Sime AL 56 Fault Code: Causes, Fixes & Repair Costs
What does the Sime AL 56 fault code mean?
AL 56 on a Sime boiler is a lock caused by the temperature difference between flow and return exceeding its maximum limit (Sime notes it in the context of open-vent installations). The board watches the spread between water leaving the boiler and water coming back; a spread beyond design limits means heat is being generated far faster than the system is carrying it away — classic poor-circulation territory: a struggling pump, restriction, air, or closed valves. Rather than let the heat exchanger cycle against thermal stress, the boiler locks and displays AL 56. Sime's manual directs persistent cases to the Technical Assistance Centre, because the diagnosis spans the system as much as the boiler.
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
- Circulation restriction or failing pump Common
A tired pump, seized rotor, blocked strainer or sludged pipework slows flow through the heat exchanger, so flow temperature races ahead of return and the ΔT limit trips.
- Air in the system Common
Airlocks — especially after refilling — impede circulation and produce erratic, widened temperature spreads until bled out.
- Closed or partially closed valves Sometimes
Isolation or zone valves left partly shut starve the circuit; the boiler sees the same abnormal spread. Worth checking after any recent system work.
- Sensor error exaggerating the spread Sometimes
A drifting flow or return probe can widen the apparent ΔT without a real circulation problem; engineers verify both probes as part of the diagnosis.
How to fix it
- Check pressure and bleed radiators DIY safe
Top the system up to around 1–1.2 bar cold if low, and bleed any airlocked radiators — the two homeowner-safe actions that restore circulation.
- Check any accessible valves are open DIY safe
If pipework valves near the boiler or a recently serviced zone valve are visibly part-closed, open them fully. Do not dismantle anything.
- Reset once with the OK/RESET button DIY safe
Hold the OK RESET button for more than 3 seconds. Sime boilers allow a limited number of consecutive resets (six) before locking out on AL 28, so if the code returns after one or two attempts, stop and book an engineer rather than using them all up.
- Have a Gas Safe registered engineer carry out Sime's documented remedy Gas Safe engineer
The engineer will verify pump performance, check for restrictions and sludge, confirm both temperature probes read accurately, and deal with whatever is throttling circulation — flushing the system if needed. Sime directs persistent AL 56 cases to their Technical Assistance Centre.
Parts you may need
- Circulation pump · from £130
- Flow/return probe (NTC) · from £20
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–£400, depending on the underlying cause.
Frequently asked questions
What does AL 56 mean on a Sime boiler?
The temperature gap between the water leaving the boiler and the water returning has exceeded its maximum limit, so the boiler locked out to protect the heat exchanger. It almost always means circulation is poor — pump, air, sludge or a closed valve — or occasionally that a temperature probe is misreading.
Can I fix AL 56 myself?
Sometimes: restore pressure to about 1–1.2 bar, bleed the radiators, make sure accessible valves are open, and reset once. If the code returns, the pump and system need a professional look — repeated locking against a real circulation fault stresses the boiler.
Why does the manual mention open vent?
Sime associates this particular check with open-vent (heat-only) installations, where system design and circulation arrangements differ from sealed combis. The practical meaning for you is unchanged: the water is not moving heat away fast enough.