Baxi Promax Combi Fault codes & diagnostics

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

Combi Natural Gas 24-33 kW 2006-2015 Discontinued

Replaced by: Potterton Promax Ultra Combi

This boiler uses indicator light combinations

Note which lights are on, off, or flashing and at what speed. Match the combination to the cards below. Do not reset until you have noted the pattern. How to read fault codes →

Severity at a glance

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

All 16 documented codes

Tap any card for details

LED / flash-pattern codes
Indicator Neons

Your boiler is using a combination of flashing or solid lights on the front panel to tell you it has detected a problem and may need a manual reset.

High DIY-safe £100-350

What to check first

  1. Check if the gas supply is active (e.g., check your gas meter or other gas appliances)
  2. Turn the selector switch to the 'Reset' position and hold for 5 seconds
  3. Check that your central heating system pressure is between 1 and 1.5 bar on the gauge

Parts commonly replaced

  • Printed Circuit Board (PCB)
  • Air Pressure Switch
  • Overheat Thermostat

Related codes

Full explanation for Indicator Neons
Overheat indicator

Your boiler has become too hot and has switched itself off as a safety precaution.

High DIY-safe £100-280

What to check first

  1. Check that all radiator valves are open to allow water to flow
  2. Ensure the system pressure is between 1.0 and 1.5 bar on the gauge
  3. Press the white Reset button on the front of the control panel

Parts commonly replaced

  • Overheat Thermostat
  • Circulation Pump
  • Main Heat Exchanger cleaning
Full explanation for Overheat indicator
Digital codes
E01

Your boiler is unable to light a flame, which means you will not have any heating or hot water at the moment.

High DIY-safe £100-£350

What to check first

  1. Check if other gas appliances in your home, like a hob, are working to confirm your gas supply is active.
  2. If you have a prepayment meter, check that there is enough credit.
  3. Check if any external gas valves have been accidentally turned off.
  4. Press and hold the 'R' or reset button for at least five seconds to try and restart the boiler.

Parts commonly replaced

  • Ignition Electrode
  • Gas Valve
  • Spark Leads
  • PCB

Related codes

Full explanation for E01
E02

Your boiler has shut down because its internal temperature got too hot, acting as a safety precaution to prevent damage.

High DIY-safe £100-250

What to check first

  1. Check that your radiator valves are open and not turned to zero.
  2. Ensure there is enough water pressure in the system (typically 1.0 to 1.5 bar).
  3. Press and hold the Reset button for 5 seconds to see if the boiler restarts.

Parts commonly replaced

  • Overheat Thermostat
  • Circulation Pump
  • Central Heating Filter
Full explanation for E02
E03

Your boiler has detected a problem with how it releases exhaust gases or how much air is getting into the system, causing it to shut down for safety.

High Engineer only £100-£250

Parts commonly replaced

  • Air Pressure Switch
  • Venturi Tube
  • Fan
  • Flue Sensor
Full explanation for E03
E05

Your boiler has detected a problem with the internal sensor that monitors the temperature of the water used to heat your radiators.

High Engineer only £90-160

Parts commonly replaced

  • Central Heating (CH) Flow NTC Sensor
  • Connecting Wiring Harness

Related codes

Full explanation for E05
E06

Your boiler's hot water sensor has stopped working correctly, which means the system cannot accurately measure or control the temperature of your hot water.

Medium Engineer only £90-£160

Parts commonly replaced

  • Domestic Hot Water (DHW) Temperature Sensor
  • Wiring Harness

Related codes

Full explanation for E06
E10

Your boiler has detected that there is not enough water in the system to operate safely and has shut down as a precaution.

High DIY-safe £0-120

What to check first

  1. Locate the filling loop (a silver flexible hose) usually found underneath the boiler.
  2. Slowly open the one or two taps/valves on the filling loop until you hear water moving.
  3. Watch the pressure gauge until it reaches between 1 and 1.5 bar, then close the valves tightly.
  4. Reset the boiler using the selector switch or reset button to clear the fault.

