Sime AL 93 Fault Code: Causes, Fixes & Repair Costs
What does the Sime AL 93 fault code mean?
AL 93 on a Sime boiler is a combustion set cannot be reached error — the sibling of AL 90, distinguished internally by the combustion controller but identical in practical meaning and remedy. The self-regulating combustion system drove its corrections and the measured burn never converged on its target, so the boiler locked out. Sime's documented diagnostics are the same set as for AL 90: the ignition/detection electrode, the flue inlet and outlet for blockage, the air diaphragm, and the gas calibration and supply — followed by the automatic self-calibrating procedure once the underlying cause has been put right.
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
- Flue or air-path restriction Common
A restricted air path stops the mixture ever reaching target, whatever the controller does. First on the physical checklist.
- Electrode wear distorting the control signal Common
The controller chases a target through a degraded sensor and never appears to arrive; the burn may even be near-correct. New electrode plus recalibration is the classic resolution.
- Weak gas supply or air in the gas line Sometimes
Insufficient working pressure or entrained air caps the achievable combustion (compare AL 84); measured and purged by the engineer.
- Calibration baseline mismatch Sometimes
After parts changes without the mandatory recalibration, the stored target itself is wrong — the rerun is part of every AL 93 repair.
How to fix it
- Check the flue terminal externally DIY safe
Look for visible obstruction. That is the extent of homeowner-safe action on this code.
- Reset once with the OK/RESET button DIY safe
Hold the OK RESET button for more than 3 seconds. Sime allows a maximum of six consecutive resets before locking out on AL 28, so if the code returns after one or two attempts, stop and book an engineer.
- Have a Gas Safe registered engineer work through the combustion-control checks Gas Safe engineer
Sime's documented checklist for the combustion-control faults covers the ignition/detection electrode (condition, position and earthing), the flue inlet and outlet for blockage, the air diaphragm, the gas calibration and supply pressure, and bleeding air from the gas circuit where relevant. After rectification the engineer reruns the automatic self-calibrating procedure and confirms combustion with a flue-gas analysis.
Parts you may need
- Ignition/detection electrode · from £35
- Air diaphragm · from £15
- Gas valve (SGV) · from £150
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–£380, depending on the underlying cause.
Frequently asked questions
What does AL 93 mean on a Sime boiler?
The combustion controller could not steer the burn to its set point — the same failure as AL 90, logged as a distinct variant. Diagnosis and repair are identical: electrode, flue, air diaphragm and gas supply/calibration, then a rerun of the automatic self-calibration.
Does it matter whether I saw AL 90 or AL 93?
Not for what happens next — Sime's remedy table treats them alike, and your engineer will run the same checks. Telling them which code (and any others logged, like AL 83 or AL 62) still helps, because the sequence of codes sketches the fault's history.
Is my boiler unsafe when this code shows?
It has locked out, which is the protective state — it will not run combustion it cannot control to target. Leave it off, book a Gas Safe registered engineer, and as ever: any smell of gas means leave the property and call 0800 111 999.