Sime Brava Slim HE Regular Fault codes & diagnostics

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

Heat Only Natural Gas 2016-present
Download the Sime Brava Slim HE Regular 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.
1 Emergency 36 High 2 Medium
Just want the most common Sime Brava Slim HE Regular faults?
The top 6 problems in plain English — with causes, DIY checks and repair costs.
Fault codes

All 39 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 external filling loop (the flexible silver hose) usually found underneath the boiler or nearby.
  2. Open the one or two taps on the filling loop slowly until you hear water moving.
  3. Watch the pressure gauge on the boiler until it reaches between 1.0 and 1.2 bar, then close the taps fully.

Parts commonly replaced

  • Filling loop valve
  • Pressure sensor
  • Expansion vessel

Related codes

Full explanation for AL 02
AL 05

Your boiler has detected a problem with the internal temperature sensor that monitors the water leaving the unit, preventing it from heating correctly.

High Engineer only £90-160

Parts commonly replaced

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

Related codes

Full explanation for AL 05
AL 06

Your boiler has failed to light, which means it cannot provide the heat or hot water you need.

High DIY-safe £100-300

What to check first

  1. Check if other gas appliances, like a hob, are working to ensure gas is reaching the house
  2. Check if your gas meter has credit (if using a prepayment meter)
  3. Hold the 'Reset' button for 3 seconds to attempt a restart

Parts commonly replaced

  • Ignition electrode
  • Flame sensing electrode
  • Gas valve

Related codes

Full explanation for AL 06
AL 07

Your boiler has switched itself off because it has detected that the water inside is getting too hot.

High DIY-safe £100-£220

What to check first

  1. Check that your radiator valves are open and not turned to zero.
  2. Attempt to reset the boiler by turning the selector switch to the reset position and back.
  3. Ensure your system pressure is correct if a gauge is visible, and bleed any air from your radiators.

Parts commonly replaced

  • Safety Thermostat (Limit Statutory)
  • Overheat Sensor
  • Circulation Pump

Related codes

Full explanation for AL 07
AL 08

Your boiler is struggling to detect the flame it needs to stay lit, which means it will stop working to keep you safe.

High Engineer only £100-280

Parts commonly replaced

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

Related codes

Full explanation for AL 08
AL 09

Your boiler is overheating because water is not moving through the system properly.

High Engineer only £120-£350

Parts commonly replaced

  • Circulation Pump
  • System Filter
  • Pump Impeller

Related codes

Full explanation for AL 09
AL 10

Your boiler has detected an issue with its internal settings or a secondary sensor, which is preventing it from running properly.

High Engineer only £100-220

Parts commonly replaced

  • Auxiliary temperature sensor
  • Wiring harness
  • Main PCB

Related codes

Full explanation for AL 10
AL 11

Your boiler has detected an issue with the internal electrical signal that controls the gas flow, meaning it cannot stay alight safely.

High Engineer only £120-£280

Parts commonly replaced

  • Gas valve modulator coil
  • Writing harness
  • Main PCB

Related codes

Full explanation for AL 11
AL 12

Your boiler's internal computer is set to the wrong ventilation mode, meaning it doesn't know if it is an open or sealed system.

High Engineer only £80-130

Parts commonly replaced

  • None - requires software configuration
  • Main PCB

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

  • Flue NTC 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 exhaust gases, and it has stopped running as a safety precaution.

High Engineer only £120-210

Parts commonly replaced

  • Exhaust sensor (SF)
  • Electrical wiring harness
  • Main PCB

Related codes

Full explanation for AL 14
AL 15

Your boiler has stopped working because the internal control board cannot communicate with the fan that clears exhaust gases.

High Engineer only £120-£350

Parts commonly replaced

  • Fan wiring loom
  • Fan
  • Main PCB

Related codes

Full explanation for AL 15
AL 18

Your boiler has detected that waste water cannot drain away properly, likely due to a blockage in the outlet pipe or internal trap.

High DIY-safe £80-160

What to check first

  1. Check if the external white plastic drain pipe has frozen during cold weather.
  2. If frozen, pour warm (not boiling) water over the external pipe or apply a heat pack to thaw it.
  3. Ensure the internal condensate trap is not visibly blocked with debris.
  4. Reset the boiler once the blockage is cleared.

Parts commonly replaced

  • Condensate trap
  • Condensate electrode
  • Drain pipe insulation

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 £100-250

What to check first

  1. Leave the boiler powered on but do not touch any buttons for at least 1 hour.
  2. After 60 minutes have passed, the lockout will clear automatically.
  3. Attempt one single reset to see if the original fault has cleared.

Parts commonly replaced

  • Ignition Electrode
  • Gas Valve
  • PCB

Related codes

Full explanation for AL 28
AL 37

Your boiler has detected that the electrical power supply from the national grid is far too low for it to operate safely.

Medium Engineer only £0-120

Parts commonly replaced

  • No parts usually required
  • Printed Circuit Board (if fault is internal)

Related codes

Full explanation for AL 37
AL 40

Your boiler has detected an issue with the electrical power supply coming into your home, meaning it cannot operate safely with the current frequency of the electricity.

High Engineer only £100-350

Parts commonly replaced

  • Printed Circuit Board (PCB)
  • External power surge protector

Related codes

Full explanation for AL 40
AL 41

Your boiler is failing to keep its flame lit and has locked itself out for safety after several unsuccessful attempts.

High Engineer only £120-280

Parts commonly replaced

  • Ignition electrode
  • Detection electrode
  • Flame sensing lead
  • Gas valve

Related codes

Full explanation for AL 41
AL 42

Your boiler's control panel has detected that one of the buttons is stuck or the main circuit board is receiving a constant signal from the controls.

