Sime Brava Slim HE System Fault codes & diagnostics

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

System Natural Gas 2016-present
Download the Sime Brava Slim HE System manual The official installation & service manual (PDF) — the exact document these fault codes were verified against. PDF

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Severity at a glance

Each cell = one fault code. Hover to identify.
3 Emergency 35 High 1 Medium
Just want the most common Sime Brava Slim HE System faults?
The top 6 problems in plain English — with causes, DIY checks and repair costs.
Fault codes

All 39 documented codes

Tap any card for details

AL 02

Your boiler has stopped working because the water pressure inside the system has dropped too low to operate safely.

High DIY-safe £0-120

What to check first

  1. Locate the silver filling loop (braided hose) usually found underneath the boiler.
  2. Slowly open the small valve handles to allow water into the system.
  3. Watch the pressure gauge and close both valves once it reaches between 1.0 and 1.2 bar.

Parts commonly replaced

  • Filling loop
  • Expansion vessel recharge
  • Pressure sensor

Related codes

Full explanation for AL 02
AL 05

Your boiler has stopped working because a sensor that monitors the temperature of the water leaving the unit has failed or disconnected.

High Engineer only £90-160

Parts commonly replaced

  • Flow temperature sensor (NTC thermistor)
  • Sensor wiring harness

Related codes

Full explanation for AL 05
AL 06

Your boiler has stopped working because it cannot light the gas or cannot detect that a flame has started.

High DIY-safe £120-£350

What to check first

  1. Check if your gas supply is active by testing another gas appliance like a hob.
  2. Ensure your gas emergency control valve (usually by the meter) is open.
  3. Press and hold the 'R' or reset button for a few seconds to restart the ignition sequence.

Parts commonly replaced

  • Ignition electrode
  • Detection electrode
  • Gas valve
  • Ignition lead

Related codes

Full explanation for AL 06
AL 07

Your boiler has switched itself off because it has detected that the internal water temperature is too high, often caused by trapped air or a blockage preventing water flow.

High DIY-safe £120-£250

What to check first

  1. Check that your radiator valves are open to ensure water can circulate.
  2. Carefully bleed your radiators to remove any trapped air from the system.
  3. Attempt to reset the boiler by turning the selector dial to the 'R' position for a few seconds.

Parts commonly replaced

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

Related codes

Full explanation for AL 07
AL 08

Your boiler is having trouble sensing the flame required to heat your water, which means it has shut down for safety.

High Engineer only £120-280

Parts commonly replaced

  • Ignition electrode
  • Ionisation probe
  • Gas valve
  • PCB

Related codes

Full explanation for AL 08
AL 09

Your boiler has stopped working because water is not moving through the internal pipework as it should.

High Engineer only £180-£350

Parts commonly replaced

  • Circulation Pump
  • Central Heating Filter
  • Pump Impeller

Related codes

Full explanation for AL 09
AL 10

Your boiler's electronic control system is confused because it is receiving conflicting signals from an internal sensor or is incorrectly set up for your home's pipework.

High Engineer only £90-180

Parts commonly replaced

  • Auxiliary temperature sensor
  • Control board PCB

Related codes

Full explanation for AL 10
AL 11

Your boiler has lost communication with the internal component that controls the gas flow, meaning it cannot ignite safely to provide heating or hot water.

High Engineer only £120-£280

Parts commonly replaced

  • Gas valve modulator
  • Wiring harness
  • Main PCB

Related codes

Full explanation for AL 11
AL 12

Your boiler's computer has been set up with the wrong internal settings, causing it to shut down because it doesn't recognize how it is built.

High Engineer only £80-150

Parts commonly replaced

  • Main PCB (if settings cannot be saved)

Related codes

Full explanation for AL 12
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 (smoke probe)
  • Main PCB
  • Heat exchanger cleaning

Related codes

Full explanation for AL 13
Showing 11–20 of 39
AL 14

Your boiler has detected a problem with the sensor that monitors the temperature of the waste gases, causing the system to shut down for safety.

High Engineer only £120-190

Parts commonly replaced

  • Exhaust sensor (SF)
  • Wiring harness connectors

Related codes

Full explanation for AL 14
AL 15

Your boiler has detected a communication problem with its internal fan, which means it cannot safely clear away exhaust fumes and will not start.

High Engineer only £120-£280

Parts commonly replaced

  • Fan wiring harness
  • Combustion fan
  • Main PCB

Related codes

Full explanation for AL 15
AL 18

Your boiler has detected that waste water cannot drain away properly, usually because the discharge pipe is blocked or the internal trap is full.

