Baxi Assure Fault codes & diagnostics

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

Combi Natural Gas 24-36 kW 2020-present
Choose your boiler type

The Assure range comes in 6 types

Pick the one you have. The fault codes, GC numbers and manual differ by type.

524 COMBI 2 65 codes · 1 GC number 25 Combi 19 codes · 2 GC numbers 30 Combi 19 codes · 2 GC numbers 36 Combi 19 codes · 2 GC numbers 530 COMBI 2 65 codes · 1 GC number 536 COMBI 2 65 codes · 1 GC number
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Note it down before pressing reset — resetting clears the code immediately. How to read fault codes →

Severity at a glance

Each cell = one fault code. Hover to identify.
28 High 1 Medium 2 Low
Just want the most common Baxi Assure faults?
The top 6 problems in plain English — with causes, DIY checks and repair costs.
Fault codes

All 31 documented codes

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09

Your boiler has detected a temporary internal issue that has caused it to stop working, but it can likely be resolved by a simple manual reset.

Medium DIY-safe £80-120 Shared across the Assure range

What to check first

  1. Locate the reset button on the front control panel.
  2. Press and hold the reset button for approximately 3 seconds.
  3. Wait a few minutes to see if the boiler restarts and the fault code disappears.

Parts commonly replaced

  • None
  • Printed Circuit Board (PCB)

Related codes

Full explanation for 09
15

Your boiler has experienced a temporary glitch and needs to be manually restarted to get it back up and running.

Low DIY-safe Shared across the Assure range

What to check first

  1. Locate the 'Reset' button on the front control panel of your boiler.
  2. Press and hold the button for 2 to 3 seconds.
  3. Wait a few minutes to see if the boiler fires up and the fault code clears.

Related codes

Full explanation for 15
20

Your boiler has detected an internal communication error between its internal parts, meaning it has safely shut itself down to prevent potential damage.

High Engineer only £150-300 Shared across the Assure range

Parts commonly replaced

  • Printed Circuit Board (PCB)
  • Wiring harness
  • Gas valve

Related codes

Full explanation for 20
28

Your boiler is struggling to read its own internal water temperature correctly, which is preventing it from heating your home or water safely.

High Engineer only £150-250 Shared across the Assure range

Parts commonly replaced

  • NTC Sensor
  • Wiring harness
  • Printed Circuit Board

Related codes

Full explanation for 28
40

Your boiler has detected a general internal fault that is preventing it from starting up correctly.

High Engineer only £150-300 Shared across the Assure range

Parts commonly replaced

  • Printed Circuit Board (PCB)
  • Wiring harness
  • Gas valve

Related codes

Full explanation for 40
103

Your boiler's communication system with its optional safety add-ons is experiencing a glitch, causing the unit to stop working as a precaution.

High Engineer only £120-250 Shared across the Assure range

Parts commonly replaced

  • Safety Assist accessory module
  • Wiring harness
  • PCB

Related codes

Full explanation for 103
104

Your boiler's internal safety communication system is experiencing a fault, which has caused it to stop working to ensure your safety.

High Engineer only £150-250 Shared across the Assure range

Parts commonly replaced

  • PCB
  • Wiring harness
  • Safety sensor link

Related codes

Full explanation for 104
107

Your boiler has lost communication with an external safety device, such as a smart control or external sensor, preventing it from operating correctly.

High Engineer only £120-250 Shared across the Assure range

Parts commonly replaced

  • External safety sensor
  • Wiring harness
  • PCB

Related codes

Full explanation for 107
110

Your boiler has detected that the water inside the system has become too hot and has automatically shut down to prevent damage.

High Engineer only £150-300 Shared across the Assure range

Parts commonly replaced

  • NTC Sensor
  • Primary Heat Exchanger
  • Central Heating Pump

Related codes

Full explanation for 110
118

Your boiler has stopped working because the water pressure inside the heating system has dropped too low.

High DIY-safe £0-£120 Shared across the Assure range

What to check first

  1. Locate the external filling loop, usually a silver braided hose with one or two valves underneath the boiler.
  2. Slowly open the valve(s) to allow mains water into the system until the pressure gauge reads between 1.0 and 1.5 bar.
  3. Close the valve(s) firmly and ensure the boiler resets itself.

Parts commonly replaced

  • Filling loop
  • Pressure relief valve
  • Pressure sensor

Related codes

Full explanation for 118
Showing 11–20 of 31
125

Your boiler is struggling to pump water around your heating system effectively, meaning it cannot properly heat your home or taps.

