Sime AL 72 Fault Code: Causes, Fixes & Repair Costs
What does the Sime AL 72 fault code mean?
AL 72 on a Sime boiler flags incorrect positioning of the delivery (flow) sensor: the board's plausibility checks have concluded that the flow probe is not reading like a probe properly attached to the delivery pipe. Rather than a probe that has failed electrically (that is AL 05), this is a probe whose readings are physically implausible — typically because it has slipped from its pocket or clip, is no longer in firm contact with the pipe, or was refitted in the wrong position after service work. Sime's documented remedy is to check that the delivery sensor is attached to the delivery pipe. A sensor reading air instead of pipework responds slowly and coolly, and the combustion-control logic notices.
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
- Probe dislodged from the delivery pipe Common
Contact-type probes clip to the pipe; vibration or disturbance during service can leave one hanging loose, reading ambient rather than water temperature. Refitting it snugly, with its clip and any thermal contact restored, is the fix.
- Probe refitted incorrectly after work Common
After heat-exchanger or pipework jobs the probe may be replaced in the wrong position or without proper contact. AL 72 appearing soon after a repair points straight back at that work.
- Probe or wiring degradation mimicking displacement Sometimes
A degrading probe can drift into the same implausible-reading territory; the engineer confirms position first, then tests the probe electrically.
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 whether the boiler was recently worked on DIY safe
AL 72 shortly after a service or repair strongly suggests the probe was not refitted correctly — mention it when you call, and to the engineer who did the work.
- Have a Gas Safe registered engineer carry out Sime's documented remedy Gas Safe engineer
The engineer will confirm the delivery probe is seated on the delivery pipe with proper contact — Sime's documented check for AL 72 — refit or re-clip it as needed, and verify its readings track a heat cycle correctly. If position is fine, the probe itself is tested and replaced.
Parts you may need
- Delivery (flow) probe NTC · from £20
- Probe retaining clip · from £5
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 £80–£200, depending on the underlying cause.
Frequently asked questions
What does AL 72 mean on a Sime boiler?
The boiler believes its flow (delivery) temperature sensor is not properly attached to the delivery pipe — its readings look like a probe measuring air, not water. It is usually a physically dislodged or badly refitted probe rather than an electrical failure, and the fix is to reseat it correctly.
How is AL 72 different from AL 05?
AL 05 is the delivery sensor failing electrically (open, short, out of range). AL 72 is the same sensor apparently healthy but positioned wrongly, so its readings are implausible. The distinction matters because AL 72 is often fixed by refitting rather than replacing.
Is this serious?
It is cheap to fix but should not be ignored: the flow reading feeds temperature limits and combustion control, so the boiler restricts operation while it cannot trust it. A short engineer visit normally resolves it.