High DIY-safe £80-140

What to check first

  1. Locate the plastic pipe leading from the boiler to the outside drain.
  2. Check if the end of the pipe is submerged in water or blocked by debris/ice.
  3. If the pipe is frozen, gently pour warm (not boiling) water over it to melt the ice.
  4. Restart the boiler using the reset button.

Parts commonly replaced

  • Condensate siphon
  • Condensate electrode
  • Drain pipework

Related codes

Full explanation for AL 18
AL 28

Your boiler has been manually reset too many times in a short period and has temporarily locked itself for safety.

High DIY-safe £80-250

What to check first

  1. Leave the boiler powered on and do not touch any buttons for 60 minutes.
  2. After one hour has passed, the lockout will clear automatically.
  3. Attempt one final reset; if the original fault returns, do not attempt further resets and call an engineer.

Parts commonly replaced

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

Related codes

Full explanation for AL 28
AL 37

Your boiler is receiving too little electricity from the mains supply to operate safely and has shut down as a precaution.

High Engineer only £0-120

Parts commonly replaced

  • Main PCB

Related codes

Full explanation for AL 37
AL 40

Your boiler has detected an issue with the electrical power coming into your home and has shut down as a safety precaution.

High Engineer only £100-280

Parts commonly replaced

  • Main PCB
  • Surge Protector

Related codes

Full explanation for AL 40
AL 41

Your boiler has tried to light several times but cannot maintain a stable flame, causing the system to lock out for safety.

High Engineer only £120-£280

Parts commonly replaced

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

Related codes

Full explanation for AL 41
AL 42

Your boiler has detected that one of its control buttons is stuck or pressed down for too long, preventing the system from working correctly.

Medium DIY-safe £100-£350

What to check first

  1. Carefully press and release each button on the control panel to see if any are stuck in the 'in' position.
  2. Gently wipe the control panel with a dry cloth to remove any debris or sticky residue.
  3. Turn the boiler off at the fused spur switch, wait 30 seconds, and turn it back on to reset the electronics.

Parts commonly replaced

  • Main PCB (Printed Circuit Board)
  • Control Interface/Bezel

Related codes

Full explanation for AL 42
AL 43

Your boiler has lost its connection to the smart thermostat or remote control, meaning it is not receiving instructions on when to turn the heating on.

High DIY-safe £80-180

What to check first

  1. Check that your smart thermostat or remote controller is powered on.
  2. Replace batteries in the wall thermostat if applicable.
  3. Restart your home internet router if the thermostat relies on Wi-Fi.
  4. Ensure any modular receivers plugged into the boiler are pushed in firmly.

Parts commonly replaced

  • OpenTherm Gateway
  • Remote Thermostat
  • PCB (Printed Circuit Board)

Related codes

Full explanation for AL 43
AL 72

Your boiler has detected that a temperature sensor is not properly attached to the main internal pipe, meaning it cannot accurately monitor the water's heat.

High Engineer only £90-160

Parts commonly replaced

  • Delivery temperature sensor (NTC)
  • Sensor retaining clip

Related codes

Full explanation for AL 72
Showing 21–30 of 39
AL 81

Your boiler has detected an issue with the air or gas mixture during ignition, often caused by a blocked vent or a flue problem, and has shut down for safety.

High Engineer only £100-280

Parts commonly replaced

  • Air pressure switch
  • Gas valve
  • Flue seals
  • Fan

Related codes

Full explanation for AL 81
AL 83

Your boiler is struggling to burn gas correctly and efficiently, which can cause the system to shut down for safety.

High Engineer only £100-280

Parts commonly replaced

  • Ignition electrode
  • Flame sensing electrode
  • Gas valve
  • Flue seals

Related codes

Full explanation for AL 83
AL 96

Your boiler has switched itself off because it has detected an obstruction in the exhaust pipe that prevents waste gases from leaving the building safely.

Emergency Engineer only £100-250

Parts commonly replaced

  • Flue terminal
  • Flue seals
  • Pressure switch
  • Fan

Related codes

Full explanation for AL 96
ALL 30

Your boiler has lost the ability to accurately measure the temperature of the water returning to it, so it has shut down as a precaution to prevent overheating.