High Engineer only £150-350 Shared across the Assure range

Parts commonly replaced

  • Central heating pump
  • Diverter valve
  • System filter

Related codes

Full explanation for 125
128

Your boiler has stopped because the flame inside it went out unexpectedly while it was trying to heat your home.

High Engineer only £120-250 Shared across the Assure range

Parts commonly replaced

  • Ignition electrode
  • Gas valve
  • PCB

Related codes

Full explanation for 128
133

Your boiler is failing to light because it is not receiving enough gas or is unable to spark correctly to create a flame.

High Engineer only £120-250 Shared across the Assure range

Parts commonly replaced

  • Ignition electrode
  • Gas valve
  • Printed circuit board

Related codes

Full explanation for 133
134

Your boiler is failing to light because it cannot detect a flame, which means your heating and hot water have stopped working.

High Engineer only £120-250 Shared across the Assure range

Parts commonly replaced

  • Ignition electrode
  • Gas valve
  • Flame rectification probe

Related codes

Full explanation for 134
135

Your boiler has stopped working because it cannot light its internal flame or is not receiving a consistent supply of gas.

High Engineer only £120-250 Shared across the Assure range

Parts commonly replaced

  • Ignition electrode
  • Gas valve
  • Flame rectification probe

Related codes

Full explanation for 135
160

Your boiler has detected a problem with the fan or its internal electronics, meaning it has safely shut itself down to prevent further issues.

High Engineer only £150-300 Shared across the Assure range

Parts commonly replaced

  • Fan assembly
  • Wiring harness
  • Printed Circuit Board (PCB)

Related codes

Full explanation for 160
321

Your boiler has detected an internal issue with one of its main sensors, meaning it cannot operate safely and has shut itself down.

High Engineer only £150-300 Shared across the Assure range

Parts commonly replaced

  • Flue gas sensor
  • NTC thermistor
  • Printed Circuit Board (PCB)

Related codes

Full explanation for 321
384

Your boiler has detected an internal communication error and has safely shut itself down to prevent any further issues.

High Engineer only £120-250 Shared across the Assure range

Parts commonly replaced

  • Printed Circuit Board (PCB)
  • Wiring harness

Related codes

Full explanation for 384
Bat

Your boiler's internal back-up battery for the safety monitoring system is low or failing and needs replacing.

Low Engineer only £70-130 Shared across the Assure range

Parts commonly replaced

  • Safety Assist battery pack

Related codes

Full explanation for Bat
E0

Your boiler's internal computer parts are having trouble talking to each other, which means the system cannot operate correctly.

High Engineer only £150-£450 Shared across the Assure range

Parts commonly replaced

  • Printed Circuit Board (PCB)
  • Wiring Harness
  • Communication Interface

Related codes

Full explanation for E0
Showing 21–30 of 31
E4

Your boiler is struggling to move water around the system properly before it starts the ignition process.

High DIY-safe £150-£350 Shared across the Assure range

What to check first

  1. Check that your central heating valves under the boiler are fully open
  2. Ensure all radiator valves (TRVs) are turned to the highest setting
  3. Check the boiler pressure gauge is between 1 and 1.5 bar; top up if it is too low
  4. Restart the boiler using the reset button

Parts commonly replaced

  • Circulation Pump
  • Water Pressure Sensor
  • Flow/Return Thermistor

Related codes

Full explanation for E4
E15

Your boiler has detected an issue with the internal gas valve, meaning it cannot safely control the fuel flow and has shut down as a precaution.

High Engineer only £220-380 Shared across the Assure range

Parts commonly replaced

  • Gas valve
  • Printed Circuit Board (PCB)
  • Wiring harness

Related codes

Full explanation for E15
E20

Your boiler is having trouble correctly measuring the temperature of the water used to heat your radiators.

High Engineer only £90-160 Shared across the Assure range

Parts commonly replaced

  • Central heating NTC temperature sensor
  • Connecting wiring harness

Related codes

Full explanation for E20
E28

Your boiler has stopped working because the water pressure is either too low or the system has become too hot.

High DIY-safe £0-120 Shared across the Assure range

What to check first

  1. Check the pressure gauge on the front of the boiler; it should be between 1 and 1.5 bar.
  2. If it is low, use the filling loop underneath the boiler to top up the water pressure.
  3. Press the 'R' or Reset button once the pressure is restored to restart the boiler.

