Sime AL 89 Fault Code: Causes, Fixes & Repair Costs
What does the Sime AL 89 fault code mean?
AL 89 on a Sime boiler means the combustion feedback signal is unstable: the flame-sensing signal the combustion controller uses to regulate the burn is fluctuating too much to control against. These boilers do not just detect that a flame exists — they continuously read its quality through the detection electrode and adjust the gas/air mixture in response. When that feedback becomes noisy or erratic, closed-loop control is impossible and the boiler stops. Sime's checklist is the combustion family's usual suspects: the ignition/detection electrode first, then the flue for blockage, the air diaphragm and the gas calibration — because each of those can make a real flame genuinely unstable, or make a stable flame look unstable to a worn electrode.
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
- Worn or contaminated detection electrode Common
The electrode's surface degrades with age and deposits; its signal grows noisy long before flame detection fails outright. It is the first check on Sime's list and the most common repair for AL 89.
- Flue disturbance destabilising the flame Common
A partial blockage or recirculation at the terminal makes the flame itself unstable — the feedback is faithfully reporting a real problem in the air path.
- Gas/air mixture drift Sometimes
A degraded air diaphragm or drifted calibration leaves the mixture marginal, producing an unsteady burn. Recalibration after correcting the cause restores stable feedback.
- Electrode lead or earthing fault Sometimes
A damaged sensing lead or poor earth injects electrical noise into the signal. Checked alongside the electrode itself.
How to fix it
- Glance at the flue terminal DIY safe
The one safe external check: look for obstruction at the terminal. Everything else on this code is combustion-side work.
- 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–£350, depending on the underlying cause.
Frequently asked questions
What does AL 89 mean on my Sime boiler?
The flame-quality signal the combustion controller relies on has become too unstable to control against, so the boiler stops. The usual cause is a worn detection electrode; a disturbed flue, a marginal gas/air mixture or a noisy sensing lead produce the same symptom. It is an engineer diagnosis along Sime's documented checklist.
Is the flame actually unstable, or just the reading?
Either — and that is exactly what the engineer determines. A worn electrode makes a good flame look bad; a blocked flue or drifted mixture makes the flame genuinely bad. The checklist (electrode, flue, diaphragm, calibration) separates the two.
How much is the typical fix?
Most AL 89 cases end with a new electrode and a recalibration — the lower half of £100–£350. Flue remediation or valve/board involvement pushes higher. Regional prices vary; get a written quote.