Andrews E63 Fault Code: Causes, Fixes & Repair Costs
What does the Andrews E63 fault code mean?
The E63 code on Andrews MAXXflo commercial condensing water heaters signals that the return water temperature sensor has developed a short circuit. The unit's control system continuously monitors this sensor to track the temperature of water returning to the appliance; when the sensor reading falls outside the expected range due to a short, the system locks out as a safety measure. On multi-burner MAXXflo units, the display alternates between the fault letter and the number of the affected burner module, helping an engineer pinpoint exactly which module requires attention. Related codes in the same sequence include E64 (flow sensor short circuit) and E78 (too many user resets), which can be useful context if more than one code appears.
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 return NTC thermistor Common
The return sensor itself has developed an internal short circuit, sending an implausible signal to the control board. This is the most straightforward cause and usually means the sensor needs replacing outright.
- Damaged or corroded wiring Common
The cable run between the return sensor and the PCB can suffer from chafing, heat damage, or corrosion at connectors. A short anywhere in this harness will produce the same E63 lockout as a failed sensor.
- Moisture or water ingress at the sensor connections Sometimes
Condensate or system water finding its way into the sensor plug or terminal block creates a short circuit path. This is particularly likely if there has been a recent leak near the sensor pocket.
- Scale build-up in hard water areas Sometimes
In areas with high water hardness, mineral deposits can accumulate around the sensor pocket and affect heat transfer, potentially stressing the sensor over time until it fails electrically.
- Master or slave PCB fault Rare
If the sensor and wiring both test correctly, the control board itself may be misinterpreting the sensor signal or has developed a fault on the sensor input circuit. This is the least common but most costly explanation.
How to fix it
- Do not repeatedly reset the appliance DIY safe
Andrews E codes indicate a lockout that requires the underlying fault to be fixed before a reset will hold. Pressing reset more than two or three times without resolving the cause risks triggering an E78 (too many resets) lockout on top of the E63, and can mask important diagnostic information for the engineer.
- Note which burner module number is displayed DIY safe
On multi-burner MAXXflo units the display alternates between 'E' and a number indicating the affected burner module. Write this down before calling an engineer — it will help them go straight to the right module and speed up diagnosis.
- Check for any obvious water leaks near the appliance DIY safe
A visual inspection around the base and pipework of the unit can reveal whether water ingress may have contributed to the sensor fault. Do not open the casing — simply note what you can see from the outside and report it to the engineer.
- Arrange a visit from a Gas Safe registered engineer Gas Safe engineer
Because the E63 fault involves internal sensor wiring and potentially the control board, all further diagnosis and repair must be carried out by a Gas Safe registered engineer with experience of commercial Andrews MAXXflo equipment. Consider contacting Andrews Water Heaters (part of Baxi Heating UK) directly for details of approved service engineers familiar with the MAXXflo control system.
- Engineer to inspect wiring harness and sensor connections Gas Safe engineer
The engineer will check the wiring between the return sensor and the PCB for signs of damage, loose terminals, or corrosion. A disconnected or poorly seated plug is sometimes all that is responsible for the short circuit reading.
- Engineer to test and replace the return NTC sensor if faulty Gas Safe engineer
Using a multimeter to measure resistance across the sensor terminals, the engineer can confirm whether the thermistor itself has shorted. If so, it will be replaced with an Andrews-approved part and all connections re-secured.
- Engineer to investigate the PCB if the sensor and wiring are sound Gas Safe engineer
Should the sensor and harness both prove serviceable, the master or slave control board will need testing. A faulty board may require repair or full replacement, which on a commercial MAXXflo unit can be a significant cost — the engineer should quote before proceeding.
- Engineer to reset and recommission the appliance Gas Safe engineer
Once the root cause has been resolved, the engineer will perform a controlled reset of the affected burner module, verify the return sensor is reading correctly, and carry out a full functional check before returning the appliance to service.
Parts you may need
- Return NTC temperature sensor (thermistor) · from £45
- Sensor wiring harness / loom · from £55
- MAXXflo master control PCB · from £420
- Sensor pocket / thermowell · from £30
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–£500, depending on the underlying cause.
Frequently asked questions
Can I reset the Andrews E63 fault myself and keep using the water heater?
You can attempt a single reset using the reset button on the affected burner module, but the E63 is a lockout fault that will return immediately if the underlying sensor fault has not been fixed. Repeatedly resetting without a repair can trigger an E78 (too many resets) lockout, which adds another layer of complication. The appliance should be treated as out of service until a Gas Safe engineer has diagnosed and resolved the sensor fault.
How do I know which burner module is affected on my MAXXflo?
The MAXXflo display alternates between the letter 'E' and a digit representing the burner module number. Make a note of this number before calling your engineer — on a multi-burner unit (for example a CWH90 or CWH120) this identifies exactly which module has locked out, allowing the rest of the appliance to potentially remain in service while awaiting repair, depending on how your system is configured.
What is the difference between E63 and E64 on an Andrews MAXXflo?
Both are sensor short circuit lockouts, but they relate to different parts of the water circuit. E63 points to the return sensor (monitoring water coming back into the unit), while E64 indicates a short circuit on the flow sensor (monitoring water leaving the unit). The diagnostic and repair process is similar for both — wiring inspection followed by sensor testing — but the physical location of the sensor differs, so an engineer familiar with the MAXXflo layout will know exactly where to look for each code.
Why are Andrews MAXXflo repairs more expensive than a standard domestic boiler?
The MAXXflo is a commercial-grade condensing storage water heater designed for hotels, care homes, schools, and large residential schemes. Replacement parts — particularly control boards — are more specialised and priced accordingly. Labour costs can also be higher because the engineer needs specific knowledge of the MAXXflo control system. For a sensor replacement the total bill is typically in the £150–£300 range, but if the PCB is implicated you could be looking at £400–£600 or more for parts alone on some models. Contacting Andrews Water Heaters (Baxi Heating UK) for an approved service engineer is advisable to ensure the correct parts and expertise.