, so parts can be pricier and harder to source quickly than standard domestic boiler components — it is worth asking your engineer to confirm part availability before booking the repair.
Does an Andrews MAXXflo repair have to be done by a Gas Safe engineer?
Yes. Andrews specify that all installation and servicing must be carried out by competent, qualified personnel. In the UK, any work involving the gas valve, burner components, or associated control systems must legally be done by a Gas Safe registered engineer with suitable commercial heating competencies. Working on this equipment without the correct registration is illegal, potentially dangerous, and will void any remaining warranty or service agreement.