Sime AL 57 Fault Code: Causes, Fixes & Repair Costs
What does the Sime AL 57 fault code mean?
AL 57 on a Sime boiler is a lock raised by the flow temperature (FT) check — the board's supervision of how the flow temperature behaves during operation (noted, like AL 56, in the context of open-vent installations). If the flow temperature moves in a way the check does not accept — climbing too fast, overshooting, or failing to respond as combustion control expects — the boiler locks rather than continue. Sime's documented remedies are to carry out the automatic self-calibrating procedure, and to contact the Technical Assistance Centre if the fault persists: the check is tied into the boiler's self-calibrating combustion management, so recalibration is genuinely part of the fix rather than a ritual.
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
- Combustion management needing recalibration Common
On these self-calibrating boilers, drift between the combustion settings and reality (after component changes, gas-quality shifts or long service intervals) can make flow temperature behaviour fail the check. Running the self-calibration — Sime's own first remedy — realigns it.
- Poor circulation heating the flow too fast Common
A weak pump, air or restriction lets flow temperature spike faster than the check allows, much like AL 56. Circulation basics are checked alongside calibration.
- Flow probe drift Sometimes
A delivery sensor reading high or laggy distorts the very measurement the check supervises. Engineers verify the probe against the resistance table.
How to fix it
- Check pressure and bleed radiators DIY safe
Rule out the innocent causes first: top up to around 1–1.2 bar cold and bleed any airlocked radiators, then reset once.
- 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 run Sime's automatic self-calibrating procedure — the manual's stated remedy — after confirming circulation and the delivery probe are sound, and complete a flue-gas analysis. Persistent cases go to Sime's Technical Assistance Centre for deeper diagnosis.
Parts you may need
- Delivery (flow) probe NTC · from £20
- Circulation pump · 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 £100–£350, depending on the underlying cause.
Frequently asked questions
What does AL 57 mean on a Sime boiler?
The boiler's flow-temperature supervision has locked it out — the temperature of water leaving the boiler behaved in a way the check would not accept. Sime's stated fix is to run the automatic self-calibrating procedure; underlying circulation problems or a drifting flow probe produce the same lock and are checked at the same visit.
What is the self-calibrating procedure?
Sime's combustion-control boilers tune their own gas/air settings through an automatic calibration routine (started from the panel and taking up to about 15 minutes, run by an engineer). Several fault codes, AL 57 included, list rerunning it as the documented remedy because the boiler's checks are built around a correctly calibrated baseline.
Can I run the self-calibration myself?
No — Sime intends it as part of commissioning and servicing by a qualified engineer, paired with a flue-gas analysis to confirm combustion is correct afterwards. Your safe steps are pressure, bleeding and a single reset.