Medium DIY-safe £100-£280

What to check first

  1. Carefully press and release each button on the control panel to ensure none are physically stuck down.
  2. Gently wipe the control surface with a dry cloth to remove any debris or sticky residue.
  3. Switch the boiler off at the fused spur switch, wait 30 seconds, and turn it back on to reset the electronics.

Parts commonly replaced

  • Control Interface / Button Membrane
  • Main PCB (Printed Circuit Board)

Related codes

Full explanation for AL 42
AL 43

Your boiler has lost its connection to your smart thermostat or external heater controls, meaning it cannot receive instructions to turn on.

High DIY-safe £80-220

What to check first

  1. Check that your wall thermostat is powered on and has fresh batteries if applicable.
  2. Ensure the thermostat wiring or receiver box is securely plugged in and hasn't been accidentally bumped.
  3. Turn the boiler power off and back on at the fused spur switch to attempt to re-establish the connection.

Parts commonly replaced

  • OpenTherm Thermostat/Receiver
  • Control Interface Cable
  • Main PCB

Related codes

Full explanation for AL 43
AL 72

Your boiler has detected that a temperature sensor is not sitting correctly on its pipe, meaning it cannot accurately monitor the water temperature.

High Engineer only £90-160

Parts commonly replaced

  • Flow temperature NTC sensor
  • 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 supply during ignition and has shut down for safety.

High Engineer only £120-280

Parts commonly replaced

  • Gas Valve
  • Electrode
  • Flue Seals
  • Fan

Related codes

Full explanation for AL 81
AL 96

Your boiler has detected a blockage or restriction in the flue pipe which is preventing exhaust gases from escaping safely, causing the system to shut down.

Emergency Engineer only £100-250

Parts commonly replaced

  • Flue terminal / Cowl
  • Pressure switch
  • Fan assembly
  • Venturi tube

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 sensor
  • Wiring harness

Related codes

Full explanation for ALL 30
ALL 44

Your boiler is failing to light the burner because it has detected a technical issue with the internal ignition system.

High Engineer only £150-300

Parts commonly replaced

  • Gas valve
  • Ignition transformer
  • Main circuit board

Related codes

Full explanation for ALL 44
ALL 77

Your boiler has detected an electrical issue with the internal gas control valve, preventing it from operating safely.

High Engineer only £180-350

Parts commonly replaced

  • Gas valve
  • Main 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 shut down to prevent a fault.

High Engineer only £180-350

Parts commonly replaced

  • Gas valve
  • PCB

Related codes

Full explanation for ALL 78
ALL 79

Your boiler has detected an issue with the internal gas valve's electrical signal, which is preventing it from safely supplying gas to the burner.

High Engineer only £150-300

Parts commonly replaced

  • Gas valve
  • Main control board

Related codes

Full explanation for ALL 79
ALL 80

Your boiler is having trouble communicating with an internal valve that controls the flow of water, preventing the system from heating up properly.

High Engineer only £150-250

Parts commonly replaced

  • Wiring harness
  • Gas valve
  • Main PCB

Related codes

Full explanation for ALL 80
ALL 82

Your boiler has detected multiple failed attempts to light the flame and has locked itself out for safety reasons.

High Engineer only £150-300

Parts commonly replaced

  • Ignition electrode
  • Gas valve
  • Flame rectification probe

Related codes

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

Your boiler has detected that it is not receiving enough gas to operate safely and has shut itself down to prevent damage.

High Engineer only £100-250

Parts commonly replaced

  • Gas valve
  • Gas meter regulator

Related codes

Full explanation for ALL 84
ALL 88

Your boiler's electronic brain has detected a critical internal fault and has shut itself down to prevent potential damage.

High Engineer only £250-450

Parts commonly replaced

  • Main Control PCB
  • Wiring harness

Related codes

Full explanation for ALL 88
ALL 89

Your boiler is struggling to maintain a stable flame because it cannot correctly detect the quality of the combustion process, causing it to shut down for safety.

High Engineer only £150-300

Parts commonly replaced

  • Flame rectification electrode
  • Gas valve
  • PCB

Related codes

Full explanation for ALL 89
ALL 90

Your boiler is unable to achieve the correct air and gas mixture required for safe and efficient burning.

High Engineer only £150-350

Parts commonly replaced

  • Gas valve
  • Flue fan
  • Air pressure switch
  • Main PCB

Related codes

Full explanation for ALL 90
ALL 92

Your boiler has detected that it is struggling to breathe properly due to air trapped in the combustion system, preventing it from heating your home safely.

High Engineer only £150-300

Parts commonly replaced

  • Flue seal
  • Air pressure switch
  • Fan assembly

Related codes

Full explanation for ALL 92
ALL 93

Your boiler is failing to mix the gas and air correctly to create a safe flame, so it has stopped working to prevent unsafe operation.

High Engineer only £150-350

Parts commonly replaced

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

Related codes

Full explanation for ALL 93
ALL 95

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

High Engineer only £150-300

Parts commonly replaced

  • Ignition electrode
  • Flame rectification probe
  • PCB

Related codes

Full explanation for ALL 95
ALL 98

Your boiler's computer is having trouble starting up correctly, which means it cannot operate your heating or hot water safely.

High Engineer only £200-400

Parts commonly replaced

  • Main control board (PCB)

Related codes

Full explanation for ALL 98
ALL 99

Your boiler's main control computer is experiencing a critical internal error and has stopped working to keep the system safe.

High Engineer only £250-450

Parts commonly replaced

  • Main 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 has lost its fine-tuned settings and needs to run a setup process to ensure the gas is burning safely and efficiently.
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