Parts commonly replaced

  • Filling loop valves
  • Expansion vessel (recharge or replace)
  • Water pressure switch
Full explanation for E10
E25

Your boiler has shut down because it is getting too hot inside, likely because the water is not moving around the system correctly.

High Engineer only £150-£350

Parts commonly replaced

  • Circulation pump
  • Main heat exchanger
  • Diverter valve
Full explanation for E25
E35

Your boiler is sensing a flame inside the system even when it hasn't been told to light one, which causes it to shut down for safety.

High Engineer only £120-350

Parts commonly replaced

  • Flame sensing electrode
  • Gas valve
  • Printed Circuit Board (PCB)
Full explanation for E35
Flame Failure

Your boiler has failed to light or maintain a flame, meaning your heating and hot water have stopped working for safety reasons.

High DIY-safe £100-£350

What to check first

  1. Check that your gas supply is turned on and that other gas appliances are working
  2. Press the red 'Flame Failure' reset button once to restart the ignition sequence
  3. If you have a prepay gas meter, ensure it has sufficient credit

Parts commonly replaced

  • Ignition Electrode
  • Flame Sensing Probe
  • Gas Valve
  • PCB
Full explanation for Flame Failure
Flame Failure Reset Button

Your boiler has failed to light the gas flame and needs manual intervention to attempt to restart the ignition process.

High DIY-safe £100-350

What to check first

  1. Check that your gas pre-payment meter (if applicable) has credit.
  2. Ensure other gas appliances in the home, like a hob, are working correctly.
  3. Press and hold the reset button for five seconds to see if the boiler restarts.

Parts commonly replaced

  • Ignition Electrode
  • Gas Valve
  • Flame Sensing Lead
  • PCB
Full explanation for Flame Failure Reset Button
Overheat

Your boiler has switched itself off because it has become too hot to operate safely.

High DIY-safe £100-250

What to check first

  1. Locate the white Reset button on the front control panel
  2. Press the Reset button once to restart the boiler
  3. Ensure all radiator valves are open and that the system pressure is at 1.5 bar

Parts commonly replaced

  • Overheat Thermostat
  • Circulation Pump
  • Heat Exchanger
Full explanation for Overheat
Pressure Gauge (below 0.5 bar)

Your boiler has stopped working because the water pressure is too low for it to operate safely.

High DIY-safe £0-120

What to check first

  1. Locate the internal or external filling loop (the silver braided hose beneath the boiler).
  2. Slowly open the one or two taps/valves until you hear water entering the system.
  3. Watch the pressure gauge and close the taps once the needle reaches between 1.0 and 1.5 bar.
  4. Restart the boiler if it does not automatically reset.

Parts commonly replaced

  • Filling loop
  • Pressure sensor
  • Expansion vessel
Full explanation for Pressure Gauge (below 0.5 bar)
Red reset button illuminated

Your boiler has failed to ignite and has locked itself for safety, meaning you currently have no heating or hot water.

High DIY-safe £100-350

What to check first

  1. Check that your gas supply is turned on at the meter
  2. Check if other gas appliances are working (like a hob)
  3. Press the red illuminated button once to attempt a reset

Parts commonly replaced

  • Ignition Electrode
  • Gas Valve
  • Spark Lead
Full explanation for Red reset button illuminated
Safety Thermostat neon

Your boiler has shut itself down because it has become too hot inside, likely due to a circulation issue or a lack of water pressure.

High DIY-safe £90-280

What to check first

  1. Check the pressure gauge and top up to 1.5 bar if it is too low
  2. Ensure all radiator valves are open and not blocked by furniture
  3. Allow the boiler to cool down, then press and hold the reset button for 5 seconds

Parts commonly replaced

  • Overheat Thermostat
  • Central Heating Pump
  • Diverter Valve
Full explanation for Safety Thermostat neon