Johnson Starley C10d Fault codes & diagnostics

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

Regular Natural Gas 10.4 kW 2001-2007 Discontinued

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34 High 2 Medium
Just want the most common Johnson Starley C10d faults?
The top 6 problems in plain English — with causes, DIY checks and repair costs.
Fault codes

All 36 documented codes

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13

Your boiler has been manually reset too many times in a single hour and has locked itself as a safety precaution.

High DIY-safe £0-120

What to check first

  1. Locate the main electrical power switch for the boiler (usually a fused spur near the unit).
  2. Switch the power OFF.
  3. Wait approximately 30 seconds.
  4. Switch the power back ON to clear the lockout.

Parts commonly replaced

  • None (Software reset)
  • Control PCB (if fault persists)
Full explanation for 13
21

Your boiler's main control panel has developed an internal electrical error and can no longer process information correctly.

High Engineer only £250-£400

Parts commonly replaced

  • ESYS Control Board
  • Main PCB
Full explanation for 21
25

Your boiler's internal computer parts are having trouble communicating because their software versions do not match.

High Engineer only £280-£450

Parts commonly replaced

  • ESYS Electronic Control Board
  • Main PCB

Related codes

Full explanation for 25
26

Your boiler is struggling to keep its burner lit and has shut down for safety after failing to maintain a flame multiple times.

High Engineer only £120-350

Parts commonly replaced

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

Related codes

Full explanation for 26
30

Your heater has detected a technical fault with its internal temperature sensor, causing the system to shut down for safety.

High Engineer only £100-190

Parts commonly replaced

  • Duct air temperature sensor (NTC)
  • Sensor wiring harness

Related codes

Full explanation for 30
31

Your boiler has lost connection with its internal air temperature sensor, which means it cannot safely monitor the heat level and has stopped working.

High Engineer only £90-180

Parts commonly replaced

  • Duct air temperature sensor (thermistor)
  • Wiring harness

Related codes

Full explanation for 31
34

Your boiler is not receiving the correct amount of electricity from your home's power supply to run safely and has shut down as a precaution.

High Engineer only £90-250

Parts commonly replaced

  • Electrical Wiring
  • Printed Circuit Board (PCB)
  • External Voltage Regulator
Full explanation for 34
43

Your boiler's internal thermometer has developed an electrical fault, meaning the system can no longer safely monitor how much heat is returning to the unit.

High Engineer only £120-£240

Parts commonly replaced

  • Return air temperature sensor (thermistor)
  • Wiring harness
Full explanation for 43
45

Your boiler has detected an electrical fault with the sensor that monitors the exhaust gas temperature, causing it to shut down for safety.

High Engineer only £120-210

Parts commonly replaced

  • Flue gas temperature sensor
  • Wiring harness

Related codes

Full explanation for 45
46

Your boiler has shut down because it cannot detect a signal from the sensor that monitors exhaust gases, preventing it from running safely.

High Engineer only £120-£210

Parts commonly replaced

  • Flue Gas Temperature Sensor (NTC)
  • Wiring Harness

Related codes

Full explanation for 46
Showing 11–20 of 36
57

Your heater has automatically shut itself down because it is getting too hot and needs to cool down for safety.

Medium DIY-safe £90-180

What to check first

  1. Check that all warm air vents and registers are open and not blocked by furniture or curtains.
  2. Inspect the air filter and clean or replace it if it is dusty or clogged.
  3. Ensure the return air grille is clear of obstructions to allow proper airflow.

Parts commonly replaced

  • Air filter
  • Limit thermostat
  • Circulating fan motor

Related codes

Full explanation for 57
A01

Your boiler's main control board has lost connection with the display screen, meaning they can no longer talk to each other to operate the heating.

High Engineer only £180-350

Parts commonly replaced

  • Main PCB
  • Display Interface
  • Wiring Loom
Full explanation for A01
b18

Your heater has automatically switched off because the air in the vents has become too hot, usually caused by blocked airflow or a lack of ventilation.

