Sime Format DGT HE System Fault codes & diagnostics

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

System Natural Gas 2010-2018 Discontinued
GC Number 41-283-18
Download the Sime Format DGT HE System 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 System 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 inside the system has dropped 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 silver flexible hose with one or two small taps.
  2. Slowly open the tap(s) until you hear water flowing and watch the pressure gauge on the boiler panel.
  3. Close the taps tightly once the needle reaches between 1.0 and 1.2 bar.

Parts commonly replaced

  • Filling loop hose
  • Pressure sensor
  • Expansion vessel re-pressurisation

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 radiators, causing the system to stop working for safety.

High Engineer only £90-160

Parts commonly replaced

  • Flow Temperature Sensor (NTC)
  • Wiring Harness

Related codes

Full explanation for AL 05
AL 06

Your boiler is not detecting a flame, which means it cannot light itself to provide heating or hot water.

High DIY-safe £120-280

What to check first

  1. Check if other gas appliances in your home, such as a hob, are working to confirm gas supply.
  2. Ensure your gas meter has credit if you use a prepay meter.
  3. Press the 'Reset' button on the boiler control panel to see if it restarts.

Parts commonly replaced

  • Ignition Electrode
  • Flame Sensing Electrode
  • Gas Valve

Related codes

Full explanation for AL 06
AL 07

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

High DIY-safe £90-180

What to check first

  1. Check that all radiator valves are fully open
  2. Bleed all radiators to remove trapped air from the system
  3. Once the boiler has cooled down, press the 'R' button to reset the unit

Parts commonly replaced

  • Safety thermostat (High limit stat)
  • Circulation pump
  • Overheat sensor

Related codes

Full explanation for AL 07
AL 08

Your boiler is struggling to light or detect a flame, which means it will shut down to prevent wasting gas.

High Engineer only £120-£280

Parts commonly replaced

  • Ignition electrode
  • Flame sensing probe
  • Gas valve
  • PCB

Related codes

Full explanation for AL 08
AL 10

Your boiler's control system is confused about how it should be working, likely due to a faulty sensor or a setting error in its internal computer.

High Engineer only £90-180

Parts commonly replaced

  • Auxiliary Temperature Sensor
  • Control PCB

Related codes

Full explanation for AL 10
AL 13

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

High Engineer only £120-£220

Parts commonly replaced

  • Smoke probe (Exhaust sensor)
  • Main heat exchanger cleaning
  • 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 £120-190

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 responsible for venting exhaust gases is not spinning at the correct speed, so it has safely shut itself down to prevent unsafe operation.

High Engineer only £150-300

Parts commonly replaced

  • Fan assembly
  • Air pressure switch
  • PCB

Related codes

Full explanation for AL 15
AL 18

Your boiler has detected that the waste water pipe or internal drainage trap is blocked, preventing the unit from draining properly.

High DIY-safe £80-160

What to check first

  1. Check the white plastic pipe leading outside to see if it is frozen or blocked by debris.
  2. If frozen, carefully pour warm (not boiling) water over the external pipe to thaw it.
  3. Inspect the internal condensate trap (siphon) if accessible and clean it of any visible sludge or debris.
  4. Reset the boiler once the blockage is cleared.

Parts commonly replaced

  • Condensate siphon/trap
  • Condensate electrode
  • Drainage pipework

Related codes

Full explanation for AL 18