Sime Format DGT HE Combi Fault codes & diagnostics

10 fault codes with plain-English explanations, severity ratings, DIY guidance, and repair cost estimates.

Combi Natural Gas 2010-2018 Discontinued
GC Numbers 47-283-2447-283-2547-283-26
Download the Sime Format DGT HE Combi manual The official installation & service manual (PDF) — the exact document these fault codes were verified against. PDF

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10 High
Just want the most common Sime Format DGT HE Combi faults?
The top 6 problems in plain English — with causes, DIY checks and repair costs.
Fault codes

All 10 documented codes

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AL 02

Your boiler has stopped working because the water pressure is too low and needs to be topped up.

High DIY-safe £0-120

What to check first

  1. Locate the filling loop, which is usually a flexible silver hose under the boiler.
  2. Slowly open the small valve (or valves) on the hose until you hear water moving.
  3. Watch the pressure gauge and close the valves tightly once the needle reaches between 1.0 and 1.2 bar.

Parts commonly replaced

  • Filling loop
  • Pressure sensor
  • Expansion vessel

Related codes

Full explanation for AL 02
AL 05

Your boiler has detected a problem with the temperature sensor that monitors the water being sent to your taps and radiators, causing it to stop working as a safety precaution.

High Engineer only £90-160

Parts commonly replaced

  • Central Heating (SM) Temperature Sensor
  • NTC Thermistor
  • Wiring Harness

Related codes

Full explanation for AL 05
AL 06

Your boiler has failed to light a flame, which means you will not have any heating or hot water until the issue is resolved.

High DIY-safe £120-350

What to check first

  1. Check if other gas appliances in your home (like a hob) are working to ensure gas is reaching the property
  2. Ensure your gas meter has credit if you use a topped-up prepay meter
  3. Press the 'Reset' button on the boiler control panel once to see if the fault clears

Parts commonly replaced

  • Ignition electrode
  • Flame sensing electrode
  • Gas valve
  • Printed Circuit Board (PCB)

Related codes

Full explanation for AL 06
AL 07

Your boiler has overheated, causing a safety switch to shut the system down to prevent damage.

High DIY-safe £100-220

What to check first

  1. Allow the boiler to cool down for at least 30 minutes
  2. Check that all radiator valves are fully open
  3. Check the boiler pressure is between 1.0 and 1.5 bar
  4. Press the selector switch to the 'R' (Reset) position for a few seconds

Parts commonly replaced

  • High limit safety thermostat
  • System pump
  • Overheat sensor (NTC)

Related codes

Full explanation for AL 07
AL 08

Your boiler is struggling to light or detect a flame, meaning it cannot safely heat your water or radiators.

High DIY-safe £100-280

What to check first

  1. Check that your gas emergency control valve is open and you have credit on your gas meter
  2. Turn other gas appliances like a hob on to confirm gas is flowing to the property
  3. Press and hold the 'R' reset button for a few seconds to see if the boiler restarts

Parts commonly replaced

  • Ignition electrode
  • Flame sensing probe
  • Gas valve
  • Printed Circuit Board (PCB)

Related codes

Full explanation for AL 08
AL 10

Your boiler has detected an internal sensor error or a configuration mismatch, meaning it cannot accurately monitor the system's operation and has stopped working for safety.

High Engineer only £90-180

Parts commonly replaced

  • Auxiliary NTC Sensor
  • Main PCB
  • Wiring Loom

Related codes

Full explanation for AL 10
AL 13

Your boiler has detected that the exhaust gases are too hot or the sensor monitoring them has failed, causing the system to shut down for safety.

High Engineer only £120-£220

Parts commonly replaced

  • Exhaust sensor (SF)
  • Smoke probe
  • Main PCB

Related codes

Full explanation for AL 13
AL 14

Your boiler has detected an issue with the sensor that monitors exhaust gases, and it has shut down for safety purposes.

High Engineer only £120-£200

Parts commonly replaced

  • Exhaust sensor (SF)
  • Smoke probe
  • Wiring harness

Related codes

Full explanation for AL 14
AL 15

Your boiler has detected that the internal fan is spinning at the wrong speed, so it has safely shut down to prevent improper combustion.

High Engineer only £150-300

Parts commonly replaced

  • Fan assembly
  • Air pressure switch
  • Venturi tube
  • PCB

Related codes

Full explanation for AL 15
AL 18

Your boiler has stopped working because the pipe that removes waste water is blocked, causing water to back up inside the unit.

High DIY-safe £80-160

What to check first

  1. Check the plastic pipe leading from the boiler to the outside drain for signs of freezing or blockages.
  2. If the external pipe is frozen, gently pour warm (not boiling) water over it to melt the ice.
  3. Ensure the internal siphon/trap is clear of debris if accessible without opening the sealed boiler casing.
  4. Reset the boiler by turning the selector switch to the 'R' position for a few seconds.

Parts commonly replaced

  • Condensate trap
  • Condensate pipework
  • Electrode (if damaged by moisture)

Related codes

Full explanation for AL 18