Andrews E13 Fault Code: Causes, Fixes & Repair Costs
What does the Andrews E13 fault code mean?
The E13 code on an Andrews boiler signals that the differential water flow switch is reading as closed when it should be open. In other words, the boiler's control board is receiving a signal suggesting water is circulating when it may not be — or the switch itself has become stuck in the closed position. This is closely related to the E12 code: where E12 indicates the switch is open when it should be closed, E13 is the opposite condition. Either way, the boiler will typically lock out as a precaution to prevent overheating or damage until the root cause is identified and resolved.
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 stuck differential flow switch Common
The flow switch itself may have jammed in the closed position due to wear, limescale build-up, or debris lodged around the mechanism. This is the most direct cause of an E13 reading, as the component is physically failing to return to its open state.
- Circulation pump failure or seizure Common
A failing or seized pump can create abnormal pressure differentials across the flow switch, causing it to report an incorrect closed signal. Pumps can seize during long periods of inactivity, particularly after summer months when the heating hasn't been used.
- Sludge, scale, or blockage in the system Sometimes
Magnetite sludge, limescale, or debris accumulating in the heat exchanger or pipework can restrict flow in a way that sends misleading signals to the flow switch. This is more common in older systems that have never been power-flushed or had inhibitor added.
- Wiring fault or loose connection Sometimes
Corroded terminals, a loose connector, or damaged wiring between the flow switch and the PCB can cause the control board to receive a false closed signal even if the switch itself is functioning correctly.
- PCB fault Rare
In rarer cases, the printed circuit board may be misinterpreting or incorrectly processing the signal from the flow switch, producing an E13 code when the switch is actually operating normally.
How to fix it
- Note the fault code and attempt a single boiler reset DIY safe
Before doing anything else, write down the exact code displayed. Press the reset button as described in your Andrews boiler's user manual. Only reset once or twice — repeated resets without resolving the cause are not recommended and can mask the fault. Observe whether the boiler attempts to fire normally or immediately returns to E13.
- Check that your gas supply is working DIY safe
Confirm other gas appliances in the property are working normally. If there is no gas supply, contact your gas supplier rather than continuing to investigate the boiler.
- Check and top up system pressure if low DIY safe
Look at the boiler's pressure gauge. Most Andrews boilers operate correctly between 1.0 and 1.5 bar when cold. If it reads below 1.0 bar, use the filling loop to top up the pressure to around 1.2–1.5 bar, then attempt another reset. Low pressure alone can sometimes contribute to poor circulation readings.
- Bleed radiators to remove trapped air DIY safe
Air locked in the system can disrupt water circulation and contribute to flow switch errors. Use a radiator bleed key to release air from each radiator in turn, starting on the ground floor. Re-check the system pressure afterwards and top up if needed.
- Have a Gas Safe engineer inspect and test the flow switch Gas Safe engineer
A registered engineer will be able to test the differential flow switch electrically and mechanically to determine whether it is genuinely stuck closed or sending an incorrect signal. If the switch has failed, it will need to be replaced — this is not a homeowner task as it involves opening the boiler casing and working on internal components.
- Have the circulation pump checked and replaced if necessary Gas Safe engineer
The engineer should verify that the pump is spinning freely and delivering adequate flow. A seized or weak pump is a very common cause of this fault. Pump replacement is typically straightforward for an experienced engineer and usually more cost-effective than attempting a repair on the old unit.
- Ask the engineer to inspect wiring and PCB connections Gas Safe engineer
If the switch and pump are both in good working order, the engineer should check all wiring between the flow switch and the PCB for signs of corrosion, loose terminals, or damage. A faulty PCB would be considered only after other causes have been ruled out.
- Consider a power flush if sludge or blockage is suspected Gas Safe engineer
If the engineer identifies restricted flow caused by magnetite sludge or scale build-up, a power flush of the system may be recommended. This clears the pipework and heat exchanger, restoring correct circulation and reducing the likelihood of the fault recurring. Adding a suitable inhibitor afterwards is strongly advised.
- Contact a Gas Safe registered engineer if the fault persists Gas Safe engineer
If the E13 code keeps returning after basic homeowner checks, you should arrange a visit from a Gas Safe registered engineer as soon as possible. Always verify your engineer's registration at gassaferegister.co.uk before allowing any work to begin on the boiler.
Parts you may need
- Differential water flow switch · from £55
- Circulation pump (compatible replacement) · from £85
- PCB (Andrews compatible) · from £220
- System inhibitor (e.g. Fernox F1) · from £18
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
What is the difference between the Andrews E12 and E13 fault codes?
These two codes relate to the same component — the differential water flow switch — but describe opposite conditions. E12 means the switch is open when it should be closed, suggesting the boiler isn't detecting flow when it expects to. E13 is the reverse: the switch is reading as closed when it should be open, indicating a stuck or faulty switch, a pump issue, or a wiring problem sending an incorrect signal to the control board.
Can I fix the Andrews E13 fault myself?
Homeowners can safely try a boiler reset, top up system pressure via the filling loop, and bleed radiators to remove trapped air. These steps occasionally resolve the fault if low pressure or air locks are contributing factors. However, if the fault persists, the repair involves opening the boiler casing to access the flow switch, pump, or wiring — all of which must be handled by a Gas Safe registered engineer. Do not attempt to open the boiler yourself.
How much does it typically cost to fix an E13 fault on an Andrews boiler?
For most homeowners, the repair costs between £150 and £450 including parts and labour. A flow switch replacement tends to sit at the lower end of this range, while a circulation pump replacement typically costs more. If the fault turns out to be a PCB issue, costs can be higher — PCBs for Andrews boilers can cost £200–£350 for the part alone plus engineer labour — though this is a less common cause of E13. Emergency or out-of-hours call-outs will add to any of these figures.
Will a power flush fix my Andrews E13 fault?
A power flush can help if the E13 fault is being caused or worsened by sludge or scale restricting flow through the system. It won't fix a mechanically failed flow switch or a seized pump on its own, but it can restore correct circulation and reduce the likelihood of repeat faults. A Gas Safe engineer will be able to advise whether a power flush is appropriate once they have diagnosed the root cause.