Andrews E12 Fault Code: Causes, Fixes & Repair Costs
What does the Andrews E12 fault code mean?
The E12 fault code on Andrews boilers indicates that the differential water flow switch is reading as open when the boiler expects it to be closed during normal operation. In plain terms, the boiler cannot confirm that water is circulating adequately through the system, so it locks out as a safety measure. This is distinct from the E13 code, which triggers when the same switch is stuck in the closed position — E12 is specifically the 'switch still open' variant. Andrews boilers share the same E-code platform as Baxi (both sit under the BDR Thermea group), so engineers familiar with Baxi fault logic will recognise this code immediately.
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
- Low system water pressure Common
If the boiler's system pressure has dropped below its minimum operating threshold, the differential flow switch may not close because there is insufficient water pressure to register adequate flow. This is one of the first things to rule out, as it is easily checked by the homeowner on the pressure gauge.
- Blocked or seized circulation pump Common
If the pump has seized, is running very slowly, or is blocked by sludge and debris, water will not circulate at the required rate. The flow switch will remain open because it detects no meaningful flow, triggering E12.
- Faulty differential water flow switch Common
The flow switch itself can wear out, become blocked with debris, or fail electrically. When it fails in the open position, the boiler receives a permanent 'no flow' signal even when water is actually circulating correctly.
- Wiring fault in the flow switch circuit Sometimes
A loose connection, corroded terminal, or damaged wire between Terminals 6 and 8 on the Andrews control board can interrupt the signal from the flow switch, causing the boiler to incorrectly interpret the switch as open.
- Sludge or debris blockage in the system Sometimes
Accumulated magnetite sludge or limescale in the pipework or heat exchanger can restrict water flow to the point where the switch cannot detect adequate circulation, even if the pump itself is functioning correctly.
- External controls or pressure switch interrupting the circuit Rare
External time controls, zone valves, or a separate external water pressure switch that has tripped or been inadvertently turned off can cut water flow to the boiler, keeping the differential switch open.
How to fix it
- Check the system pressure gauge DIY safe
Look at the pressure gauge on the boiler — it should typically read between 1 and 1.5 bar when cold. If it has dropped below 1 bar, top up the system using the filling loop (usually a flexible braided hose beneath the boiler with two inline valves). Open both valves slowly until the pressure reaches around 1.2–1.5 bar, then close both valves firmly. Do not overfill beyond 2 bar.
- Verify that external controls have not shut the system down DIY safe
Check that your programmer, room thermostat, and any zone valves are all set correctly and calling for heat. A zone valve that has stuck closed or a timer set to 'off' can prevent water from circulating, leaving the flow switch open.
- Reset the boiler DIY safe
Press and hold the reset button (refer to your Andrews boiler manual for its exact location) for around 3 seconds. Allow the boiler to restart and observe whether it fires up normally or displays E12 again. Do not reset more than 2–3 times — repeated resets without identifying the cause can mask a more serious problem.
- Have a Gas Safe engineer inspect the circulation pump Gas Safe engineer
A registered engineer will check whether the pump is running, test its speed and output, and confirm whether it is moving water around the system. A seized or failing pump is one of the most common causes of E12 and will need professional testing and, if necessary, replacement.
- Have the wiring between Terminals 6 and 8 checked Gas Safe engineer
An engineer will inspect the wiring loom and terminal connections associated with the differential flow switch circuit. Corroded or loose connections at these terminals can send a false 'open' signal to the controller and are a straightforward repair once identified.
- Have the differential water flow switch tested and replaced if faulty Gas Safe engineer
The engineer will test the flow switch electrically and mechanically. If it is found to be defective — stuck open, blocked internally, or failed — it will need to be replaced with the correct Andrews-compatible component.
- Ask about a powerflush if sludge is suspected Gas Safe engineer
If the engineer finds evidence of magnetite sludge or debris restricting flow, a powerflush of the central heating system may be recommended. This clears blockages from pipework, radiators, and the heat exchanger and can prevent repeat faults.
- Call a Gas Safe registered engineer if the fault persists Gas Safe engineer
If the above homeowner checks do not resolve the E12 code, contact a Gas Safe registered engineer. You can verify any engineer's registration at gassaferegister.co.uk before they begin work. Do not continue operating the boiler in lockout.
Parts you may need
- Differential water flow switch · from £30
- Circulation pump (Andrews/Baxi compatible) · from £85
- Pump capacitor · from £12
- Control board / PCB · 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 £120–£380, depending on the underlying cause.
Frequently asked questions
What is the difference between the Andrews E12 and E13 fault codes?
Both codes point to a problem with the differential water flow switch, but they indicate opposite failure states. E12 means the switch is stuck open — the boiler cannot detect adequate water flow. E13 means the switch is stuck closed — it is permanently signalling flow even when none is expected. E12 is generally associated with flow or circulation problems, while E13 is more often a failed switch or wiring fault keeping it energised.
Can I fix the Andrews E12 fault code myself?
There are a couple of safe checks a homeowner can do: verify the system pressure is between 1 and 1.5 bar and top it up if needed, check that all external controls are calling for heat, and try resetting the boiler once or twice. Beyond those steps, the work involves internal boiler components — the pump, flow switch, wiring, and potentially the PCB — which must be handled by a Gas Safe registered engineer.
How much does it cost to fix an E12 fault on an Andrews boiler?
Most E12 repairs fall in the range of £120–£380 in the UK, depending on the fault found. Topping up pressure costs nothing. A flow switch replacement, including parts and labour, is typically at the lower end. A circulation pump replacement with labour usually comes to around £250–£350. If the PCB is involved, costs can rise to £400 or more, but that is less common with this particular code.
Could a powerflush fix my Andrews E12 fault?
If the underlying cause is a sludge or debris blockage restricting water flow, then yes — a powerflush can restore adequate circulation and resolve the fault. However, a powerflush (typically £300–£600 for a full system) is only worth doing once a Gas Safe engineer has confirmed that sludge is the root cause. If the pump or flow switch has actually failed, a powerflush alone will not clear the E12 code.