Sime AL 74 Fault Code: Causes, Fixes & Repair Costs
What does the Sime AL 74 fault code mean?
AL 74 on a Sime boiler reports a fault with the second delivery sensor. Sime's combustion-control models monitor flow temperature with more than one probe — the second delivery sensor gives the board an independent reading used for cross-checking and safety supervision. When that probe fails electrically or its readings stop agreeing with reality, the board raises AL 74. Sime's documented remedy is to check the second delivery sensor's operation and its position, mirroring the checks for the primary probe: is it seated properly on the pipe, is its wiring sound, and does its resistance track temperature correctly?
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 or drifting second delivery probe Common
The sensor element ages just like the primary probe: drift, open or short circuits all trigger AL 74. Replacement is straightforward once confirmed.
- Probe out of position Sometimes
Sime's remedy explicitly includes checking position — a dislodged probe reads implausibly and fails the cross-check even though it is electrically healthy.
- Wiring or connector fault Sometimes
A damaged lead or corroded connector between probe and board mimics sensor failure and is ruled out by measurement at both ends.
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 when the code appears DIY safe
Constant from cold, or only once hot? Intermittent, heat-related faults often point at connectors; constant ones at the probe. This helps the engineer.
- Have a Gas Safe registered engineer carry out Sime's documented remedy Gas Safe engineer
The engineer will check the second delivery sensor's seating and contact, measure it against the resistance table, inspect the wiring, and replace the probe if confirmed faulty — Sime's documented operation-and-position check. Flow readings are then verified through a full cycle.
Parts you may need
- Delivery 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
Why does my Sime boiler have two flow sensors?
The combustion-control models cross-check flow temperature with an independent second probe — if one drifts or fails, the disagreement is caught rather than trusted. AL 74 means that second probe has failed its checks, so the safety net itself needs repairing.
Can the boiler run on just one sensor?
Not normally — the cross-check is part of the boiler's temperature supervision, so it restricts operation until the second probe reads believably again. That is deliberate, protective behaviour.
Is AL 74 expensive?
Usually one of the cheaper repairs: the probe is an inexpensive part and access is reasonable, so most fixes land toward the bottom of £90–£220. A wiring repair is similar.