High DIY-safe £90-180

What to check first

  1. Check all heating vents and grilles around the house to ensure they are fully open and not blocked by furniture or curtains.
  2. Inspect the air filter on the unit and clean or replace it if it is clogged with dust.
  3. Ensure the intake and exhaust terminals outside your property are clear of debris, leaves, or snow.

Parts commonly replaced

  • Air filter
  • Overheat thermistor
  • Limit thermostat

Related codes

Full explanation for b18
b19

Your boiler has shut down because it detected that the exhaust gases are getting too hot, which is a safety measure to prevent damage or dangerous fumes.

High Engineer only £120-350

Parts commonly replaced

  • Heat exchanger
  • Flue thermistor
  • Condensate trap

Related codes

Full explanation for b19
b24

Your boiler has stopped working because the internal fan is not spinning at the correct speed to safely clear away exhaust fumes.

High Engineer only £180-£450

Parts commonly replaced

  • Fan motor
  • Main control board (PCB)
  • Fan wiring harness

Related codes

Full explanation for b24
b25

Your boiler has switched itself off because it has reached a dangerously high temperature and needs to cool down for safety.

High Engineer only £120-280

Parts commonly replaced

  • High Limit Thermostat
  • Main Heat Exchanger
  • Circulation Pump

Related codes

Full explanation for b25
b26

Your boiler has switched itself off because the internal drain for waste water is full and cannot empty properly.

High DIY-safe £80-160

What to check first

  1. Locate the external plastic condensate pipe leaving your property.
  2. Check for any blockages at the end of the pipe or ice if the weather is freezing.
  3. Thaw any frozen sections using a hot water bottle or warm (not boiling) water.
  4. Restart the boiler using the reset button.

Parts commonly replaced

  • Condensate trap
  • Internal discharge pipe work
  • Condensate pump

Related codes

Full explanation for b26
E01

Your boiler is failing to light the gas flame despite several attempts, meaning your heating and hot water will not work.

High DIY-safe £120-350

What to check first

  1. Check if your gas supply is active by testing other gas appliances like a hob.
  2. Ensure your gas prepayment meter has sufficient credit.
  3. Press the 'Reset' button on the boiler control panel for 3 seconds.

Parts commonly replaced

  • Ignition Electrode
  • Gas Valve
  • Flame Sensing Probe
  • Printed Circuit Board (PCB)

Related codes

Full explanation for E01
E02

Your boiler thinks a flame is burning even though it hasn't been told to start yet, which is causing it to lock itself down for safety.

High Engineer only £120-280

Parts commonly replaced

  • Flame Sensing Electrode
  • Gas Valve
  • Main Control PCB

Related codes

Full explanation for E02
E03

Your boiler has shut down because it has reached an unsafe temperature and needs to cool down before it can be reset.

High DIY-safe £100-220

What to check first

  1. Check that all radiator valves are fully open
  2. Ensure the system pressure is between 1 and 1.5 bar
  3. Allow the boiler to cool down and try pressing the reset button

Parts commonly replaced

  • Overheat Thermostat
  • Circulation Pump
  • Thermistor

Related codes

Full explanation for E03
Showing 21–30 of 36
E04

Your boiler has detected that the internal fan, which safely removes waste gases, is not spinning at the correct speed to operate safely.

High Engineer only £200-£450

Parts commonly replaced

  • Fan Assembly
  • Main PCB
  • Venturi Tube

Related codes

Full explanation for E04
E05

Your boiler has lost connection with the sensor that monitors the temperature of the exhaust gases, causing it to stop running for safety.

High Engineer only £120-£210

Parts commonly replaced

  • Flue thermistor (NTC sensor)
  • Wiring harness connection

Related codes

Full explanation for E05
E06

Your boiler has detected an electrical fault with the sensor that monitors exhaust gases, causing the system to shut down for safety.

High Engineer only £120-210

Parts commonly replaced

  • Flue thermistor (NTC sensor)
  • Wiring harness
  • Main control board

Related codes

Full explanation for E06
E07

Your boiler's internal temperature sensor has stopped sending signals, causing the system to stop working as a safety precaution.