Parts commonly replaced

  • Filling loop
  • Pressure sensor
  • Safety thermostat

Related codes

Full explanation for E28
E110

Your boiler has got too hot or is struggling to maintain a steady flame, causing it to shut down for safety.

High DIY-safe £100-£350 Shared across the Assure range

What to check first

  1. Check that your gas pre-pay meter has credit and the emerald lever is open
  2. Ensure all radiator valves are fully open to allow water to circulate
  3. Let the boiler cool down for 30 minutes, then press and hold the 'R' or Reset button for 5 seconds

Parts commonly replaced

  • Overheat Thermostat
  • Circulation Pump
  • Gas Valve

Related codes

Full explanation for E110
E118

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

High DIY-safe £0-120 Shared across the Assure range

What to check first

  1. Locate the filling loop (usually a silver braided hose with small black lever valves under the boiler).
  2. Slowly open the valves until you hear water flowing and the pressure gauge needle moves between 1.0 and 1.5 bar.
  3. Close both valves tightly and press the 'Reset' button on the boiler control panel.

Parts commonly replaced

  • Filling loop
  • Expansion vessel
  • Pressure sensor

Related codes

Full explanation for E118
E125

Your boiler has detected that water isn't moving through the system properly, which is causing it to stop to prevent overheating.

High DIY-safe £120-£350 Shared across the Assure range

What to check first

  1. Check that all radiator valves are fully open
  2. Ensure the boiler pressure is between 1.0 and 1.5 bar on the gauge
  3. Bleed all radiators to remove trapped air from the system

Parts commonly replaced

  • Central heating pump
  • Flow and return sensors (NTC thermistors)
  • Diverter valve

Related codes

Full explanation for E125
E128

Your boiler has detected that the flame has gone out while it was running, which could be caused by an issue with the gas supply or a faulty internal fan.

High DIY-safe £150-450 Shared across the Assure range

What to check first

  1. Check that your gas emergency control valve is open and other gas appliances are working
  2. Check if your condensate pipe has frozen if it is cold outside
  3. Press the Reset button for five seconds

Parts commonly replaced

  • Fan
  • Gas Valve
  • Ignition Electrode

Related codes

Full explanation for E128
E130

Your boiler has sensed it is getting too hot too quickly, likely because the water is not moving through the system properly or there is air trapped inside.

High DIY-safe £100-£350 Shared across the Assure range

What to check first

  1. Check that your system pressure is between 1.0 and 1.5 bar on the pressure gauge
  2. Ensure all radiator valves are fully open to allow water to flow
  3. Bleed your radiators to remove any trapped air that might be blocking circulation
  4. Press the Reset button for five seconds

Parts commonly replaced

  • Circulation Pump
  • Overheat Thermostat
  • Flow/Return Sensor

Related codes

Full explanation for E130
E133

Your boiler has failed to ignite or stay lit, which usually means the gas supply has been interrupted or the condensate pipe is frozen.

High DIY-safe £120-300 Shared across the Assure range

What to check first

  1. Check that your gas stop tap is open and your credit (if on a prepay meter) hasn't run out.
  2. Inspect the white plastic pipe outside; if it is frozen, defrost it by pouring warm (not boiling) water over it.
  3. Press and hold the Reset button for five seconds.

Parts commonly replaced

  • Gas Valve
  • Ignition Electrode
  • Printed Circuit Board (PCB)
  • Hot Water NTC Sensor

Related codes

Full explanation for E133
Showing 31–31 of 31
E160

Your boiler's internal fan, which safely exhausts fumes, is spinng at the wrong speed or has stopped working entirely.

High Engineer only £250-£450 Shared across the Assure range

Parts commonly replaced

  • Fan assembly
  • Printed Circuit Board (PCB)
  • Wiring harness

Related codes

Full explanation for E160
Status & diagnostic codes

4 operating states & engineer diagnostics

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

000 Status Your boiler is currently in standby mode and is simply waiting for a signal to provide heating or hot water. 100 Status Your boiler is currently running at its absolute maximum capacity and is not modulating down as it should. 304 Diagnostic Your boiler is currently running a special testing or setup procedure designed for engineers, rather than operating in normal heating mode. 312 Status Your boiler is currently running a self-cleaning cycle to remove trapped air from the system and will return to normal operation shortly.
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