Sime AL 30 Fault Code: Causes, Fixes & Repair Costs
What does the Sime AL 30 fault code mean?
AL 30 on a Sime boiler indicates a fault with the return temperature sensor — the probe measuring the temperature of water coming back from the radiators into the boiler (on T/system variants, Sime's table calls it the boiler sensor). The control board continuously reads this probe to manage the spread between flow and return, protect the heat exchanger and drive its combustion control; when the probe's reading goes open, shorted or implausible, the board raises AL 30. Sime's documented remedy is direct: replace the return probe. In practice an engineer will first confirm the probe rather than its wiring or connector is at fault, since a chafed lead produces the same symptom.
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
- Failed return probe (NTC) Common
The sensor element drifts or fails with age and thermal cycling. Sime's own solution column for AL 30 is simply to replace the return probe — it is the expected outcome.
- Wiring or connector fault Sometimes
A loose connector or damaged lead between probe and board mimics a dead sensor. Engineers measure the probe's resistance at both ends of the circuit before fitting parts.
- Board input fault Rare
Rarely, a healthy probe reads wrongly at the PCB because the measuring input has failed — identified once a known-good probe still misreads.
How to fix it
- 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.
- Note any symptoms before the fault DIY safe
Odd cycling, radiators heating unevenly or kettling beforehand suggest the probe had been drifting — worth telling the engineer.
- Have a Gas Safe registered engineer carry out Sime's documented remedy Gas Safe engineer
The engineer will measure the return probe against Sime's resistance/temperature table, inspect its wiring, and replace the probe — Sime's documented remedy for AL 30. After replacement they should watch flow and return readings track correctly through a heat cycle.
Parts you may need
- Return sensor 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 £90–£220, depending on the underlying cause.
Frequently asked questions
What does AL 30 mean on a Sime Murelle?
The return temperature probe has failed its checks — the board no longer trusts the reading of water returning from the system. Sime's manual gives a one-line remedy: replace the return probe. It is one of the simpler, cheaper repairs on these boilers.
Can the boiler run without the return sensor?
No — the reading feeds the boiler's temperature management and combustion control, so it suspends normal operation rather than run blind. That is protective behaviour, not damage.
Is this a DIY job?
No. The probe sits on the primary pipework inside the sealed casing of a gas appliance; diagnosis and replacement belong to a Gas Safe registered engineer. It is normally a quick visit.