High Engineer only £90-160

Parts commonly replaced

  • Return air thermistor
  • Wiring harness connector

Related codes

Full explanation for E07
E08

Your boiler has detected an electrical fault with the internal sensor that monitors the temperature of the air returning to the unit, causing it to shut down.

High Engineer only £120-220

Parts commonly replaced

  • Return air thermistor
  • Sensor wiring harness
  • Main control board

Related codes

Full explanation for E08
E11

Your boiler is failing to detect the flame needed to heat your water, so it has shut down for safety.

High Engineer only £120-280

Parts commonly replaced

  • Flame rectification probe
  • Ignition electrode
  • PCB (Printed Circuit Board)

Related codes

Full explanation for E11
E12

Your boiler's main internal computer has developed a fault and can no longer control the heating system safely.

High Engineer only £280-£450

Parts commonly replaced

  • Main Printed Circuit Board (PCB)

Related codes

Full explanation for E12
E15

Your boiler has detected that the exhaust gases are getting too hot and has shut down as a safety precaution to prevent internal damage.

High Engineer only £120-350

Parts commonly replaced

  • Heat exchanger
  • Flue thermistor
  • Main PCB

Related codes

Full explanation for E15
E16

Your boiler has detected that a safety sensor for air circulation is stuck, preventing the heating process from starting safely.

High Engineer only £120-220

Parts commonly replaced

  • Air pressure switch
  • Venturi tube
  • Fan assembly

Related codes

Full explanation for E16
E17

Your boiler has detected that a safety sensor hasn't moved into the correct position to allow the fan and burner to start safely.

High Engineer only £120-250

Parts commonly replaced

  • Air pressure switch
  • Fan assembly
  • Venturi tube

Related codes

Full explanation for E17
Showing 31–36 of 36
E19

Your boiler has stopped working because waste water cannot leave the system, likely due to a full or faulty pump or a blocked drainage pipe.

High DIY-safe £120-250

What to check first

  1. Check the external white plastic drainage pipe for any blockages or ice if it is cold outside.
  2. Ensure any visible condensate collection tanks are not full or overflowing.
  3. Gently pour warm water over the external pipe if you suspect it is frozen.

Parts commonly replaced

  • Condensate pump
  • Condensate trap
  • Internal drainage hosing
Full explanation for E19
E27

Your boiler's internal temperature sensor has lost its connection or stopped working, causing the system to shut down for safety.

High Engineer only £120-210

Parts commonly replaced

  • Duct thermistor (NTC sensor)
  • Wiring harness
  • Main control board

Related codes

Full explanation for E27
E28

Your boiler's temperature sensor responsible for monitoring the warm air flow has detected an electrical fault and is preventing the system from running safely.

High Engineer only £120-£190

Parts commonly replaced

  • Duct thermistor (NTC sensor)
  • Wiring harness

Related codes

Full explanation for E28
E31

Your boiler has sensed that the air inside the system is getting dangerously hot and has shut down to protect itself.

High DIY-safe £120-280

What to check first

  1. Check all warm air vents (registers) throughout the house are open and not blocked by furniture.
  2. Inspect the air filter and clean it if it is dusty or clogged.
  3. Restart the boiler using the reset button once the unit has cooled down.

Parts commonly replaced

  • Air filter
  • Circulation fan motor
  • Duct temperature sensor (Thermistor)

Related codes

Full explanation for E31
E32

Your boiler's internal fan, which moves warm air around your home, has stopped working correctly and needs professional attention.

High Engineer only £180-350

Parts commonly replaced

  • Air circulation fan motor
  • Main PCB
  • Fan capacitor

Related codes

Full explanation for E32
Lt

Your heater is temporarily shutting down because it is getting too hot and needs to cool down for safety.

Medium DIY-safe £60-140

What to check first

  1. Check all warm air grilles/vents are open and not blocked by furniture or curtains
  2. Inspect the air filter and clean or replace it if it is dusty
  3. Turn the heater off and back on to reset once it has cooled down

Parts commonly replaced

  • Air filter
  • Limit thermostat
  • Fan motor
Full explanation for Lt