Andrews A03 Fault Code: Causes, Fixes & Repair Costs
What does the Andrews A03 fault code mean?
The A03 fault code on Andrews boilers (including Andrews Water Heaters commercial units) signals a protective lockout, most commonly triggered by one of three underlying issues: overtemperature protection activating when the flow temperature climbs too high, a fan fault where the flue-gas extraction fan is not running correctly, or in some model variants, an ignition failure. Because Andrews produces a range of commercial and domestic hot water appliances — many sharing control platforms with Ariston and Vokera-derived systems — the precise trigger can differ between models. In all cases the boiler or water heater shuts itself down as a safety measure. Do not attempt to override this lockout; the cause must be identified and corrected before normal operation can resume.
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
- Overtemperature — flow temperature exceeded safe limit Common
The most frequently seen trigger for A03 on Andrews and related appliances. If the boiler or water heater cannot shed heat fast enough — due to poor circulation, a blocked heat exchanger, or a faulty high-limit thermostat — the flow temperature climbs above the safe threshold (often cited as around 103°C on Ariston-platform units) and the safety cutout activates. Low system pressure can contribute by reducing water volume and accelerating temperature rise.
- Fan fault — flue-gas extraction fan not performing correctly Common
The fan creates a draught that expels combustion gases safely through the flue. If its motor windings are burnt out, its bearings have deteriorated, or its speed drops below the expected range, the boiler's PCB detects an anomaly and locks out to prevent harmful gases accumulating. The fan does not need to stop entirely; even running too slowly can trigger A03. Blocked flue terminals or duct restrictions can also load the fan beyond its normal operating point.
- Air in the system or low water pressure Common
Trapped air creates hot spots and disrupts flow, causing localised overheating and spurious temperature sensor readings. Low system pressure (below approximately 1 bar on a sealed system) reduces water circulation and compounds the overtemperature risk. Both issues can make A03 appear without any component having actually failed.
- Faulty or incorrectly positioned temperature sensor Sometimes
A damaged NTC thermistor or a sensor that has shifted out of its pocket will send inaccurate temperature data to the PCB. The controller may interpret a normal flow temperature as dangerously high, or vice versa, triggering a precautionary lockout. Wiring corrosion on sensor leads produces identical symptoms.
- Circulation pump failure or obstruction Sometimes
If the pump is seized, running at reduced speed, or blocked with scale or sludge, water cannot circulate quickly enough to keep temperatures stable. The boiler overheats within the heat exchanger, tripping the high-limit protection and displaying A03.
- PCB fault generating a false fault signal Rare
All sensors, the fan, and the pump communicate with the PCB. A failing control board can misread correct signals as fault conditions, producing an A03 lockout even when the fan or temperature sensor is functioning normally. This is a less common cause and is typically only confirmed after other components have been ruled out.
- Gas valve fault preventing stable ignition Rare
On model variants where A03 relates to an ignition fault, a sticking or faulty gas valve may prevent the burner from lighting or holding a flame. In documented UK field cases, disconnecting and reconnecting igniter wiring while the valve was inspected formed part of the engineer's diagnostic process.
How to fix it
- Check the system pressure gauge DIY safe
Look at the pressure gauge on the appliance or boiler panel. On a sealed central heating system it should read between 1 and 1.5 bar when cold. If it reads below 1 bar, low pressure may be contributing to the fault. Refer to your boiler's user manual for instructions on using the filling loop to bring pressure back up to approximately 1.2 bar, then attempt a single reset.
- Bleed radiators and purge air from the system DIY safe
Using a radiator bleed key, open each radiator bleed valve in turn (starting from the ground floor) until water runs steadily with no air sputtering out. Recheck system pressure afterwards and top up if it has dropped. This removes trapped air that can cause localised overheating and trigger the A03 protection.
- Check the flue terminal is unobstructed DIY safe
Go outside and visually inspect the flue outlet. Make sure it is not blocked by debris, bird nesting material, or — in cold weather — a build-up of ice. Do not probe inside the flue or touch any components; this is a visual check only. A blocked terminal restricts draught, loads the fan, and can contribute to an A03 lockout.
- Attempt a single boiler reset DIY safe
Once pressure has been checked and the system bled, press and hold the reset button as described in your user manual. Allow the appliance to complete its full start-up sequence before deciding whether the fault has cleared. If the A03 code returns immediately, do not keep resetting — repeated resets without fixing the underlying cause will not help and may worsen the situation.