High Engineer only £120-220

Parts commonly replaced

  • Return NTC thermistor
  • Wiring harness

Related codes

Full explanation for ALL 30
ALL 44

Your boiler is failing to light because it cannot detect that the gas valve is opening correctly to fuel the flame.

High Engineer only £150-£300

Parts commonly replaced

  • Gas valve
  • Main PCB

Related codes

Full explanation for ALL 44
ALL 77

Your boiler has detected an electrical problem with the component that controls the gas supply to the burner, meaning it has safely shut down to prevent unsafe operation.

High Engineer only £180-350

Parts commonly replaced

  • Gas valve
  • PCB

Related codes

Full explanation for ALL 77
ALL 78

Your boiler has detected an electrical issue with the gas control valve, meaning it cannot safely regulate the fuel supply and has stopped working to prevent a risk.

High Engineer only £180-350

Parts commonly replaced

  • Gas valve
  • Main control board

Related codes

Full explanation for ALL 78
ALL 79

Your boiler has detected an issue with the electrical signal to the gas valve, meaning it cannot safely control the fuel supply to ignite the burner.

High Engineer only £150-300

Parts commonly replaced

  • Gas valve
  • PCB

Related codes

Full explanation for ALL 79
ALL 80

Your boiler's internal computer has lost connection with the part that controls the flow of gas or water, meaning it cannot safely start up.

High Engineer only £150-250

Parts commonly replaced

  • Gas valve
  • Wiring harness
  • Main PCB

Related codes

Full explanation for ALL 80
ALL 82

Your boiler has detected multiple failed attempts to ignite or maintain a stable flame and has locked itself out to prevent potential gas safety issues.

Emergency Engineer only £150-350

Parts commonly replaced

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

Related codes

Full explanation for ALL 82
Showing 31–39 of 39
ALL 84

Your boiler is struggling to receive enough gas to run properly, so it has shut down to prevent unsafe operation.

High Engineer only £120-250

Parts commonly replaced

  • Gas valve
  • Gas regulator
  • Gas meter

Related codes

Full explanation for ALL 84
ALL 88

Your boiler's main control unit has detected an internal fault and has shut itself down to prevent damage to its electronic systems.

High Engineer only £250-450

Parts commonly replaced

  • Main printed circuit board (PCB)

Related codes

Full explanation for ALL 88
ALL 89

Your boiler has detected that the air and gas mixture used to create heat is not burning correctly and has shut down for your safety.

Emergency Engineer only £150-300

Parts commonly replaced

  • Ionisation electrode
  • Gas valve
  • PCB

Related codes

Full explanation for ALL 89
ALL 90

Your boiler is struggling to burn gas properly and has shut down for safety reasons because it cannot achieve the correct flame settings.

High Engineer only £150-300

Parts commonly replaced

  • Gas valve
  • Flue seal
  • Air pressure switch
  • Venturi

Related codes

Full explanation for ALL 90
ALL 92

Your boiler is struggling to get the right balance of air and gas for safe burning and has automatically locked itself out to prevent damage.

High Engineer only £150-300

Parts commonly replaced

  • Gas valve
  • Air pressure switch
  • Flue assembly

Related codes

Full explanation for ALL 92
ALL 93

Your boiler is failing to burn the gas and air mixture correctly, meaning it has shut itself down to ensure your safety.

High Engineer only £150-300

Parts commonly replaced

  • Gas valve
  • Venturi
  • Fan assembly
  • Air pressure switch

Related codes

Full explanation for ALL 93
ALL 95

Your boiler is struggling to keep a steady flame, causing it to repeatedly shut down and restart for safety reasons.

High Engineer only £150-300

Parts commonly replaced

  • Ionisation probe
  • Ignition lead
  • Printed Circuit Board (PCB)

Related codes

Full explanation for ALL 95
ALL 98

Your boiler's computer is failing to start up correctly, preventing it from functioning safely.

High Engineer only £250-450

Parts commonly replaced

  • Main PCB

Related codes

Full explanation for ALL 98
ALL 99

Your boiler's main computer brain has encountered a critical internal error and needs to be reset or professionally inspected to safely restore operation.

High Engineer only £200-450

Parts commonly replaced

  • Printed Circuit Board (PCB)
  • Wiring harness

Related codes

Full explanation for ALL 99
Status & diagnostic codes

1 operating states & engineer diagnostics

Not faults — these are normal operating states. Tap to expand.

AL 62 Diagnostic Your boiler needs to run a self-check to ensure it is burning gas safely and efficiently, usually following a repair or a change in gas supply.
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