Sime AL 04 Fault Code: Causes, Fixes & Repair Costs
What does the Sime AL 04 fault code mean?
The AL 04 code appears on Sime boilers when the control board loses confidence in the readings from the domestic hot water (DHW) temperature sensor — also known as an NTC thermistor or temperature probe. This small component monitors the temperature of the hot water circuit by changing its electrical resistance as the water temperature rises or falls. When the PCB detects a reading that is implausible, inconsistent, or completely absent, it triggers AL 04 and locks the boiler out as a protective measure. The boiler will not restart until the underlying sensor issue is resolved.
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 DHW NTC thermistor Common
The sensor itself has developed an open circuit, a short circuit, or has simply degraded over time. Either scenario causes the PCB to receive a signal that falls outside the expected range, triggering the lockout. This is the most frequent root cause and is usually confirmed by testing the sensor's resistance with a multimeter.
- Loose, corroded, or damaged wiring Common
The wiring harness connecting the DHW sensor to the control board can develop loose terminals or corrosion at the connector, particularly in older boilers or those installed in damp environments. The intermittent or absent signal mimics a failed sensor and produces the same AL 04 lockout. An engineer will inspect and reseat all connections before condemning the sensor itself.
- PCB fault Rare
If the sensor and its wiring both test as healthy, the printed circuit board may have lost its ability to correctly read the sensor circuit. PCB faults are relatively rare but can produce exactly the same symptom, meaning the fault code persists even after a new sensor is fitted.
How to fix it
- Reset the boiler once DIY safe
Press and hold the reset button on your Sime boiler for the required time as shown in your user guide. A one-off reset can clear a transient signal glitch. Do not attempt more than two or three resets in succession — persistent AL 04 after resetting means a physical fault needs attention.
- Check that the boiler has power and the gas supply is on DIY safe
Confirm the boiler's mains switch is on, the consumer unit has not tripped, and the gas isolation valve at the meter is in the open position. These simple checks rule out unrelated supply issues before calling an engineer.
- Arrange for a Gas Safe engineer to inspect the DHW sensor wiring Gas Safe engineer
An engineer will isolate the appliance from mains power and close the gas supply before opening the casing. They will check each connector on the DHW sensor circuit for corrosion, loose pins, or chafed cable. Reseating or replacing a corroded connector can sometimes clear the fault at minimal cost.
- Have the engineer test the DHW NTC sensor with a multimeter Gas Safe engineer
The sensor's resistance is measured and compared against the manufacturer's specification at a known water temperature. An out-of-range reading confirms the sensor has failed and needs replacement. This is a straightforward test that takes only a few minutes.
- Replace the DHW NTC thermistor if it has failed Gas Safe engineer
Genuine or compatible Sime DHW sensors are relatively inexpensive parts. The engineer will fit the correct replacement, reseal any connections, and power the boiler back up to verify the fault code has cleared and hot water is functioning normally.
- Have the PCB investigated if AL 04 persists after a new sensor Gas Safe engineer
If the fault code returns with a confirmed-good sensor in place, the control board itself is the likely culprit. PCB diagnosis and replacement is specialist work and the most costly outcome of an AL 04 fault. Your engineer may advise whether repair or boiler replacement is more economical at this stage.
- Call a Gas Safe registered engineer if you have not already done so Gas Safe engineer
All work beyond the basic homeowner checks above — including opening the boiler casing, testing components, and replacing parts — must be carried out by a Gas Safe registered engineer. It is a legal requirement in the UK and essential for your safety and insurance.
Parts you may need
- Sime DHW NTC thermistor / temperature sensor · from £20
- Wiring harness connector / terminal repair kit · from £10
- Sime PCB (control board) · from £280
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–£220, depending on the underlying cause.
Frequently asked questions
Can I fix Sime AL 04 myself?
The only safe DIY steps are resetting the boiler once or twice and confirming your gas supply and power are on. Everything else — opening the casing, testing the sensor with a multimeter, and replacing components — must be done by a Gas Safe registered engineer. Attempting internal boiler repairs without Gas Safe registration is illegal and potentially dangerous.
How much does it cost to fix a Sime AL 04 fault in the UK?
A straightforward DHW sensor replacement typically costs £100–£220 all-in, covering the engineer's call-out, around an hour of labour, and the sensor part itself. If the PCB turns out to be the problem, the board alone can cost £200–£350 in parts, pushing the total bill significantly higher. Getting a written quote before work starts is always advisable.
Will the boiler produce any hot water while AL 04 is showing?
No. Because AL 04 is a lockout fault, the boiler shuts down completely and will not fire up for central heating or domestic hot water until the fault is cleared and the boiler successfully restarted. You will need to arrange alternative hot water arrangements until an engineer has resolved the issue.
Could a boiler service have prevented AL 04?
Quite possibly, yes. An annual service includes inspecting sensor connections and checking component readings, so a corroded terminal or a sensor drifting out of specification can be caught before it causes a full lockout. Keeping your boiler on a regular service schedule is the most cost-effective way to avoid unexpected breakdowns.