- Engineer to inspect and test the fan assembly Gas Safe engineer
A Gas Safe registered engineer will check fan speed against the manufacturer's specification, inspect motor windings and bearings, look for mechanical obstructions within the fan housing, and verify that all electrical connectors to the fan harness are secure and corrosion-free. A faulty fan must be replaced like-for-like; this is not a DIY task on a gas appliance.
- Engineer to test the temperature sensors and high-limit thermostat Gas Safe engineer
The engineer will use a multimeter to check NTC resistance values against the temperature–resistance curve for the specific sensor fitted, confirm the sensor is correctly seated in its pocket, and inspect wiring for damage or corrosion. A defective high-limit thermostat will be replaced at this stage.
- Engineer to check the circulation pump and system flow rate Gas Safe engineer
The pump will be tested for correct rotation, adequate flow rate, and signs of seizure or sludge ingress. If the system has not been chemically flushed recently, the engineer may recommend a power flush to clear magnetite and scale that could be restricting flow.
- Engineer to inspect the gas valve and ignition components (where A03 relates to ignition) Gas Safe engineer
If the model variant is displaying A03 as an ignition lockout, the gas valve, igniter electrode, and flame-sense electrode will all be examined. The engineer will confirm correct gas supply pressure and that the valve opens and closes as commanded by the PCB.
- Engineer to test or replace the PCB if all other components pass Gas Safe engineer
If the fan, sensors, pump, and gas valve all prove serviceable, the PCB becomes the suspect. The engineer may run a diagnostic routine, swap the board for a tested unit, or arrange a manufacturer warranty repair depending on the appliance's age and service history.
- Call a Gas Safe registered engineer if the fault persists or you cannot identify the cause Gas Safe engineer
Andrews commercial appliances and domestic water heaters should be serviced by a Gas Safe registered engineer experienced with commercial equipment. Contact Andrews (Baxi Commercial) technical support or your regular heating engineer. Do not continue operating an appliance that keeps locking out on A03 — it is shutting down for a safety reason.
Parts you may need
- Flue-gas extraction fan (model-specific) · from £140
- NTC flow temperature sensor · from £25
- High-limit thermostat / overheat cutout · from £30
- Circulation pump (15–50 series) · from £85
- PCB / control board (model-specific) · from £220
- Gas valve (model-specific) · from £130
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
Is the Andrews A03 fault code always the same fault, or does it vary by model?
It can vary. On Andrews appliances sharing an Ariston-derived platform the A03 code typically signals an overtemperature lockout, meaning the flow temperature has exceeded a safe threshold. On units built on Vokera-related control systems it most commonly points to a fan fault. Some variants use A03 for ignition failures. Check your specific model's user manual or contact Andrews (Baxi Commercial) technical support with your appliance serial number to confirm the exact meaning before any repair work is carried out.
Can I fix the Andrews A03 fault myself?
There are a few checks a homeowner can safely carry out: verify the system pressure is between 1 and 1.5 bar, bleed the radiators to remove trapped air, and visually inspect the flue terminal for blockages. One reset attempt after these checks is reasonable. However, any work involving the fan, sensors, pump, gas valve, or PCB must be carried out by a Gas Safe registered engineer. On commercial Andrews water heaters, even routine fault diagnosis is best left to a qualified specialist.
How much does it cost to repair an Andrews boiler showing A03?
For the most common repairs — fan replacement or sensor and thermostat work — most UK homeowners pay between £150 and £450 including parts and labour. A straightforward sensor swap typically sits at the lower end; a fan replacement usually falls in the £200–£350 range. If the PCB turns out to be faulty, costs can rise to £400–£600 or more depending on the model, as commercial-grade boards tend to be more expensive than domestic equivalents. If two major components need replacing simultaneously it is worth obtaining a quote for a new or refurbished appliance for comparison.
Why does the Andrews A03 fault keep coming back after a reset?
A reset does not fix the underlying problem — it simply clears the lockout condition so the appliance can attempt to start again. If A03 returns quickly or repeatedly, it means the root cause (overheating, fan degradation, a failing sensor, or another issue) is still present. Repeated resets without a repair can mask a developing fault and, in some cases, cause the boiler to go into a deeper lockout state. Book a Gas Safe engineer to diagnose and resolve the fault properly rather than relying on resets.